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SPATIAL ECOLOGY OF THE GOPHER TORTOISE (Gopherus polyphemus) IN COASTAL SAND DUNE HABITAT: BURROW SITE SELECTION, HOME RANGE AND

SEASONAL ACTIVITY PATTERNS

By

ANTHONY YIN KUN LAU

A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2011

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© 2011 Anthony Yin Kun Lau

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To the tortoises

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ACKNOWLEDGMENTS

This study would not be possible without the support of a large group of people. I

am indebted to my advisors Ken Dodd and Ray Carthy for guidance, mentorship, and

constructive and helpful criticism throughout this study. I also thank my two remaining

committee members Jerry Johnston and Kenneth Krysko for their invaluable input and

support.

I thank Joan Berish with the Florida Fish and Wildlife Conservation Commission

(FWC) for inspiring my interest in sand dune “hoover chickens”, sharing her expertise

with me through interesting discussions, and lending equipment for trapping tortoises. I

am grateful to the Gopher Tortoise Council for providing financial support for this study.

I thank the staff at Guana Tolomato Matanzas National Estuarine Research Reserve

(GTMNERR), particularly Joseph Burgess, Emily Montgomery, Randy Altman, and

Scott Eastman for providing access to study sites, GIS and tortoise data, and logistical

support throughout the study period.

I am indebted to the many volunteers that have helped me in the field under the

scorching Florida sun, particularly Alice Chow, Stephen Harris, Robert Lara, David

Markus, Ryo Sakurai, Irina Skinner, Chad Teal, Nick Tolopka, and Travis Thomas.

Special thanks goes to Cody Godwin and Eric Suarez, who helped me track tortoises

when I was unable to do so.

I am thankful for fellow Wildlife Ecology and Conservation (WEC) graduate

students and building 150 office-mates, particularly Willandia Didier-Chaves, Jackson

Frechette, Santiago Espinosa, Fang-Yuan Hua, Gloria Lentijo, Mauricio Nunez-

Regueiro, Eduardo Andres Silva Rodriguez, Jose Soto, and Marianela Velilla for

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friendship and statistical assistance. I thank Jason Fidorra, Theresa Walters and Natalie

Williams for providing helpful comments on my proposal and earlier drafts of this thesis.

I thank the faculty and staff of the Department of Wildlife Ecology and

Conservation, who provided a great learning environment and helped me throughout

the process of obtaining funding, meeting deadlines, and staying out of trouble.

I thank Akiko Arai and my dog Piper for their companionship and mental support

throughout this study. Last but not least, I am thankful to my family for allowing me to

follow my passions and pursue a career in tortoise hunting and cooter (turtle) chasing.

This research was conducted under FWC permit LSSC-10-0005 and University of

Florida Animal Research Committee permit 015-09WEC.

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TABLE OF CONTENTS page

ACKNOWLEDGMENTS .................................................................................................. 4

LIST OF TABLES ............................................................................................................ 8

LIST OF FIGURES .......................................................................................................... 9

ABSTRACT ................................................................................................................... 10

CHAPTER

1 INTRODUCTION .................................................................................................... 12

Habitat Selection ..................................................................................................... 12 Hierarchy in Habitat Selection .......................................................................... 13

Habitat Selection in Reptiles ............................................................................. 14 Fossorial Reptiles ................................................................................................... 16 Status, Ecology, and Conservation of Gopher Tortoises ........................................ 17

Project Objectives ................................................................................................... 19 Description of the Habitat and Study Site ............................................................... 20

2 FACTORS INFLUENCING BURROW SITE SELECTION OF GOPHER TORTOISEs IN COASTAL SAND DUNE HABITAT ............................................... 32

Background ............................................................................................................. 32 Influences of Anthropogenic Disturbances ....................................................... 34 Habitat Selection of Gopher Tortoises .............................................................. 35

Methods .................................................................................................................. 37 Study Area ........................................................................................................ 37

Burrow Survey and Available Burrow Sites ...................................................... 38 Explanatory Variables ...................................................................................... 38

Data Analysis .......................................................................................................... 40 Burrow Site Selection ....................................................................................... 40 Model Selection ................................................................................................ 40

Results .................................................................................................................... 41 Discussion .............................................................................................................. 42

3 HOME RANGE, SEASONAL ACTIVITY AND MOVEMENT PATTERNS OF GOPHER TORTOISE IN COASTAL SAND DUNE HABITAT ................................. 55

Background ............................................................................................................. 55 Methods .................................................................................................................. 58

Study Area ........................................................................................................ 58

Radio-Telemetry ............................................................................................... 59 Data Analysis .......................................................................................................... 60

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Results .................................................................................................................... 60 Movement Distances ........................................................................................ 61 Burrow Use and Activity Patterns ..................................................................... 61

Annual Home Range ........................................................................................ 62 Dispersal Pattern .............................................................................................. 62

Discussion .............................................................................................................. 62

4 CONCLUSIONS ..................................................................................................... 71

Status of Sand Dune Tortoises ............................................................................... 71

Effects of Human Activities ..................................................................................... 73

Sea Level Rise ........................................................................................................ 74

Management Implications ....................................................................................... 75 Coastal Habitat Management ........................................................................... 75 Feasibility As Relocation Sites ......................................................................... 76 Public Education ............................................................................................... 77

Future Research ..................................................................................................... 78

APPENDIX

A HABITAT CHARACTERISTICS (BURROW AND RANDOM LOCATIONS) OF ADULT GOPHER TORTOISES IN COASTAL SAND DUNES, GUANA TOLOMATO MATANZAS NATIONAL ESTUARINE RESEARCH RESERVE, SOUTH PONTE VEDRA BEACH, FLORIDA, USA (2010-2011) ............................ 83

B SPEARMAN’S NON-PARAMETRIC CORRELATION MATRIX FOR VARIABLES CONSIDERED IN BURROW SITE SELECTION MODEL ................. 84

LIST OF REFERENCES ............................................................................................... 85

BIOGRAPHICAL SKETCH ............................................................................................ 94

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LIST OF TABLES

Table page 2-1 Potential factors influencing burrow site selection .............................................. 48

2-2 Burrow site selection models .............................................................................. 49

2-3 Relative importance of burrow site selection factors ........................................... 50

3-1 Mean annual home range (ha) of adult Gopher Tortoises .................................. 66

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LIST OF FIGURES

Figure page 1-1 Map of Gopher Tortoise study site ...................................................................... 24

1-2 Aerial photographs of a portion of the northern coastal sand dune ................... 25

1-3 Representative topographic profile and land cover maps ................................... 26

1-4 Aerial photographs of a portion of the southern coastal sand dune (1/4) ........... 27

1-5 Aerial photographs of a portion of the southern coastal sand dune (2/4) ........... 28

1-6 Aerial photographs of a portion of the southern coastal sand dune (3/4) ........... 29

1-7 Aerial photographs of a portion of the southern coastal sand dune (4/4) ........... 30

1-8 A female Gopher Tortoise (Gopherus polyphemus) foraging ............................. 31

2-1 Probability of burrow site selection as a function of soil resistance .................... 51

2-2 Probability of burrow site selection as a function of distance to edge ................. 52

2-3 Probability of burrow site selection as a function of % herbaceous cover .......... 53

2-4 Probability of burrow site selection as a function of no. of tortoise burrows ........ 54

3-1 A Gopher Tortoise with a radio transmitter ......................................................... 67

3-2 Seasonal movement patterns (mean distance traveled per move + SE) ............ 68

3-3 Seasonal movement patterns (mean number of burrows used + SE) ................ 69

3-4 Mean annual home range (MCP) + SE of adult Gopher Tortoises ..................... 70

4-1 Size class distribution of Gopher Tortoise burrows ............................................. 80

4-2 Monthly beach visitor counts (January 2009 – December 2010) ........................ 81

4-3 The Florida coastline after a sea level rise of 3 meters ...................................... 82

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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science

SPATIAL ECOLOGY OF THE GOPHER TORTOISE (Gopherus polyphemus) IN

COASTAL SAND DUNE HABITAT: BURROW SITE SELECTION, HOME RANGE AND SEASONAL ACTIVITY PATTERNS

By

Anthony Yin Kun Lau

December 2011

Chair: C Kenneth Dodd, Jr. Cochair: Raymond Carthy Major: Wildlife Ecology and Conservation

The Gopher Tortoise (Gopherus polyphemus) has been suggested to be a

keystone species in southeastern upland sandhill ecosystems, and its habitat

requirements have been well documented. Relatively few studies have been conducted

on populations that occur in coastal sand dunes. Due to their close proximately to the

ocean and highly fragmented habitat, coastal populations of Gopher Tortoises are

affected by unique factors that are not observed in upland populations.

Ideal Gopher Tortoise habitats in uplands consist of well-drained soil, open

canopy, and abundant herbaceous ground vegetation. In this study, burrow site

selection of Gopher Tortoises in a coastal sand dune site was quantitatively modeled

and significant biological, environmental, and anthropogenic factors that may influence

burrow site selection at fine and coarse spatial scales were identified. Land cover type,

distance to edge, soil resistance, percentage herbaceous cover, slope angle, and

number of tortoise burrows have significant influences on burrow site selection

probability.

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Radiotelemetry techniques were used to characterize habitat use, movement and

seasonal activity patterns, and areal requirements of adult Gopher Tortoises. Twenty

tortoises were fitted with radio-transmitters and tracked for up to 12 months. Home

range size, distances moved, number of burrows used, and duration of winter inactivity

were determined and used to compare with previous studies conducted in upland

habitats. Home range size was determined using minimum convex polygon (MCP) and

fixed kernel methods. Sand dune Gopher Tortoises exhibited significantly smaller home

ranges and used fewer number of burrows than upland tortoises. Dispersal away from

sand dune habitats was not observed probably because of the proximity of a heavily

used highway paralleling the coast.

Conservation needs and management implications for sand dune Gopher

Tortoises are discussed. Coastal sand dunes may serve as short-term alternative

recipient sites for Gopher Tortoises affected by human development.

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CHAPTER 1 INTRODUCTION

Habitat selection across multiple spatial and temporal scales is a central topic in

wildlife ecology. Extensive research has been conducted on a variety of taxa, and the

insight gained from these studies has dramatically improved our understanding of

spatial and resource requirements of many species. Consequently, conservation

planning for habitat preservation and restoration has been improved for many imperiled

taxa and ecosystems.

Relatively few quantitative studies have been conducted on habitat selection, as

opposed to simple habitat use, of burrowing terrestrial reptiles largely because of their

cryptic behavior. Conservation priorities for these species include a better

understanding of: 1) the process of habitat selection, 2) the integral roles of these

species in their ecosystems, and 3) anthropogenic influences on habitat selection and

movement patterns. Further, there is a need to effectively communicate an appreciation

of these roles to the public and to resource managers.

Habitat Selection

The relationship between animals and their environments has been a subject of

interest to biologists for centuries, and the association is a fundamental question in

ecology and conservation (Martin 1998, Groom et al. 2006). Animals, unlike plants,

have the ability to actively choose their habitat. In addition to climatic conditions and

availability of food and breeding sites, animals frequently are able to recognize distinct

features and actively select optimal habitat; this process is termed habitat selection.

Some general models have been developed to demonstrate this process: ideal free

distribution (Fretwell and Lucas 1970); despotic distribution (Fretwell 1972, Van Horne

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1983); ideal preemptive distribution (Pulliam and Danielson 1991). Habitat selection has

considerable influences on survivorship and reproductive success of a species (Thomas

and Taylor 1990, Block and Brennan 1993). Many migratory species, such as pelagic

mammals and song birds, require distinct habitats for overwintering, nursing, feeding,

and mating; whereas most pond-breeding amphibians depend on both aquatic and

terrestrial habitats (Semlitsch 2000). Understanding what habitat features are important

for wildlife in different life stages will allow conservationists to protect, improve, and

restore habitats.

