(c) 2001 w.h. freeman and company chapter 13: population structures robert e. ricklefs the economy...

(c) 2001 W.H. Freeman and Company

Chapter 13: Population Structures

Robert E. RicklefsThe Economy of Nature, Fifth Edition


Chapter Opener


Habitat Fragmentation and Landscape Ecology

Fragmentation of formerly continuous habitats is happening worldwide, caused by: clearing of forests construction of roads channeling of rivers

Landscape ecology addresses how size and arrangement of habitat patches influence: activities of individuals growth and regulation of populations interactions between species


Figure 13.1


Consequences of Habitat Fragmentation

Plants and animals can use a habitat patch only if they have access to it.

Changes in human land use patterns have reduced such access.

Small, isolated populations in habitat fragments are vulnerable to extinction: they suffer from loss of genetic diversity they are subject to random perturbations 扰

动


Forest Fragmentation in the Amazon

When habitats are fragmented, most areas are closer to edges, often with negative consequences.

Forest fragmentation in the Amazon basin results in greater exposure of trees within 100 m of forest edges, resulting in: drying of vegetation excessive wind damage losses of up to 15 tons of biomass per hectare

annually


Brown-Headed Cowbirds 褐头牛鹂 and Kentucky Warblers

Cowbirds are nest parasites of other birds, such as the Kentucky warbler. Cowbirds: prefer open farms and fields venture into the edges of forests in search of host nests

In one study, parasitism on warblers was as high as 60% within 300 m of forest edges: warblers had to be 1.5 km from the forest edge to

escape parasitism

Nest predators also venture into forest from edges.


Figure 13.2


Many species are in decline.

Warblers and many other forest birds of eastern North America have declined in recent years.

Populations of most species have persisted for millions of years: many species are now threatened with

extinction, a cause for great concern to understand this problem and potential

solutions we must explore population structures


Terminology 1

A population is made up of the individuals of a species within a particular area: each population lives in patches of suitable

habitatHabitats naturally exist as a mosaic of

different patches: many populations are thus broken into

somewhat isolated subpopulations


Terminology 2

Population structure refers to: the density and spacing of individuals within

suitable habitat the proportions of individuals in various age

classes mating system genetic structure

Populations exhibit dynamic behavior, changing through time because of births, deaths, and movements of individuals.


铁线莲 sp.


Geographic distributions are determined by suitable habitats.

The distribution of a species is its geographic range.

Consider the geographic range of sugar maple in eastern North America: broader outlines of the range are

determined by climatic factors within its range, the species only occurs in

suitable habitats (absent from many habitats such as marshes and serpentine barrens)


沟酸浆

Ecological niche modeling predicts the distributions of species

The range of physical conditions over which species can persist is referred to as the fundamental niche of the species.

Within this range of conditions, predators, pathogens, and competitors can limit the distribution of a species to a smaller realized niche.

The modeler starts by mapping the known occurrences of a species in geographic space, then catalogs the combination of ecological conditions — generally temperature and precipitation — at the locations where the species has been recorded. The catalog of ecological conditions is a species’ ecological envelope.



黑藻


Barriers to long-range dispersal limit geographic distribution.

Introduced species often expand successfully into new regions: 160 European starlings were introduced near New

York City in 1890 and 1891; within 60 years, the North American population of starlings covered more than 3 million square miles

Other examples of successful introductions: dogs in Australia, pigs and rats in Pacific islands fast-growing pines and eucalyptus trees worldwide


蓝鳍金枪鱼


Dispersion of Individuals within Populations

Dispersion of individuals within a population describes their spacing with respect to one another.

A variety of patterns is possible: clumped (individuals in discrete groups) evenly spaced (each individual maintains a

minimum distance from other individuals) random (individuals distributed independently

of others within a homogeneous area)


Figure 13.5


Causes of Dispersion

Even spacing may arise from direct interactions among individuals: maintenance of minimum distance between

individuals or direct competition for limited resources may cause this pattern

Clumped distribution may arise from: social predisposition to form groups clumped distribution of resources tendency of progeny to remain near parents


Figure 13.6


Figure 13.7


Populations exist in heterogeneous landscapes.

Uniform habitats are the exception rather than the rule: most populations are divided into

subpopulations living in suitable habitat patches

Degree to which members of subpopulations are isolated from one another depends on: distances between subpopulations nature of intervening environment mobility of the species


Causes of Isolation

Whether or not areas of unfavorable habitat are substantial barriers 实质障碍 to mobility depends on: distance between subpopulations nature of intervening habitat mobility of the species


Mobility and Isolation

The extent of isolation of subpopulations depends on the mobility of the species: snail kites 食螺鸢 in southern Florida are

linked into a single population Geckos 壁虎 in Australia are separated by

agriculture into subpopulations between which there is little movement of individuals

Several models address patchiness...