Hierarchy in Habitat Selection

Some of the earlier literature described habitat selection as a two-staged process

(Svardson 1949, Hilden 1965). In the first stage, individuals select among different

environments using general features over a broad landscape scale. In the second

stage, individuals respond to more subtle and specific habitat features within the

selected landscape. Habitat selection may be influenced by a wide array of factors or

features such as visual habitat cues, habitat structure, bioenergetics, and the presence

of competitors, predators, and prey. The complicated, multivariate nature of the habitat

selection process has given rise to the concept of the n-dimensional niche (Hutchinson

1957).

Habitat selection of nonbreeding migratory birds has been suggested to be a multi-

stage hierarchical decision-making process (Hutto 1985). Johnson (1980) proposed a

similar scheme that consists of four orders of selection. First order selection, at the

broadest scale, represents the selection of the geographic or physical range of a

species restricted by climatic conditions and geographic continents. Second order

selection represents the home range of an animal within its geographic range. Third

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order selection represents a particular area within an animal’s home range where it

chooses to hunt, forage, rest, mate or nest. Fourth order selection, at the finest scale,

represents specific microhabitat features (i.e., a particular perch or a log) within a

particular site (i.e., feeding ground or nesting ground) determined in third order

selection. In vertebrates, habitat selection studies often focus on second order (i.e.,

home range/territory) and third order selection (i.e., nesting ground/feeding ground)

because manipulation and replication at the scale in which these selections operate is

more feasible than those at larger scales.

The hierarchical nature of habitat selection has also been an emphasis in

landscape ecology, where the scale of ecological processes such as habitat selection

and habitat use are explicitly considered (Lima and Zollner 1996). Since then, habitat

selection has been studied in many taxa across different ecosystems at multiple spatial

scales. These studies have improved understanding and knowledge of habitat

requirements, species interactions, home range use and migratory behavior of many

wildlife species. Since habitat selection directly affects a species’ fitness, improved

understanding of habitat selection processes undoubtedly contributes to wildlife

managers’ success with conservation and management.

Habitat Selection in Reptiles

As noted by Rouse et al. (2011), habitat selection in reptiles may be influenced by

many factors, such as foraging mode, mating system and reproductive mode. Reptiles,

because of their ectothermic, terrestrial lifestyle, may have different habitat selection

processes and habitat requirements than similar sized endothermic birds or mammals.

Most reptiles are incapable of regulating their body temperature (Tb) metabolically and

rely on basking to increase their Tb; as such, appropriate basking locations play an

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important role in reptile habitat selection. Burrowing is another important aspect of

reptile body temperature regulation. Because many lizards, snakes, and tortoises live in

underground burrows, which serve as shelters from predators and sites for

thermoregulation, the sub-surface physical characters of habitats (e.g., soil friability and

moisture content) become important components of habitat use. Few studies have

focused on fine scale habitat selection by burrowing reptiles due to the fact that many

reptiles, especially squamates, are fossorial or semi-fossorial. Further, the paucity of

studies is partially due to the difficulty of observing free-living reptiles under natural

conditions and finding their nests, burrows, or underground retreats (Burger and

Gochfeld 1991, López et al. 1998, Dodd and Barichivich 2007).

Habitat selection of tortoises has been studied by many authors using

radiotelemetry techniques. Kazmaier et al. (2001) studied Texas Tortoises (Gopherus

berlandieri) in semiarid shrublands and found that Texas Tortoises exhibited broad-

scale selection for home ranges and avoided areas with high (riparian) and low (old-

field) canopy cover. In Mediterranean semiarid shrublands, Greek Tortoises (Testudo

graeca) select habitats with simplified vegetation structures (re-colonization shrublands

and small non-irrigated fields) at the landscape scale and areas with high and complex

vegetation cover at the home range scale (Anadon et al. 2006). Guzman and Stevenson

(2008) studied Yellow-footed Tortoises (Chelonoidis denticulata) in the Peruvian

Amazonian forest and found that C. denticulata have strong preference for open-canopy

swamp in floodplain forest and bamboo- or grass-dominated vegetation in terra firma

forest. The results from these studies suggest that habitat selection of tortoises is

mostly influenced by canopy cover and vegetation structure.

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The effects of intra- and interspecies interaction and fine-scale resource

distribution on habitat selection of subterranean reptiles are seldom examined. A more

thorough understanding of the role of these factors (temperature, species interactions,

and resources distribution) on habitat selection of reptiles is needed in order to

efficiently conserve these species.

Fossorial Reptiles

Fossorial reptiles are an important component of many ecosystems. For example,

tortoises of the genus Gopherus excavate extensive underground burrows that provide

shelter for many vertebrate and invertebrate species. More than three hundred species

of animals have been documented using Gopher Tortoises (G. polyphemus) burrows

(Young and Goff 1939, Jackson and Milstrey 1989, Kent et al. 1997). Some of these

animals are obligate burrow commensals found nowhere else (Mushinsky et al. 2006).

Due to the importance of Gopher Tortoise burrows to the biodiversity of upland

ecosystems, Gopher Tortoises are viewed as a keystone species and an ecosystem

engineer (Eisenburg 1983, Mushinsky et al. 2006). Some snakes (e.g., Masticophis

spp., Pituophis spp., Crotalus spp., and Drymarchon spp.) are apex predators in their

respective ecosystems and the decline in these species may indirectly alter the

population and community structure of their prey and ecosystems. Additionally, fossorial

reptiles recycle micronutrients to the top soil layer in the process of digging and

burrowing and may affect above ground vegetation community structure (Abaturov

1972).

Fossorial reptiles face a number of threats globally. For example, Gopher Tortoise

populations in the southeastern United States have declined due to rapid development

in the region, human harvesting for food, and habitat degradation caused by fire

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suppression (Mushinsky et al. 2006). Consequently, populations of associated burrow

commensals such as the Gopher Frog (Lithobates capito), Florida Mouse (Podomys

floridanus) and Eastern Indigo Snake (D. couperi) have declined. In addition, these

species are generally neither charismatic nor popular among the general public, making

it difficult to garner public support for conservation projects of fossorial or semi-fossorial

reptiles. For example, Kellert (1980) found that the general public is less willing to

modify energy projects for the sake of protecting Eastern Indigo Snakes (D. couperi)

compared with charismatic animals such as Pumas (Puma concolor) or Bald Eagles

(Haliaeetus leucocephalus). Dodd (1987) noted the general public’s prejudice against

snakes, or “ophidiophobia,” may be an impediment in snake conservation.

Status, Ecology, and Conservation of Gopher Tortoises

The Gopher Tortoise, Gopherus polyphemus (Daudin 1802), is the only testudinid

that occurs in eastern North America (east of the Mississippi River) and is one of the

most studied tortoise species in the world. The Gopher Tortoise is so named because of

its impressive burrowing ability; Gopher Tortoise burrows on average are 4.5 m in

length and 2 m in depth (Hansen 1963). This long lived, moderate-sized herbivorous

tortoise usually occurs in habitats with well drained, sandy soils with open canopy that

support herbaceous plant growth. It ranges from extreme southeastern South Carolina,

south through peninsular Florida and west through southern Georgia, Alabama,

Mississippi and southeastern Louisiana (Ernst and Lovich 2009). The Gopher Tortoise

is emblematic of many southeastern fire-adapted ecosystems as it often thrives in well-

managed, frequently burned upland ecosystems. Some of the most notable places that

support large populations of Gopher Tortoises are Ocala National Forest and Merritt

Island National Wildlife Refuge in Florida. In addition to upland pine forest and sandy

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scrubs, Gopher Tortoises also can be found in coastal sand dunes and on barrier

islands off the coast of Florida and Georgia.

Populations of Gopher Tortoises appear to be declining range-wide, mainly

outside of protected areas (Hermann et al. 2002, Smith et al. 2006). Populations are

affected by a number of threats. Habitat destruction and degradation, habitat

fragmentation, and upper respiratory tract disease (URTD) have been cited as the

greatest threats to Gopher Tortoise survival (Mushinsky et al. 2006). In Florida, tortoise

habitats are constantly being replaced by housing developments, shopping centers,

pine plantations and agriculture. More roads are being built to support the ever-growing

human population. Consequently, tortoise habitats and populations are becoming more

fragmented as roads act as effective barriers to tortoise movements.

In the past, tortoises have been slaughtered for food and some populations in

northwestern Florida may have been hunted to extirpation (J. E. Berish, personal

communication). Human predation on Gopher Tortoises is still taking place in parts of its

range despite its protected status (Mushinsky et al. 2006, T. M. Thomas, personal

communication).

The Gopher Tortoises’ role as an ecosystem engineer and its long lifespan (up to

50-70 years) has inspired many ecologists to study their ecology. Several long-term

studies have been conducted to determine population density and abundance, size-

class structure, sex ratio, survival and fecundity (Landers et al. 1980, Landers et al.

1982, Auffenberg and Franz 1982, Diemer 1992b). Burrowing, grazing, and mound

building by Gopher Tortoises have been found to increase vegetative species richness

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and promote environmental heterogeneity in an oak-pine sandhill forest (Kaczor and

Hartnett 1990).

The Gopher Tortoise is a protected species throughout its range and is listed as

“vulnerable” on the International Union for Conservation of Nature (IUCN) Red List of

Threatened Species (IUCN 2011). On the federal level, the U.S. Fish and Wildlife

Service listed populations west of Mobile and Tombigbee Rivers in Alabama,

Mississippi and Louisiana as “threatened” under provisions of the Endangered Species

Act of 1973, as amended. Currently, populations in the eastern part of its range are

being considered to be listed as “threatened” (USFWS 2011). In Florida, where this

species has been historically found in all 67 counties, it is also listed as “threatened”.

The Florida Fish and Wildlife Conservation Commission specifically prepared a

management plan (FWC 2007) to guide conservation and research efforts on this

species and its habitats in Florida.

Project Objectives

To date, few studies have incorporated anthropogenic factors such as human

disturbances and anthropogenic habitat modifications. Further, even fewer have

considered factors at multiple spatial scales (Row and Blouin-Demers 2006). Many

biological, environmental, and anthropogenic variables have been shown to affect

habitat selection of wildlife, but the impact may not be detectable in all spatial scales.

Thus, there is a need to integrate multiple factors and spatial scales into habitat

selection analyses. I therefore conducted a study on the spatial ecology of one of North

America’s most studied reptiles in a relatively understudied habitat in order to identify

biological, environmental, and anthropogenic factors that may influence Gopher

Tortoise burrow sites selection at fine and coarse spatial scales. I quantified the habitat

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characteristics of occupied versus randomly selected habitats as well as the effects of a

priori selected factors on burrow site selection. Finally, I used radio-telemetry to

determine home ranges, movements and seasonal activity patterns of adult Gopher

Tortoises. Measures of movement and spatial dispersion (Rouse et al. 2011) allowed

me to access the extent of current occupancy and to help evaluate this and other

coastal dune ecosystems as relocation sites for tortoises moved from lands slated for

development.

Description of the Habitat and Study Site

The field study was conducted between 15 January 2010 and 7 May 2011 at the

coastal sand dunes within Guana Tolomato Matanzas National Estuarine Research

Reserve (GTMNERR, 30.126111, -81.348056) on the northeastern coast of Florida, St.

Johns County (Figure 1-1). The northern section of GTMNERR includes some of the

only pristine, undeveloped dunes on the eastern coast of Florida. The ca. 78 ha study

area (7.1 km long and 90-145 m wide) is composed mostly of beach dune and coastal

strand habitat (FNAI 2010). Beach dune is an herbaceous community built by a number

of coastal specialist grasses such as Sea Oats (Uniola paniculata) and Bitter

Panicgrass (Panicum amarrum), whose stems trap the sand grain blown off the beach

and form the foundation of the dune. Other plants commonly seen in the beach dune

are Railroad Vine (Ipomoea pescaprae spp. brasiliensis), Seacoast Marshelder (Iva

imbricata), Saltmeadow Cordgrass (Spatina patens), Beach Morning Glory (Ipomoea

imperati), Indian Blanket (Gaillardia pulchella), Bull Thistle (Cirsium horridulum) and

Prickly Pears (Opuntia spp.). Coastal strand is the woody plant community located

further inland and directly behind the beach dune. It is dominated by dense Saw

Palmetto (Serenoa repens) and dwarfed Cabbage Palm (Sabal palmetto) on the

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seaward edge, and Red Bay (Persea borbonia), Red Cedar (Juniperus virginiana),

Yaupon Holly (Ilex vomitoria), and Tough Bully (Sideroxylon tenax) further inland.

Habitat heterogeneity and vegetation community succession in this site is driven

mainly by water, wind and occasionally fire. In the beach dune, plants on the foredune

are constantly exposed to salt spray and sand burial from onshore wind blowing from

the Atlantic Ocean, while plants on the upper dune are subjected to the same stresses

in addition to occasional inundation by storm tides and destruction by strong waves. The

plants in the beach dune are adapted to withstand these stresses. In the coastal strand,

salt spray blown off the ocean maintains a low, even canopy of hardwood. The natural

fire frequency in this community is unknown. However, the major vegetative component

(Saw Palmetto) is highly flammable and fires typically spread rapidly in the coastal

strand when started.

The study area is bordered by a high-use road (Florida State Road A1A) to the

west, the Atlantic Ocean to the east, and private housing developments to the north and

south. It includes some of the only pristine and undeveloped dunes on the eastern coast

of Florida. The beach is accessible by humans through three elevated public beach

accesses, located at 0.66 km, 1.76 km, and 5.46 km north of the southern-most point of

the study site, respectively. In addition to these three boardwalks, there are foot paths

that dissect the beach dune at various locations. These paths are believed to be created

by frequent foot traffic of beachgoers before the public beach accesses were made.

Public access to the beach dune is restricted to protect the vegetation community of the

coastal beach dune.

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The study area can be further divided into two sections by width, topographic

profile, and vegetation community structure (Figure 1-3). The northern section (15.7 ha)

consists of the area north of the northern-most public beach access (Figure 1-2),

whereas the southern section (62.3 ha) consists of the remaining area (Figure 1-4, 1-5,

1-6, and 1-7). The overall width of the northern section (min = 124 m, max = 145 m) is

wider than the southern section (min = 90 m, max = 122 m). The northern section has

the same number of blow-outs and unofficial paths (n = 4) as the southern section

despite being significantly shorter. The overall topographic profile in the southern

section is steeper than the northern section, and the vegetative community appears to

be less heterogeneous.

In addition to natural disturbances, the study site has experienced large scale

anthropogenic disturbances in the past. In the early 19th century, the Ponte Vedra

Beach area, then known as Mineral City, was mined for rutile and ilmenite, which

contain titanium and zirconium, respectively. Other minerals such as zircon, aluminum

silicate, and monazite have also been mined. The mining lasted until the early 1930s

when the state government pressured the mining company to stop mining operations

because of beach destruction (R. E. Moore, personal communication). During the late

1940s, free range cattle were a problem faced by the Community Association of Ponte

Vedra Beach, who installed electric fences to keep the cattle off of the beach. These

activities may have had long lasting effects on the vegetation community structure of the

sand dune, creating large areas of openings.

There are two soil types found in the study site: the Fripp series and the Beaches

series. The Fripp series comprises deep, rapidly permeable, well drained soils that

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formed in thick sediments near beaches. The Beaches series consists of shallow, well-

drained, moderately permeable soils that formed in residuum from fined grained,

metamorphic sandstone (USDA 2010). The Fripp series occurs predominantly in the

coastal strand and in the upper part of the beach dune, whereas the Beaches series is

restricted mostly to the foredune and the adjoining beach.

Gopher Tortoises are present at the site in varying densities. Based on burrow

surveys conducted by GTMNERR staff from 2005-2007 and myself during the present

study, the overall density of tortoise burrows in dunes ranges from 0.64 burrows/ha to

3.05 burrows/ha. Burrow density of the northern section (12.17 burrows/ha) is much

higher than the southern section (0.75 burrows/ha), probably due to the varying width,

topographic profile, and vegetative community structure between the two sections

(Figure 1-3). Many other animals use tortoise burrows in the sand dunes as temporary

shelters or permanent homes; during the course of this study, Eastern Coachwhip

(Masticophis flagellum, n = 3), Marsh Rabbit (Sylvilagus palustris, n = 2), and Southern

Toad (Anaxyrus terrestris, n = 1) were observed using tortoise burrows. Footprints of

Eastern Woodrats (Neotoma floridana) also have been observed in front of a tortoise

burrow.

Distinctive foraging trails created by Gopher Tortoises during their feeding forays

can be found near tortoise burrows (Figure 1-8). Gopher Tortoises may in fact be an

important stressor on fine scale heterogeneity of the herbaceous plant community in the

sand dune. As one of the major herbivores in this coastal community, tortoises may also

be an important seed disperser.

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Figure 1-1. Map of Gopher Tortoise study site in Guana Tolomato Matanzas National

Estuarine Research Reserve (GTMNERR) and adjacent counties. Florida, USA.

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Figure 1-2. Aerial photographs of a portion of the northern coastal sand dune study site

for Gopher Tortoises in Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA.

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Figure 1-3. Representative topographic profile and land cover maps of the northern (L)

and southern (R) sections of the coastal sand dune study site for Gopher Tortoises in Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA.

Legend

BURROWS

ELEVATION

CONTOUR_EL

-2 ft - 12 ft

13 ft - 17 ft

18 ft - 22 ft

23 ft - 27 ft

28 ft - 38 ft

LANDCOVER

FNAI

BEACH DUNE

COASTAL STRAND

±

0 60 12030 Meters

27

Figure 1-4. Aerial photographs of a portion of the southern coastal sand dune (1/4)

study site for Gopher Tortoises in Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA.

28

Figure 1-5. Aerial photographs of a portion of the southern coastal sand dune (2/4)

study site for Gopher Tortoises in Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA.

29

Figure 1-6. Aerial photographs of a portion of the southern coastal sand dune (3/4)

study site for Gopher Tortoises in Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA.

30

Figure 1-7. Aerial photographs of a portion of the southern coastal sand dune (4/4)

study site for Gopher Tortoises in Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA.

31

Figure 1-8. A female Gopher Tortoise (Gopherus polyphemus) foraging at the coastal

sand dune study site in Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA. (Photo courtesy of the author)

32

CHAPTER 2 FACTORS INFLUENCING BURROW SITE SELECTION OF GOPHER TORTOISES IN

COASTAL SAND DUNE HABITAT

Background

Habitat selection is a fundamental concept in ecology and has direct applications

to conservation. Disproportionate use of habitat and/or resources presumably improves

the fitness and survivorship of a species (Thomas and Taylor 1990, Block and Brennan

1993). For decades, habitat selection of wildlife has been studied extensively in

numerous species across multiple spatial and temporal scales. The insights gained from

these studies have dramatically improved our understanding of spatial and resource

requirements of many species. Consequently, conservation planning for habitat

preservation and restoration has been improved for many imperiled taxa and

ecosystems. Relatively few quantitative studies have been conducted on habitat

selection of fossorial reptiles, a vertebrate group that plays important ecological roles in

their respective ecosystems. A better understanding of the process of habitat selection

of these animals is needed in order to design conservation plans and manage protected

areas.

Reptiles, because of their ectothermic lifestyle, may have vastly different habitat

selection processes and habitat requirements than a similar sized endothermic bird or

mammal. Most reptiles are incapable of regulating their body temperature (Tb)

metabolically and rely on basking to increase their Tb. The use of burrows is another

important aspect of reptile body temperature regulation because it allows species to

control temperature by responding to changes in burrow temperature with depth through

direct contact with the substrate. Many lizards, snakes, and turtles live in underground

burrows, which serve as shelters from predators as well as retreats from harsh surface

33

temperatures or environmental conditions, such as drought. Few studies have focused

on fine scale habitat selection in burrowing reptiles, despite the fact that many reptiles,

especially squamates, are fossorial or semi-fossorial. This is due partially to the difficulty

of observing them under natural conditions and finding their nests or burrows (Burger

and Gochfeld 1991, Lopez et al. 1998, Dodd and Barichivich, 2007).

Habitat selection of tortoises has been studied by many authors using radio-

telemetry. Kazmaier et al. (2001) studied Texas Tortoises (Gopherus berlandieri) in

semiarid shrublands and found that Texas Tortoises exhibited broad-scale selection for

home ranges and avoided areas with high (riparian) and low (old-field) canopy cover. In

Mediterranean semiarid shrublands, Greek Tortoises (Testudo graeca) select habitats

with simplified vegetation structures (re-colonization shrubland and small non-irrigated

fields) at the landscape scale and areas with high and complex vegetation cover at the

home range scale (Anadon et al. 2006). Guzman and Stevenson (2008) studied Yellow-

footed Tortoises (Chelonoidis denticulata) in the Peruvian Amazonian forest and found

that C. denticulata have strong preference for open-canopy swamp in floodplain forest

and bamboo- or grass-dominated vegetation in terra firma forest. The results from these

studies suggest that habitat selection of tortoises is mostly influenced by canopy cover

and vegetation structures.

Despite these examples, the effects of intra- and interspecies interaction and fine

scale resource distribution on habitat selection of subterranean reptiles are seldom

examined. A more thorough understanding on the role of these factors (temperature,

species interactions, and resources distribution) on habitat selection of reptiles is

needed in order to efficiently conserve these species.

34

Influences of Anthropogenic Disturbances

Environmental and biological factors have significant influences on habitat

selection of wildlife at multiple spatial scales. Anthropogenic factors are also considered

to be influential in highly disturbed area, but seldom have the influences of all three

factors been studied simultaneously. Several studies (Burger and Gochfeld 1991,

Mahan and Yahner 1996, Peters and Otis 2007, Gallant et al. 2009) have attempted to

identify these factors and their relative importance. Anthropogenic disturbances could

influence habitat selection in a variety of ways. Gallant et al. (2009) studied habitat

selection of River Otters (Lontra canadensis) under different land-use regimes

(protected vs. disturbed riparian habitats) and found that environmental factors are more

influential than anthropogenic factors in describing habitat use. They suggested that

anthropogenic disturbances indirectly affect habitat selection of river otters by causing

loss of particular habitat features (e.g., beaver ponds) that are relevant to river otter

habitat requirements. Peters and Otis (2007) did not detect evidence for negative

effects of anthropogenic disturbances (level of boat disturbance) on roost-site selection

in shore birds, and noted that it is difficult to demonstrate impacts on shorebird fitness

as a result of anthropogenic disturbance. Anthropogenic disturbances may affect certain

aspects of species behavior such as social information (Berry 1986, Citta and Lindberg

2007, Betts et al. 2008) that are difficult for investigators to detect.

Many authors (Baldwin et al. 2006, Graeter et al. 2008, Blomquist and Hunter

2009) have investigated the effects of anthropogenic disturbances such as roads and

timber management practices on amphibian habitat selection and found that moisture is

the most important driver in habitat selection of amphibians at multiple spatial scales

35

and anthropogenic disturbances (e.g., clear-cuts) have different effects on habitat

selection depending on the species.