Figure 13.9


Three types of models describe the distributions of populations


Metapopulation Model

The metapopulation model views a population as a set of subpopulations occupying patches of a particular habitat: intervening habitat is referred to as the

habitat matrix:the matrix is viewed only as a barrier to

movement of individuals between subpopulations


Source-Sink Model

The source-sink model recognizes differences in quality of suitable habitat patches: in source patches, where resources are

abundant:individuals produce more offspring than needed to

replace themselvessurplus offspring disperse to other patches

in sink patches, where resources are scarce:populations are maintained by immigration of

individuals from elsewhere


Landscape Model

The landscape model considers effects of differences in habitat quality within the habitat matrix: the quality of a habitat patch can be affected

by the nature of the surrounding matrixquality is enhanced by presence of resources, such

as nesting materials or pollinatorsquality is reduced by presence of predators or

disease organisms

some matrix habitats are more easily traversed than others


Ideal Free Distributions 1

Individuals can make decisions regarding the quality of habitat patches.

Quality of a patch depends on: its intrinsic quality density of other individuals:

occupied patches may have fewer remaining resources

competing individuals may engage in conflicts


Ideal Free Distributions 2

Each individual chooses among patches to maximize its access to resources.

As a patch with high intrinsic quality fills with individuals, its resources are depleted and its apparent quality decreases: at some point, a patch with lower intrinsic quality has

greater apparent quality and it too begins to fill thus all patches reach the same apparent quality,

despite different intrinsic qualities and different densities of individuals, the ideal free distribution


Figure 13.14


Individuals move from sources to sinks.

The ideal free distribution would suggest equivalent reproductive success among individuals occupying habitat patches of differing intrinsic quality: this ideal is rarely realized:

individuals do not have perfect knowledge of patch quality

free choice may be reduced in subordinate 下级 individuals


Sources and Sinks

Some patches are population sources and others are population sinks with net movement from sources to sinks: evident in European blue tit:

populations in deciduous oak habitats are sources, with potential for rapid growth

populations in evergreen oak habitats are sinks, with potential for rapid decline


Table 13.1


Estimating population size.

Population size (number of individuals) is the ultimate measure of a population: we typically wish to know what factors cause

size to change and processes that regulate sizeTotal population size has two components:

density, the number of individuals per unit area

area occupiedDensity area occupied = size.


How do we measure density?

A total count may be feasible: suitable for small populations where individuals

can be distinctively marked often employed for endangered species,

particularly for larger animals such as mammals and birds

For sessile organisms, local density may be determined in plots, then extrapolated 推断 to entire area occupied.

Other techniques may be needed for mobile organisms.


Mark-Recapture Method

Mark-recapture methods are often used with animal populations: an initial sample is collected and all

individuals are distinctively marked marked animals are released into the

population and allowed to mix a second sample is collected and

marked and unmarked animals are tallied


Mark-Recapture Computations

For an initial marked sample of size M, a second sample of size n, containing x marked individuals, the population size N is:

N = nM/xIf 20 fish are captured, marked, and

returned to a small pond, and a second sample of 50 fish contains 6 marked fish, the population estimate is 50(20)/6 = 167.


Figure 13.15


Variation in Populations over Space and Time

Populations tend to vary in size over time.Long-term records often reveal

fluctuations that might be overlooked in shorter term: infestation by chinch bugs in Illinois monitored

over decades reveals populations fluctuations:in some years, populations averaged 1000/m2 over

an area of 300,000 km2

in other years farmers reported little damage


Figure 13.16

麦椿象


Movement of individuals maintains spatial coherence 连贯 .

Movements within populations are referred to as dispersal.

Movements between subpopulations are referred to as migrations, or more specifically: emigration (leaving a subpopulation) 迁

出 immigration (entering a subpopulation)

迁入


Monitoring DispersalMonitoring dispersal is difficult:

organisms must be marked and recaptured large areas must be covered to ensure an

adequate sampling of movementsPopulation biologists often use ingenious

巧妙的 methods to monitor dispersal: dispersal in fruit flies was investigated by

releasing individuals with a visible mutation


Figure 13.17

欧洲椋鸟在美国向西扩张


Lifetime Dispersal Distance

Average lifetime dispersal distance is a useful index of movement within populations: this indicates how far an individual moves

from its birthplace to where it settles and reproduces:a circle with radius equal to the lifetime dispersal

distance is the lifetime dispersal areathe number of individuals within this circle is the

neighborhood size of the population


Summary 1

The distribution of a population is its geographic range. Within this range, patches of suitable and unsuitable habitat may occur.

Dispersion describes spatial distributions of individuals within a population.

Most populations are divided into subpopulations; the aggregate of these is the metapopulation.


Summary 2Movements of individuals among subpopulations

are influenced by quality of the intervening habitat matrix.

Individuals should distribute themselves relative to available resources in an ideal free distribution. This is rarely realized; variations in reproductive success lead to source and sink populations.

Population biologists use various techniques to determine population size and to characterize movements of individuals within populations.

(c) 2001 w.h. freeman and company chapter 13: population structures robert e. ricklefs the economy...

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