In addition to Peters and Otis (2007), other studies have investigated the effects of

anthropogenic disturbances on wildlife in coastal systems. Bird et al. (2004) studied the

effect of light-pollution on the foraging behavior of beach mice and concluded that

artificial lights reduce the foraging frequency and efficiency of beach mice and deserve

greater consideration in conservation planning and management. Witherington and

Martin (2000) studied the effects of light pollution on hatchling sea turtles and found that

artificial lighting disorients hatchling sea turtles from reaching the ocean. Coastal areas

worldwide are under rapid human encroachment and coastal ecosystems are under

tremendous pressure. In addition to light pollution, wildlife in coastal ecosystems are

subjected to invasive exotic plants, feral/domestic predators, habitat degradation, loss

and fragmentation, sand mining, and global climate change is inducing a rising sea

level. Coastal ecosystems (marshes, mangroves, reefs and dunes) worldwide are

important to humans because they provide essential ecosystem services such as storm

buffering, fisheries production and enhanced water quality (Barbier et al. 2008). The

inhabitants of these fragile ecosystems should be protected to ensure their long-term

persistence and proper functioning of these ecosystem services.

Habitat Selection of Gopher Tortoises

The Gopher Tortoise, Gopherus polyphemus, is a moderately sized, fossorial

tortoise that occurs in a variety of habitats with well-drained, sandy soil in southeastern

North America. The species is threatened by habitat loss and fragmentation throughout

its range, and the decline of this species may consequently affect many wildlife species

that share its habitat.

36

The tortoise burrow is an important feature in many ecosystems where Gopher

Tortoises occur. The burrow and its associated apron are important for a tortoise

because they are used for resting, basking, courting, shelter from predators, nest

deposition, and as the center of feeding activities. The Gopher Tortoise burrow is used

as a shelter by many other animals, some of which (i.e., Florida Mouse, Podomys

floridanus) even modify it by excavating a separate entrance called a chimney near the

main entrance to the Gopher Tortoise burrow. The tortoise burrow provides shelter to

other animals during cold, drought and fire, and some animals are obligate commensals

that can only be found in tortoise burrows. Thus, the distribution and survivorship of

these burrow commensals is directly dependent upon the habitat modifications made by

the Gopher Tortoise.

The Gopher Tortoise occurs in habitats with well-drained, sandy soils and

abundant low growing herbaceous cover, but the exact habitat features and

mechanisms that trigger a tortoise to excavate a burrow are poorly understood.

Hatchling and young tortoises use active or inactive adult burrows near the nest from

which they hatched (Smith 1992, J. E. Berish, personal communication), although some

also construct small burrows (Butler et al. 1995, Pike 2006). Adult tortoises exhibit high

burrow fidelity and may use the same burrows over multiple years. Since tortoises

spend most of their time in a burrow, previous studies on habitat selection of Gopher

Tortoises have focused on the distribution, density, ratio of active to inactive, and

habitat characteristics of the burrows (Boglioli et al. 2000, Jones and Dorr 2004,

Baskaran et al. 2006). A better understanding of the burrow site selection process will

37

allow wildlife managers to protect and improve high quality areas that will benefit

tortoises.

In this study, I used a quantitative modeling approach to identify factors that

influence burrow site selection of Gopher Tortoises, based on a set of a priori

hypotheses that incorporated various combinations of environmental, biological, and

anthropogenic factors measured at microhabitat and home range spatial scales. The

results of this study can help wildlife managers identify critical areas in which tortoises

are likely to settle and to focus conservation efforts on these areas.

Methods

Study Area

This study was conducted in Guana Tolomato Matanzas National Estuarine

Research Reserve (GTMNERR; 30.126111, -81.348056) located on the northeastern

coast of Florida, St. Johns County. The ca. 78 ha study area (7.1 km long; 90-145 m

wide) is composed of mostly beach dune and coastal strand habitat (FNAI, 2010).

Habitat heterogeneity and vegetation community succession in this site is driven mainly

by wind. The study site is bordered by a high-use road (Florida State Road A1A) to the

West, the Atlantic Ocean to the east, and private residences to the north and south. It

includes some of the only pristine and undeveloped dunes on the eastern coast of

Florida. The beach is accessible by humans through three boardwalks, located at 0.66

km, 1.76 km, and 5.46 km north of the southern-most point of the study site. In addition

to these three boardwalks, there are numerous foot paths that dissect the beach dune

at various locations. These paths are believed to have been created by frequent foot

traffic of beachgoers before the public beach accesses were made. Public access to the

38

beach dune is restricted in order to protect the vegetation community of the coastal

beach dune.

Burrow Survey and Available Burrow Sites

Burrow surveys were conducted from January to May 2010. The entire coastal

sand dune was searched systematically to locate and categorize Gopher Tortoise

burrows following methods described by Diemer (1992b). The latitude-longitude location

of each burrow was recorded (± 3 m) using a handheld GPS unit (Garmin GPSMAP

60CSx; Garmin International Inc, Olathe, Kansas). Each burrow was classified as

active, inactive, or abandoned, according to criteria described by Auffenberg and Franz

(1982). Of 238 burrows located, 100 active burrows were randomly selected using a

random number generator to determine factors influencing burrow site selection.

For comparison with randomly selected burrow sites, an equal number of random

points (n = 100) within the study site were generated as available habitat using the

Geospatial Modelling Environment extension in ArcGIS (Spatial Ecology LLC., 2010).

Explanatory Variables

To determine factors that may influence burrow site selection of Gopher Tortoises,

three groups of explanatory variables (environmental factors, biological factors, and

anthropogenic factors) were measured in two spatial scales: microhabitat scale (1 m

radius) and home range scale (20 m radius) from the burrow opening. The 1 m radius is

assumed to be an appropriate scale for microhabitat habitat selection because tortoises

are thought to have good vision. The 20 m radius is assumed to be an appropriate scale

for home range scale selection because it is the radius of the mean annual home

ranges of Gopher Tortoises in the sand dune (Lau, unpublished data; Chapter 3 of this

39

thesis). Measurements were made at both burrow and random locations from May to

August 2010.

Five environmental factors were measured: soil resistance, elevation, slope, and

slope angle were measured at the microhabitat scale. Soil type was measured at the

home range scale. Soil resistance was measured to the closest kg per cm2 using a

handheld pocket soil penetrometer (Geotest Instrument E280; Geotest Instrument

Corp., Evanston, Illinois) with a 2.54 cm diameter adapter foot. Elevation was measured

to the closest ft (0.3048 m) using a LIDAR-generated 1 foot contour map in a GIS

software (ESRI ArcMap 9.3, Redland, California). Slope was measured as percentage

change in elevation within 1 m using a meter stick and a level. Slope angle is the

orientation of the respective slope and was measured to the closest degree using a

compass. Soil type is defined as the dominant soil type of each location, following the

USDA Soil Survey Geographic Database classifications (USDA 2010).

Nine biological factors were measured: canopy cover, % bare ground, %

herbaceous cover, % grass, % scrub and vines and % litter were measured at the

microhabitat scale. Distance to edge, land cover type, and number of tortoise burrows

were measured at the home range scale. Canopy cover is defined as the presence or

absence of canopy forming plants (> 2 m in height) at each location. Percentage bare

ground, % herbaceous cover, % grass, % scrub and vines and % litter coverage were

visually quantified to the closest % in a 1 m x 1 m quadrat at each location. At burrow

locations, the soil deposit (apron) excavated by the tortoise during its burrowing activity

was not included in the 1 m2 quadrat. Distance to edge is defined as the shortest

distance in meters from each location to a different land cover type (i.e., between

40

coastal scrub and sand dune habitat), following Williams et al. (2002). Distance to edge

was measured on aerial images of the study site using GIS software. Land cover type is

defined as the dominant land cover type of each location following the Florida Natural

Area Inventory classifications (FNAI 2010). Number of tortoise burrows is the count of

Gopher Tortoise burrows (adults and subadults) located within a 20 m radius of each

location.

Two anthropogenic factors were measured. Distance to road is the perpendicular

distance of each location to the closest road. Distance to beach access is the

perpendicular distance of each location to the closest beach access. Both distances

were measured in meters on high resolution aerial images of the study site using GIS

software.

Data Analysis

Burrow Site Selection

To identify factors that influence burrow site selection, I used logistic regression

models to predict the probability of burrow site selection as a function of factors

measured in both spatial scales. The response variable is whether or not a habitat was

selected (burrow locations) or available (random locations). I used the generalized

linear model with binomial link function in SAS (Statistical Applications Systems, Cary,

North Carolina) to estimate regression coefficients with the logistic models.

Model Selection

To address potential multi-colinearity, a Spearman’s non-parametric correlation

matrix was created to identify factors that were strongly correlated (r > |0.6|). Factors

that were strongly correlated were excluded from the final models. Distance to edge is

strongly correlated with both elevation and distance to road. Distance to road and

41

elevation were both dropped from the models because the road is parallel to the

elevation gradient and these two variables provided essentially the same information as

distance to edge. Percentage cover of woody scrubs and vines is strongly correlated

with % coverage of bare ground. The latter was dropped because it is essentially the

reciprocal of the former. Nine logistic models containing different combinations of

factors were created: 8 based on a priori hypotheses and one global model that

included all factors. The best approximating model was selected using Akaike’s

Information Criterion corrected for small sample sizes (AICc) (Burnham and Anderson

2002, Anderson 2008). Wald’s Chi-square tests were used to test for significance of

factors. A factor was considered to be significant if p < 0.05. The relative importance of

factors was determined by summing Akaike weight of the models containing said

factors. The factor with the highest summed Akaike weight was considered to be the

most important (Burnham and Anderson 2001).

Results

Gopher Tortoise burrow site selection is influenced by both environmental and

biological factors at microhabitat and home range spatial scales. The environmental +

biological model was the best and most parsimonious model, and the global model was

within 2 AICc units and therefore considered to be a statistical equivalent of the best

model (Table 2-2). The two best models contain all the same factors except distance to

beach access (an anthropogenic factor) and have a combined model weight of 0.997.

Six of twelve factors are statistically significant at α = 0.05, indicating they are

good predictors for burrow site selection (Table 2-3). Of the continuous variables, soil

resistance, distance to edge, percent herbaceous cover, and number of tortoises

burrows are statistically significant. While holding all other factors constant, burrow site

42

selection probability decreases as soil resistance increases (β = -0.93 ± 0.35, Wald χ2 =

7.05, p = 0.0079) (Figure 2-1). Burrow site selection probability also decreases as

distance to edge increases (β = -0.07 ± 0.02, Wald χ2 = 7.71, p = 0.0055) (Figure 2-2).

In contrast, burrow site selection probability increases as percentage herbaceous cover

increases (β = 0.04 ± 0.02, Wald χ2 = 5.09, p = 0.0241), while other factors are being

held at constant (Figure 2-3). Finally, burrow site selection probability increases as

number of tortoise burrows increases (β = 0.54 ± 0.14, Wald χ2 = 14.64, p = 0.0001)

(Figure 2-4).

Land cover type and distance to edge were the most important factors in

determining burrow site selection, followed by percent grass cover, percent woody

scrub and vine cover, percent herbaceous cover, percent litter cover, and presence or

absence of canopy cover (Table 2-3). Land cover type was the only significant

categorical variable. Burrow site selection probability is higher for coastal sand dune

habitat than for coastal strand.

Discussion

Gopher Tortoise burrow site selection in a coastal sand dune site was

quantitatively modeled with a combination of environmental, biological, and

anthropogenic factors at microhabitat and home range spatial scales. Out of the eight a

priori models, the “environmental + biological” model is the best fitting and the second-

most complex model, containing all but one of the explanatory variables (factors). The

high number of factors in the best fitting model indicated that most factors considered

have at least some effects on the burrow site selection of Gopher Tortoises and that the

selection process itself is complex and involves multiple spatial scales. In effect, burrow

site selection of Gopher Tortoises in coastal sand dune was related to land cover type,

43

soil resistance, percentage herbaceous ground cover, distance to edge, number of

tortoise burrows nearby, and the angle of the slope.

Land cover type, a home range scale factor, is the most important and significant

factor indicated by the combined model weight and Wald Chi-square value,

respectively. This is not surprising as there are only two available land cover types

(beach dune and coastal strand) and the majority of Gopher Tortoise burrows were

located in the beach dune. The beach dune land cover type has an open canopy and

well-drained soil, which promotes the growth of herbaceous plants and allows for sunny

nesting sites. Previous studies (Boglioli et al. 2000, Jones and Dorr 2004).suggest that

canopy closure is one of the main factors negatively influencing the distribution of

tortoise burrows. The coastal strand, due to canopy closure, provides little forage for

Gopher Tortoises (combined mean percent cover for grass and herbaceous plants =

7.29%), and few burrows (4%) are found within canopy closure. Most burrows in the

coastal strand are located very close to the edge (mean distance to edge = 1.5 m).

Gopher Tortoises and their burrows are rarely observed in the coastal strand partially

due to the low relative detectability of burrows in that habitat. Most of the coastal strand

is inaccessible to humans, and some tortoises may use it as a shelter from potential

predators (i.e., humans). When approached, tortoises would escape towards the coastal

strand when a nearby burrow is absent.

Distance to edge is another significant home range scale factor that influences

burrow site selection. Gopher Tortoises are more likely to dig burrows near the edge

between beach dune and coastal strand. The edge typically occurs in the climax of the

upper dune and is parallel to the elevation gradient. As elevation and distance to dune

44

are strongly correlated, only one factor was included in the original a priori hypotheses

and the corresponding predictive models. Elevation may also be an important factor

influencing burrow site selection. The lowest elevation for a burrow site found in the

study site was 3.1 meters above sea level whereas the lowest for a random site was 0

meters above sea level. Hansen (1963) found that the mean depth of Gopher Tortoises

burrows is 2 m. Therefore, elevation may be a limiting factor on burrow construction in

coastal sand dune because of a need to prevent salt water intrusion into the burrow.

Burrow site selection probability increases as the percentage of herbaceous

vegetation cover measured in the microhabitat scale increases. Gopher Tortoises

mostly forage within 50 m of their burrow, and most foraging activities are concentrated

on areas with high herbaceous ground vegetation. The results of this study suggest that

the amount of herbaceous vegetation cover could be a visual cue that Gopher Tortoises

use to select suitable burrow site.

Gopher Tortoises appear to select burrow sites with softer soil, but the exact

mechanism they use to determine soil resistance is unknown. At the coastal sand dune

site, the soil in the fore dune mostly consists of coarse mollusk shell-fragments,

whereas the soil in the upper dune consists of finer, smaller-grained sandy soils.

Gopherus polyphemus are prolific burrowers with morphological adaptations that

improve burrowing efficiency (Auffenberg 1966), and it is possible that these

adaptations may help the tortoise differentiate soil textures.

The number of tortoise burrows within a 20 m radius of each location correlates

positivity to the probability of burrow site selection. This result is likely an

autocorrelation. Since Gopher Tortoises excavate multiple burrows and use them

45

concurrently, and burrows are usually the center of their home ranges, it is not

surprising that there are more than one burrow within the radius of a Gopher Tortoise’s

home range.

The results of this study are consistent with the Gopher Tortoise habitat selection

literature. Jones and Dorr (2004), using a similar approach, found that Gopher Tortoise

occurrence in an intensively managed pine plantation is influenced by soil types, total

and mid-story canopy coverage, and edaphic and vegetative conditions. Active burrow

occurrence was related positively to increasing mid-story canopy closure and certain

soil types. Baskaran et al. (2006) used a similar approach to develop a model that

predicts Gopher Tortoise habitat in a five-county region in Georgia. The model they

developed predicted Gopher Tortoise habitat based on burrow associations with land

cover type, soil type, topography and hydrology. Land cover types, soil types, and

distances to streams and roads were the most significant predictors in their model.

Although the best model did not include any anthropogenic factors, the

significance of distance to beach access in the global model, which is a statistical

equivalent of the best model, approaches significance (Wald’s χ2 = 3.71, p = 0.054) and

has a positive coefficient estimate (β = 0.00121 ± 0.00063) indicating that burrow site

selection probability increases farther away from the beach access. Based on the

author’s observations at this study site, burrow density tends to be higher in areas

farther away from the beach access, and most human activities occur within <200 m

from the beach access. The only other anthropogenic factor measured in this study was

distance to road, a factor that was dropped because of high correlations with distance to

edge. Distance to road was found to be a useful predictor of the occurrence of Gopher

46

Tortoise burrows in another study (Baskaran et al. 2006), however. Pruner (2010)

studied nest-site selection of the Snowy Plover (Charadrius alexandrinus) and found

that the presence of symbolic fencing increased probability of nest-site selection.

Anthropogenic disturbances in the form of motorized vehicles, human activities, or

introduced predators should not be overlooked, as our knowledge of impacts to wildlife

associated with human disturbance is limited.

The effect of sex on habitat selection was not examined in this study as it was

logistically difficult to determine the sex of the tortoise that excavated each burrow.

Nonetheless, sex has been shown to have an effect on habitat selection, as Graeter et

al. (2008) found that female Marbled Salamanders (Ambystoma opacum) selected

clear-cut forest more frequently than males. Male Gopher Tortoises may prefer to

excavate burrows in areas with high female density, whereas females may prefer to do

so in areas with adequate nesting habitat.

The study design I employed can easily be replicated to study habitat selection of

other animals with conspicuous habitat features (such as nest or burrows) or radio-

tagged animals. The results of this study demonstrate that fine scale factors that relate

directly to the natural history of Gopher Tortoises such as percent herbaceous cover

and soil resistance have significant influences on habitat selection, and Gopher

Tortoises may use these visual and physical cues to select burrow sites. More research

should be conducted to understand these mechanisms. Research could address

whether species richness affects burrow site selection, whether tortoises are able to

recognize specific vegetative species and choose their burrow sites accordingly, and

whether tortoises can differentiate between well-drained soil and excessively-drained

47

soil for burrow selection, much as nesting sea turtles and diamondback terrapins have

been shown to do in similar habitats (Burger and Montevecchi 1975, Garmestani et al.

2000, Wood and Bjourndal 2000).

48

Table 2-1. Potential factors influencing burrow site selection for Gopher Tortoises in coastal sand dunes of Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA, 2010.

Variable Spatial scale

Environmental factors Soil resistance microhabitat Elevation microhabitat Slope microhabitat Slope angle microhabitat Soil type † home range

Biological factors % bare ground microhabitat % herbaceous cover microhabitat % grass microhabitat % woody scrub and vines microhabitat % litter microhabitat Canopy cover † microhabitat Distance to edge home range Land cover type † home range Number of tortoise burrows home range

Anthropogenic factors Distance to road home range Distance to beach access home range

† = categorical variables

49

Table 2-2. Burrow site selection models based on a priori hypotheses for Gopher Tortoises (n = 198) in the coastal sand dunes of Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA. The best models (*) are chosen based on Akaike’s Information Criterion corrected for small sample sizes (AICc), AICc is based on -2 log likelihood, which is the value of the maximized log-likelihood function of the model factors given the data set, the number of factors (K), and ΔAICc is the AICc differences relative to the smallest AICc in the model set.

Model Factors** K -2LL AICc ΔAICc w

environmental + biological* SR SL SA HE GR SV LI CA SO LC TB DE

13 125.14 153.13 0.00 0.5378

global* SR SL SA HE GR SV LI CA SO LC TB DE DA

14 123.14 153.45 0.32 0.4588

biological + anthropogenic HE GR SV LI CA LC TB DE DA

10 142.88 164.06 10.93 0.0023

biological only HE GR SV LI CA LC TB DE 9 147.55 166.51 13.38 0.0007

home range only SO LC TB DE DA 6 155.10 167.55 14.42 0.0004

environmental +anthropogenic SR SL SA SO DA 7 179.76 194.36 41.23 <0.0001

microhabitat only SR SL SA HE GR SC LI CA 9 202.19 221.16 68.03 <0.0001

environmental only SR SL SA SO 5 222.84 233.15 80.02 <0.0001

anthropogenic only DA 2 258.72 262.78 109.65 <0.0001

** SR = soil resistance, SL = soil slope, SA = slope angle, HE = % herbaceous cover, GR = % grass cover, SV = % woody scrubs and vines cover, LI = % litter cover, CA = presence or absence of canopy (>2 m) cover, SO = soil type, LC = land cover type, TB = number of tortoise burrows within 17 m radius, DE = distance to the nearest edge, DA = distance to the nearest beach access.

50

Table 2-3. Relative importance of burrow site selection factors, parameter coefficient estimates (β), for Gopher Tortoises (n = 198) in the coastal sand dunes of Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA, ranked by the sum of model weight. The most important factor (*) has the highest sum of model weight. β and P values are derived from the “environmental + biological” model.

Factors** Sum of model weight β (SE) P

LC* 1.0000 -1.91 (± 0.40) <0.0001 ***

DE* 1.0000 -0.07 (± 0.03) 0.0055**

HE 0.9996 0.04 (± 0.02) 0.0241*

GR 0.9996 0.02 (± 0.01) 0.0939

SV 0.9996 0.02 (± 0.01) 0.0539

LI 0.9996 0.03 (± 0.02) 0.1231

CA 0.9996 -0.32 (± 0.34) 0.3403

TB 0.9993 0.54 (± 0.14) 0.0001 ***

SR 0.9973 -0.93 (± 0.35) 0.0079 **

SO 0.9971 0.71 (± 0.38) 0.0659

SL 0.9967 0.97 (± 1.14) 0.3966

SA 0.9967 -0.01 (± 0.01) 0.0121 *

DA 0.4615 - -

** SR = soil resistance, SL = soil slope, SA = slope angle, HE = % herbaceous cover, GR = % grass cover, SV = % woody scrubs and vines cover, LI = % litter cover, CA = presence or absence of canopy (>2 m) cover, SO = soil type, LC = land cover type, TB = number of tortoise burrows within 17 m radius, DE = distance to the nearest edge, DA = distance to the nearest beach access. Significant codes “*” = 0.05, “**” = 0.005, “***” = 0.0005.

51

Figure 2-1. Probability of burrow site selection as a function of soil resistance from the

most parsimonious model for Gopher Tortoises in the coastal sand dunes of Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA, 2010. Solid line represents parameter coefficient estimate (β) and dash lines are ± 1 SE.

0.0

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Figure 2-2. Probability of burrow site selection as a function of distance to edge (m) (± 1

SE) from the most parsimonious model for Gopher Tortoises in the coastal sand dunes of Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA, 2010. Solid line represents parameter coefficient estimate (β) and dash lines are ± 1 SE.

0.0

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53

Figure 2-3. Probability of burrow site selection as a function of percent herbaceous

cover (± 1 SE) from the most parsimonious model for Gopher Tortoises in the coastal sand dunes of Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA, 2010. Solid line represents parameter coefficient estimate (β) and dash lines are ± 1 SE.

0.0

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54

Figure 2-4. Probability of burrow site selection as a function of number of tortoise

burrows (± 1 SE) from the most parsimonious model for Gopher Tortoises in the coastal sand dunes of Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA, 2010. Solid line represents parameter coefficient estimate (β) and dash lines are ± 1 SE.

0.0

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55

CHAPTER 3 HOME RANGE, SEASONAL ACTIVITY AND MOVEMENT PATTERNS OF GOPHER

TORTOISE IN COASTAL SAND DUNE HABITAT

Background

The concept of home range, first proposed by Burt (1943), is a fundamental

concept in spatial ecology. The home range of an animal is the area that encompasses

all the resources the animal requires to survive and reproduce. Several general

principles of home ranges of vertebrates have been proposed. McNab (1963) first

demonstrated that mammal home range size is a linear function of body size, and noted

that carnivores have larger home ranges than similar sized herbivores. Since then,

multiple formulas have been generated to empirically estimate home range sizes of

mammals based on body size and diet (Harestad and Bunnell 1979). Birds, due to their

high basal metabolic rate comparable to some mammals, exhibit a similar relationship.

Mace and Harvey (1983) found the same body size-home range size relationship in

passerine birds and rodents, and noted that birds have larger home range sizes than

similar sized rodents with similar diets, possibly due to their higher energy requirements.

Additionally, these authors found that home range size of granivores are consistently

larger than that of other herbivores.

In solitary territorial carnivorous mammals, female home ranges are determined by

the distribution and abundance of food, whereas male home ranges, during the mating

season are largely determined by the distribution of conspecific females (Erlinge and

Sandell 1986). Home range size of predator and prey density appears to be negatively

correlated in female Bobcats (Lynx rufus) (Litvaitis et al. 1986) and Canadian Lynx (L.

canadensis) (Ward and Krebs 1985). Home range sizes of males during the mating

season are expected to be larger than estimates based on energy requirements, since a

56

male’s home range may overlap with that of several females to increase the probability

of mating (Sandell 1989).

In reptiles, home range sizes are generally associated with the distribution of one

or more resources (e.g., food, shelter, mates, thermoregulation sits), and positive

correlations between home range size and body size have been observed in many

species. Sex, reproductive state, density, and food availability are some of the major

factors that influence home range size in reptiles (Zug et al. 2001). Gregory et al. (1987)

suggested that habitat structure and resource availability are the most important and

obvious factors that affect home ranges and movement patterns of snakes. For

example, snakes in the Temperate Zone require separate overwintering sites and

feeding areas that occur in widely separated habitats, and long-distance seasonal

migrations between these habitats have been observed by many authors (Larsen 1987,

Madsen and Shine 1996, Wilson et al. 2006). Hailey and Coulson (1996) studied home

range and daily movement distance of two African tortoises (Leopard Tortoise,

Geochelone pardalis, and Speke’s Hingeback Tortoise, Kinixys spekii) and found that

G. pardalis exhibited proportionally larger home ranges than K. spekii. The authors

observed frequent movements by G. pardalis to an area with sodium-rich soils and

argued that Leopard tortoises have exceptionally large home range because of the

spatial distribution of sodium-rich soils rather than energetic requirements. The home

range of the same organisms also may differ according to habitat type and quality

(Diemer 1992a, Smith et al. 1997).

Gopher Tortoises (Gopherus polyphemus) occur in a variety of habitats throughout

Florida. Although the majority of ecological studies have been conducted in sandhill

57

(longleaf pine-turkey oak-wiregrass community) or pine plantations that were once

sandhill, relatively few studies have focused on populations that occur in coastal

ecosystems (Kushlan and Mazzotti 1984, Pike 2006, Waddle et al. 2006), and

information on the spatial ecology of this species in these ecosystems is lacking.

Gopher Tortoises that live in coastal sand dunes may be challenged by threats such as

beach erosion, exposure to high salinity, urban predators, and other natural disasters

such as hurricanes and catastrophic wildfire, in addition to the typical habitat loss and

degradation experienced by inland populations. Florida is surrounded by coastal sand

dunes that have the potential to support considerable populations of Gopher Tortoises

(Auffenberg and Franz 1982).

As highly fossorial herbivores, Gopher Tortoises spend up to 98% of their time

inside their burrows and only leave to forage, court, mate, or nest (Johnston 1996, Ernst

and Lovich 2009). Individual tortoises may use multiple burrows, and intraspecific

burrow sharing is not uncommon. A typical home range of a Gopher Tortoise is small

and centers around its burrows and includes the surrounding foraging areas. Gopher

Tortoises typically feed within 50 m of the burrow (Auffenberg and Franz 1982). Males

typically have larger home ranges that overlap with the home range of several females

(Auffenberg and Iverson 1979).

Home range size has been hypothesized to be an indicator of Gopher Tortoise

habitat quality (Diemer 1992, Mushinsky and McCoy 1994). Other authors have found

that the amount of herbaceous ground cover is negatively correlated with home range

size (Auffenberg and Iverson 1979, Mushinsky and Gibson 1991). A number of home

range studies have been conducted in sandhill and pine plantations (McRae et al. 1981,

58

Diemer 1992a, Smith 1992, Smith et al. 1997, Backus 2003, Eubanks et al. 2003, Yager

et al. 2007) with varying results, but the general consensus is that Gopher Tortoise

home range varies greatly both spatially and temporally. Nevertheless, home range size

continues to be recognized as an important predictor of habitat quality.

Home range size and shape of sand dune Gopher Tortoises may be very different

compared with those occupying uplands. In terms of shape, sand dune Gopher

Tortoises likely have linear home ranges because of the linear distribution of habitat

along beach dunes. Therefore, this suggests that the overall size of coastal home

ranges might be either smaller (due to beach-related disturbances and subsequent

effects on activity) or larger (depending on forage distribution and quality) than Gopher

Tortoise home ranges in inland habitats.

In this study, radio-telemetry was used to determine the movement, seasonal

activity patterns, and annual home range of Gopher Tortoises at a coastal sand dune

site. Habitat requirements of Gopher Tortoises in this understudied habitat was inferred,

and the results of this study could help wildlife managers design better conservation

plans to protect Gopher Tortoises.

Methods

Study Area

This study was conducted in Guana Tolomato Matanzas National Estuarine

Research Reserve (GTMNERR) located on the northeastern coast of Florida, St. Johns

County. The northern section of GTMNERR includes some of the only pristine,

undeveloped dunes on the eastern coast of Florida. All Gopher Tortoises used for radio-

telemetry were captured in the 15.7 ha plot in the northernmost section of the study site,

north of the North Beach Access.

59

Radio-Telemetry

The coastal sand dune was surveyed for Gopher Tortoise burrows from January

through March 2010. Gopher Tortoises were captured using bucket pit-fall traps (Burke

and Cox 1988) and opportunistically by hand from April 2010 to May 2010. Upon

capture, standard measurements (carapace length and width, plastron length, plastron

concavity, mass, sex) were taken with calipers to the nearest mm and a digital scale to

the nearest 100 g. Males and females were distinguished visually by secondary sexual

characteristics (Landers et al. 1980). Each tortoise was assigned a unique number and

permanently marked (Cagle 1939). A total of 20 tortoises (11 females and 9 males)

were fitted with radio transmitters (Wildlife Materials, Inc., SOPR-2380). Each radio

transmitter weighed less than 5% of the body mass of the tortoise and was glued onto

the posterior edge of the carapace using quick bonding epoxy putty stick (West Marine,

Watsonville, CA) (Figure 3-1). Tortoises were released immediately at their site of

capture following measurements and transmitters attachment.

Tortoises were located once every 2 to 3 days from May to August 2010 and once

every 7-10 days from September 2010 to May 2011 using a 3-element Yagi-Uda

antenna and a TRX-1000WR receiver (Wildlife Material, Inc.). Geographic locations

were recorded with a handheld GPS with an error of +/- 3m (Garmin International Inc,

Olathe, KS), along with time of day, weather, and specific activity (e.g., walking,

foraging, resting in burrow) of the tortoise. During the inactive season from December to

February, sticks were placed in front of over-wintering burrows to determine the length

of the inactive period and to confirm survival during the winter (Diemer 1992a).

60

Data Analysis

I examined the movement patterns of adult Gopher Tortoises during the entire

study period by determining the mean distance moved between successive locations,

the mean annual home range area, the number of moves, and the total number of

burrows used following the methods of Eubanks et al. (2003). Distances moved were

calculated as the straight line distance between successive burrows or non-burrow

locations. Mean distance moved was measured per move within a calendar month (i.e.,

100 m / 4 moves = 25 m/move). Home range area (100% minimum convex polygon

[MCP] and 95% fixed kernel) were calculated using Biotas version 2.0a 3.8 (Ecological

Software Solutions LLC). The smoothing parameter in 95% kernel estimates was

selected by least-squares cross-validation (Seaman and Powell 1996).

A Shapiro-Wilk’s test for normality indicated that the values of annual home range

areas (MCP) and mean distances moved between successive locations were non-

normally distributed. All other data were normally distributed. Mean number of burrows

used and mean number of moves by males and females were compared using one-way

ANOVAs. The non-parametric Kruskal-Wallis rank sum test was used to compare mean

distance moved between successive locations and the mean annual home range areas

(MCP) by males and females. Alpha level for all analyses was 0.05 and all means were

reported with standard errors unless otherwise specified. All statistical analyses were

conducted in R version 12 (R Development Core Team 2010).

Results

From May 2010 to May 2011, 20 Gopher Tortoises were tracked 1,120 times

(mean = 56 times, range = 46 – 72, S.E. = 1.19). Tortoises on average were tracked for

a period of 325 days (range = 257 – 358, S.E. = 4.70).

61

Movement Distances

For the entire study period, female tortoises on average traveled a distance of 39.2

m per move (n = 11, range = 5.5 - 156.9, S.E. = 13.3), whereas males traveled 47.0 m

per move (n= 9, range = 24.6 - 83.4, S.E. = 7.68). Mean distance moved was not

significantly different between males and females (Kruskal-Wallis χ2 = 2.92, df = 1, p =

0.08). Both sexes were mostly inactive from January to February and traveled fewer

than 4 m per move. Movements were observed as early as March and as late as

December. In terms of mean distance traveled per move by month, both sexes

exhibited similar patterns, but males traveled significantly more distance per move in

April and August (Figure 3-2).

Burrow Use and Activity Patterns

Of 1,120 locations, tortoises were observed above ground only 26 times (2.32%),

of which 19.2% were foraging, 11.5% were basking, and 69.2% were walking. Over the

entire study period, females on average used 6 burrows (n = 11, range = 2 – 12, S.E. =

0.89), whereas males used 11 burrows (n = 9, range = 5 - 14, S.E. = 1.09). Males used

significantly more burrows than females over the course of the year (F1, 18 = 11.43, p =

0.003). In terms of number of moves during the year, females on average moved 14

times (n = 11, range = 3 – 28, S.E. = 2.52), whereas males moved 28 times (n = 9,

range = 6 - 38, S.E. = 3.10). Males moved more often than females (F1, 18 = 13.21, p =

0.002). Both sexes exhibited similar seasonal patterns in number of burrows used, but

males used almost twice as many burrows as females per month from July to August

(Figure 3-2).

62

Annual Home Range

Over the entire study period, adult Gopher Tortoises had an annual mean home

range of 0.37 ha (MCP, n = 20, S.E. = 0.14) and 0.25 ha (kernel, n = 20, S.E. = 0.02)

(Table 3-1). Home range medians did not differ between sex for MCP estimates (Males:

0.32 ha, n = 9, S.E. = 0.06; Females: 0.42 ha, n = 11, S.E. = 0.26; Kruskal-Wallis χ2 =

2.19, df = 1, p = 0.1385). For fixed kernel estimates, males had significantly larger mean

annual home range areas than females (Males: 0.31 ha, n = 9, S.E. = 0.03; Females:

0.21 ha, n = 11, S.E. = 0.03; F1, 18 = 5.50, p = 0.03) (Table 3-1).

Dispersal Pattern

During the 13 month study period, all but one radio-tagged tortoise stayed within

the 78 ha study area, and most movements were confined within the 15.7 ha sub plot

where these tortoises were originally captured. One male (#T18) used two burrows

located near a beach front house outside of the study site boundary. Most movements

were from north to south or vice versa, rather than from east to west. The longest

recorded movement was 650 m over a 12 day period by a female tortoise (Carapace

length = 26.8 cm).

Discussion

In this study, considerable amounts of individual variation exist among annual

home range areas, distance moved, number of burrows used, and number of moves.

Overall, Gopher Tortoises in coastal sand dunes have smaller home ranges than

conspecifics found in other habitat types (Table 3-4; Pike 2006). The home range of

male tortoises appears to be larger than that of females, and males used more burrows

than females. These results are consistent with those reported in the literature (McRae

63

et al. 1981, Diemer 1992a, Eubanks et al. 2003). Gopher Tortoises tend to move short

distances.

The results of this study suggest that dispersal by Gopher Tortoises out of sand

dunes is not common. During the course of this study, all radio-tagged tortoises stayed

within the coastal dune area, but 2 unmarked tortoises (1 juvenile and 1 subadult male)

were found dead on heavily-traveled State Road AIA, apparently killed by passing

vehicles while attempting to cross the road. The area west of the study site consists of

mostly coastal strand, coastal scrub and maritime hammock. Tortoise burrows have

been observed in the coastal scrub and maritime hammock but a thorough burrow

survey has not been conducted, as the area has not been burned recently and it is

physically impossible for humans to penetrate without removing obstructive plants. The

tortoises in the beach sand dune are essentially trapped within their habitat, forming an

isolated population. The dispersal ability of tortoises is limited and it is unlikely that

many tortoises can disperse out of the sand dune without being hit by a vehicle.

The minimum convex polygon is one of the most widely used methods for home

range estimation, but it tends to overestimate home ranges by including outlying

locations that are not used frequently. On the other hand, the kernel method tends to

focus on major activity centers and ignores outlying locations, which may underestimate

home range in species that spend a majority of time in fixed locations, such as the

Gopher Tortoise.

In this study, mean annual home ranges estimated by MCP for all tortoise and

females only were larger than those estimated by the kernel method (48% and 101%,

respectively; Table 3-1). As mentioned, MCP tends to overestimate home range, and

64

the mean annual home ranges of females estimated by MCP was possibly inflated due

to a female tortoises that had an exceptionally large home range (2.95 ha) due to a

single long distance movement in July (650 m). If that tortoise was removed, the mean

annual MCP home range for females becomes 0.16 ha.

This study demonstrated that the home range of sand dune Gopher Tortoises is

smaller than that of upland conspecifics. Gopher Tortoises in coastal sand dune at

GTMNERR have far smaller home ranges and used fewer burrows than Gopher

Tortoises at Kennedy Space Center, a coastal scrub site (Smith et al. 1997). McRae et

al. (1981), Diemer (1992b), Smith et al. (1997), Eubanks et al. (2003), and Yager et al.

(2007) estimated home range of adult Gopher Tortoises using similar methods, and

reported male mean annual home ranges that are 0.45, 2.92, 4.86, 2.39, 5.02, and 3.01

times larger, respectively, than estimates from the present study (Figure 3-4).

Sand dune tortoises also appear to frequent fewer burrows than upland tortoises,

possibly due to the distribution of forage plants. Based on the results of this study

(Chapter 2), Gopher Tortoise burrow selection is related to percent herbaceous cover.

By excavating burrows near their primary food source, tortoises need not travel far for

food.

Based on radio-telemetry data, burrow sharing among conspecifics of the same or

opposite sex is common at GTMNERR and was observed multiple times (n = 40). As

many as four radio-tagged tortoises have been observed sharing a single burrow

simultaneously, and two tortoises overwintered in the same burrow. Diemer (1992a) and

Epperson and Heise (2003) reported similar findings at northern Florida and southern

Mississippi sites, respectively, and indicated burrow sharing in Gopher Tortoises is not a

65

rare phenomenon. Burrow sharing by Gopher Tortoises also has been observed

commonly on Egmont Key, Florida (C.K. Dodd, Jr., personal communication).

There are few accounts of courtship and mating behaviors of Gopher Tortoises,

which typically occur in the spring to autumn in Florida (Auffenberg 1966, Ernst and

Lovich 2009, C. K. Dodd, Jr., personal communication). At this study site, courtship and

mating have been observed as late as October. The burrow defense behavior described

by Diemer (1992a) and Dodd (personal communication) was observed when a mating

pair was interrupted by the author. The female immediately retreated into the burrow

and the male blocked the entrance by turning itself sideways.

66

Table 3-1. Mean annual home range (ha) of adult Gopher Tortoises in coastal sand dunes, Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA (2010-2011), using 100% minimum convex polygon (MCP) and 95% fixed kernel methods. Values after removing a outlier tortoise are in parentheses.

n

MCP HR

SE Range Kernel

HR SE Range

All tortoises 20 0.37 0.14 <0.01-2.94 0.25 0.02 0.09-0.52

Males 9 0.32 0.06 0.13-0.63 0.31 0.03 0.23-0.52

Females 11 0.42 (0.16) 0.25 (0.05) <0.01-2.94 0.21 0.03 0.09-0.41

67

Figure 3-1. A Gopher Tortoise with a radio transmitter (Wildlife Materials, Inc., SOPR-

2380) affixed to its carapace. A unique number was painted on each individual to allow temporary identification from a distance. (Photo courtesy of author)

68

Figure 3-2. Seasonal movement patterns (mean distance traveled per move + SE) of

radio-tagged adult Gopher Tortoises (n = 20) in coastal sand dunes, Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA, May 2010 - May 2011. Solid bars represent females and empty bars are males.

0

20

40

60

80

100

120

140

M J J A S O N D J F M A M

Dis

tan

ce

Mo

ve

d (

m)

Months

69

Figure 3-3. Seasonal movement patterns (mean number of burrows used + SE) of

radio-tagged adult Gopher Tortoises (n = 20) in coastal sand dunes, Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA, May 2010 - May 2011. Solid bars are females and empty bars are males.

0

1

2

3

4

5

6

M J J A S O N D J F M A M

No. o

f B

urr

ow

s

Months

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Figure 3-4. Mean annual home range (minimum convex polygons) + SE of adult Gopher

Tortoises in previous and present studies. Solid bars are females and empty bars are males.

0.0 0.5 1.0 1.5 2.0 2.5

Present study

Yager et al. Control2007

Yager et al. Burned2007

Smith et al. 1997

McRae et al. 1981

Eubanks et al. 2003

Diemer 1992a

Mean Annual Home Range (ha)

71

CHAPTER 4 CONCLUSIONS

The Gopher Tortoise management plan prepared by FWC (2007) suggested that

site-specific studies on topics such as seasonal activity, home range size, fecundity,

and adult sex ratios may be needed in certain understudied habitats. The results

presented in this study may serve as an initial assessment of Gopher Tortoise

populations in coastal sand dunes.

Status of Sand Dune Tortoises

As mentioned in Chapter 1, the density and distribution of Gopher Tortoise at the

GTMNERR site is highly patchy. The majority of the burrows were found in the northern

section, which indicates relatively high habitat quality in the area. The southern section

contained much fewer burrows and represents marginal habitat. Burrow locations in the

southern section were farther apart from each other and most likely a result of the

relatively low tortoise density. The vegetation structure and relative openness of the

habitat inevitably have effects on burrow site selection of Gopher Tortoise in

GTMNERR, as presented in Chapter 2, but the relationships between those variables

and tortoise density is unclear. Habitat characteristics of random points did not differ

significantly between the northern and southern sections (all significant values (p)

>0.05). The disparity in tortoise density at the GTMNERR sand dunes appears to be

consistent over time, as the same trend was observed in a burrow survey conducted in

2007 (GTMNERR, unpub. data).

During the course of this study, few hatchlings or juveniles (n = 4) were

encountered at the GTMNERR site. Three were observed in the beach dune and one

was a road-kill found on State Road A1A. Based on burrow surveys conducted by the

72

author, juvenile burrows were few, and the population appears to be skewed towards

larger adults (Figure 4-1). The lack of juvenile burrows may not reflect the true

population structure because of low relative detectability of juveniles and their burrows

(Pike 2006). Juveniles are also known to use adult burrows (Diemer 1992a).

Nonetheless, the unimodal distribution of burrow widths should not be completely

dismissed, as a burrow survey conducted in similar coastal habitat in Cape Sabal,

Florida using similar methodology has produced a bimodal distribution that indicates

high recruitment (Waddle et al. 2006). The shortline Gopher Tortoise at GTMNERR

should continue to be monitored to assess its long-term status, as it represents one of

the largest populations within GTMNERR and in northeastern Florida. The coastal

population at GTMNERR may have reached its carrying capacity, meaning that it

cannot support more tortoises. Carrying capacity is positively correlated with habitat

quality, but a high quality habitat may be below its carrying capacity if an external factor

such as human exploitation reduces population size. Density can also be a misleading

indicator of habitat quality if the habitat is indeed an ecological trap (Van Horne 1983).

Little is known about the population status (i.e., stable, declining, or increasing) of

Gopher Tortoises in other coastal dune systems in Florida. Coastal beach dunes are

found throughout most of the state, but few pristine beach dunes remain undeveloped

or unaltered by human activities. On the Atlantic coast, the largest populations of

Gopher Tortoises can be found along Cape Canaveral National Sea Shore and Merritt

Island National Wildlife Refuge (Kennedy Space Center), where a large amount of land

has been under protection since the 1960s (Breininger et al. 1991, Breininger et al.

1994, Smith et al. 1997, Pike 2006). Outside of Kennedy Space Center, Gopher

73

Tortoise populations probably exist in small, isolated habitat pockets in between beach

front properties similar to the present study site. The southernmost population of this

species, at Cape Sabal, Monroe County, has experienced a steep decline in recent

years (1990 to 2001). Waddle et al. (2006) reported a 76% decline of active burrows at

that site possibly due to the reduction of habitat quality and tropical storms. Schwartz et

al. (2005) visited the same site in 2001 and suggested the population may have been

extirpated by natural causes (i.e., Hurricane Gabrielle). Large tropical storms have been

suggested to be an important cause of mortality in coastal populations of Gopher

Tortoises (Kushlan and Mazzotti 1984), and are most likely responsible for the uneven

vegetation structure and tortoise burrow density found between the northern and

southern sections at the GTMNERR site.

Effects of Human Activities

Human activities may be the greatest deterministic threat to sand dune Gopher

Tortoises. During the one year study period, three road-killed Gopher Tortoises were

observed (2 adult males, 1 juvenile). Since the coastal sand dunes study site is

essentially a closed habitat surrounded by housing developments, the Atlantic Ocean,

and State Road A1A, tortoises are confined to a narrow strip of habitat and have no

place to disperse. The only viable option for dispersal is to move west towards the

coastal scrub/maritime hammock habitat. However, this move is extremely risky as the

traffic on State Road A1A increases substantially during the tortoises’ active season

(March - November) because of the increase in number of beach users (Figure 4-2).

In addition to direct mortality caused by road traffic, sand dune tortoises may also

be affected by human activities on the beach and by introduced species or human

subsidized predators (e.g., raccoons). South Ponte Vedra Beach is a highly used

74

recreational beach throughout the year. There are three official dune overpasses and

several other unofficial trails that allow easy access throughout the beach. Access to the

dunes is prohibited by the reserve, but human activities are apparent based on the

amount of trash and litter observed in the dunes. From Chapter 2 of this thesis, distance

to beach access has been shown to be a significant factor that negatively affect burrow

site selection probability. Further, human activities may have indirect effects on tortoise

behavior that are not easily observable (e.g., reduce foraging effort; discourage

dispersal). Wild tortoises are generally nervous when they encounter humans and

terminate whatever activity they are engaged in and run for the closest shelter (e.g.,

burrows). Additionally, non-native species may negatively affect Gopher Tortoises. Feral

cats have been observed in the dunes and they may prey upon hatchling and juvenile

tortoises. The exotic Cactus Moth (Cactoblastis cactorum) has infested Prickly Pear

(Opuntia spp.) in the dunes and is causing localized mortality of the plant, which is a

favored tortoise food source and an important source of fresh water for sand dune

Gopher Tortoises.

Sea Level Rise

Global climate change and the associated sea level rise could become a major

threat to the continued survival of sand dune Gopher Tortoises. Global sea level rise

projections by 2100 vary greatly depending on sources and the climate scenarios used

in the models. The United Nations Intergovernmental Panel on Climate change

Assessment Report, one of the most cited sources of information on sea level rise,

predicted a 0.18 m to 0.59 m rise in the 21st century (IPCC 2007). However, Allison et

al. (2009) argued that sea level is likely to rise as much as 2 m by 2100 as the IPCC

models did not take into account the dynamical processes and the resulting contribution

75

to global sea level rise by Greenland and Antarctic ice sheets. Furthermore, both IPCC

(2007) and Ovenpeck and Weiss (2009) warned that global warming in this century may

be sufficient to produce a 4 to 6 m sea level rise in future centuries. Under such

scenario, the majority of the present Florida coastal area will be under water (Figure 4-

3).

As sea level continues to rise, most of the fore dune will be gradually washed out

and the dune will move inland. Sand dune wildlife, including Gopher Tortoises, will have

to move inland as well. However, State Road A1A will likely act as a barrier to the

natural migration process. In the future, wildlife managers may have to monitor coastal

wildlife migration and implement facilities that would aid migration (i.e., corridors or

culverts, Dodd et al. 2004) as the coastal habitat starts to shift inland. Manual

translocation may also be an option for focal species with limited dispersal ability such

as the Gopher Tortoises.

Management Implications

Coastal Habitat Management

In terms of habitat management in the sand dune, certain procedures intended to

make the beach more accessible to recreational users such as raking seaweed off of

the beach or building pedestrian paths should be avoided as it may destabilize the

natural plant community successions. Seaweeds are important nutrient source for some

of the pioneer species on the fore dune. In areas with strong onshore winds, pedestrian

paths that are perpendicular to the beach can promote blowouts and allow a wave of

sand to bury existing stable dune vegetation; thus, pedestrian paths should be built to

form an angle less than 90°.

76

Feasibility As Relocation Sites

Although the use of reintroduction as a conservation technique has been a

controversial subject (Burke 1991, Dodd and Seigel 1991, Reinert 1991), it is used by

the FWC to mitigate populations of Gopher Tortoise affected by human development.

One of the main objectives of the Gopher Tortoise management plan is to increase the

number of relocations to suitable, managed, and protected habitat instead of

unprotected areas (FWC 2007). However, relocation should be treated as a last resort,

as mismanaged past relocations have led to mixed gene pools (Schwartz and Karl

2005), disease transmission and outright failure.

The coastal sand dune of GTMNERR, with ample amount of herbaceous cover

and suitable soils for burrowing, appears to be a high quality habitat for Gopher

Tortoises. The lack of frequent fire in this system simplifies the management

requirements often needed in other Gopher Tortoise habitat (e.g., prescribed fire in

upland pine forest and scrub). However, in its current state, the GTMNERR site,

particularly the northern section, appears to be occupired at maximum carrying capacity

by Gopher Tortoises and therefore should not be considered as a recipient site.

The exact causes of local extirpation of the once-abundant coastal populations of

Gopher Tortoise is unclear. Disappearance could be a result of stochastic events such

as large tropical storms, and fire, or deterministic threats such as overexploitation for

food, habitat loss related to development, disease, or a combination of many causes.

Wildlife managers need to consider the viability of reintroducing Gopher Tortoises

through detailed individual habitat assessment and experimental releases before

committing to any large scale reintroduction. Priorities should be given to sites that have

77

supported Gopher Tortoises historically and still possess the optimum habitat

characteristics that will support viable populations.

Many of the undeveloped coastal areas in Florida are protected and may

potentially serve as inexpensive alternative recipient sites. Management costs for

Gopher Tortoises in coastal sand dunes are considerably cheaper than in upland

habitats, since prescribed fires and other fire-related habitat alteration (i.e., roller

chopping) are not necessary. The success rate of Gopher Tortoise relocation is

potentially affected by the time of introduction, habitat similarity between donor site and

recipient site, and sex ratios of tortoises (FWC 2007). Temporary enclosures (i.e., soft-

release) have been used at many recipient sites to increase site fidelity (Lohoefener and

Lohmeier 1986, Tuberville et al. 2005). Precautions should also be taken to prevent

introducing tortoises infected by upper tract respiratory disease (UTRD) to the recipient

site.

Public Education

Perhaps the most important task for Gopher Tortoise management in the coastal

sand dunes is to properly educate the public about the existence of these animals and

their important ecological roles. Based on conversations with some frequent beach

users, many of them did not know Gopher Tortoises can be found in the dunes, and

some people have even attempted to release tortoises into the ocean as they have

mistaken Gopher Tortoises for sea turtles. Information kiosks containing relevant

information on Gopher Tortoise ecology and threats should be added to the existing

arrays of educational materials at the GTMNERR public beach access in order to

increase public awareness.

78

Wildlife crossing signs should be installed at the highway to warn passing

motorists about crossing wildlife. During the course of this study, in addition to Gopher

Tortoises, many other wildlife species have been observed dead on the highway,

including the Black Racer (Coluber constrictor, n = 2), Eastern Diamondback

Rattlesnake (Crotalus adamanteus, n = 1), Rat Snake (Pantherophis spp., n = 3),

Striped Mud Turtle (Kinosternon baurii, n = 1), Marsh Rabbit (Sylvilagus palustris, n =

3), countless Raccoon (Procyon lotor), Virginia Opossum (Didelphis virginianus), Nine-

banded Armadillo (Dasypus novemcinctus), frogs, and numerous passerine birds.

Future Research

Future research on sand dune Gopher Tortoises should focus on quantifying

population-level parameters such as growth, recruitment, and survival rates, historic and

contemporary gene-flow patterns between adjacent areas, and viability of coastal sand

dunes as long term recipient sites for restocking. Such data are especially needed at

populations affected by human activities and at sites where tortoises have been

relocated as mitigation for ongoing development.

In order to fully understand the value of coastal dunes as recipient sites, data on

natural carrying capacity of this habitat need to be collected. Unfortunately, few coastal

sand dunes in Florida remained undeveloped. The GTMNERR study site is one of the

few remaining pristine coastal dunes in Florida that support a viable Gopher Tortoise

population, and data gathered from this study, such as burrow density and habitat

requirements, should serve as baseline data that informs management decisions on

whether or not a coastal sand dune could support viable Gopher Tortoise populations.

The habitat selection model developed in this study, with modifications such as

incorporating fine-scale bioclimatic factors, can be used to model the habitat suitability

79

of the remaining coastal sand dunes throughout Florida to identify potential sites for

tortoise relocation (Baskaran et al. 2006).

Finally, little is known about the population genetic structure of Gopher Tortoise

populations that are isolated by habitat fragmentation, such as the one found in

GTMNERR, or insular populations, such as the populations on Egmont Key, Florida.

Gopher Tortoises in these areas may have experienced population bottlenecks due to

lack of gene flow. The GTMNERR site would be a suitable site to study gene flow of

Gopher Tortoises across a human-altered landscape.

80

Figure 4-1. Size class distribution of Gopher Tortoise burrows (n = 106) at the coastal

sand dune study site at Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA.

0

5

10

15

20

25

30

35

8 12 16 20 24 28 32 36 40 44 48 52

Fre

qu

en

cy

Burrow width (cm)

81

Figure 4-2. Monthly beach visitor counts (January 2009 – December 2010) at Guana Tolomato Matanzas National Estuarine Research Reserve, South Ponte Vedra Beach, Florida, USA.

0

1000

2000

3000

4000

5000

6000

7000

J0

9F

09

M0

9A

09

M0

9J0

9J0

9A

09

S09

O09

N09

D09

J1

0F

10

M1

0A

10

M1

0J1

0J1

0A

10

S10

O10

N10

D10

Vis

ito

rs

82

Figure 4-3. The Florida coastline after a sea level rise of 3 meters. Reprinted by

permission from Wilber, Samantha. 2011. Florida under water: Global warming and rising sea levels. Creative Loafing Tampa. Available from cltampa.com/dailyloaf/archives/2011/05/31/florida-under-water-global-warming-and-rising-sea-levels/ (Accessed October 2011).

83

APPENDIX A HABITAT CHARACTERISTICS (BURROW AND RANDOM LOCATIONS) OF ADULT GOPHER TORTOISES IN

COASTAL SAND DUNES, GUANA TOLOMATO MATANZAS NATIONAL ESTUARINE RESEARCH RESERVE, SOUTH PONTE VEDRA BEACH, FLORIDA, USA (2010-2011)

84

APPENDIX B SPEARMAN’S NON-PARAMETRIC CORRELATION MATRIX FOR VARIABLES CONSIDERED IN BURROW SITE

SELECTION MODEL

SR SA SL BA HE SV LI GR EL DE DR DA TB

SR 1.00 0.09 -0.35 0.25 -0.08 -0.30 0.02 0.08 -0.50 0.47 0.45 -0.12 -0.24

SA

1.00 0.29 0.12 0.19 -0.24 0.14 0.11 -0.09 0.07 0.05 -0.10 -0.01

SL

1.00 -0.02 0.31 -0.10 0.15 0.30 0.26 -0.36 -0.24 -0.09 0.03

BA

1.00 -0.08 -0.62 -0.11 -0.15 -0.52 0.45 0.52 -0.11 -0.18

HE

1.00 -0.24 0.39 0.30 0.15 -0.23 -0.01 0.13 0.14

SV

1.00 -0.16 -0.45 0.41 -0.36 -0.53 0.17 0.06

LI

1.00 0.26 0.07 -0.12 -0.03 0.05 0.25

GR

1.00 0.12 -0.10 -0.01 -0.19 -0.04

EL

1.00 -0.80 -0.67 0.10 0.29

DE

1.00 0.74 -0.12 -0.29

DR

1.00 0.05 -0.21

DA

1.00 0.28

TB

1.00

SR = soil resistance, SL = soil slope, SA = slope angle, HE = % herbaceous cover, GR = % grass cover, SV = % woody scrubs and vines cover, LI = % litter cover, CA = presence or absence of canopy (>2 m) cover, SO = soil type, LC = land cover type, TB = number of tortoise burrows within 17 m radius, DE = distance to the nearest edge, DA = distance to the nearest beach access.

85

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BIOGRAPHICAL SKETCH

Anthony Yin Kun Lau grew up in Hong Kong, China, and moved to the United

States in 2005 to pursue a university education. He graduated with an A.A. in zoology

from Santa Fe College in 2007 and a B.S. in wildlife ecology and conservation with

honors from the University of Florida in 2009. He has been fascinated by reptiles and

amphibians since childhood after watching Sir David Attenborough’s spectacular nature

documentary series. Anthony’s research interests include landscape and molecular

ecology of reptiles and amphibians, conservation of threatened and endangered Asian

chelonians, and evolutionary biology of the order Testudines. In 2011, he received a

Master of Science in wildlife ecology and conservation from the University of Florida

under the mentorship of Drs. C Kenneth Dodd Jr. and Raymond Carthy.