nexus between ravens, tortoises, and humans · nexus between ravens, tortoises, and humans by...

Nexus between Ravens, Tortoises, and Humans

By William I. Boarman

Conservation Science Research & Consulting

Conceptual Model

Non-breeding Season/ Floaters

Mating

Nesting

Fledging

Human Influence

Tortoise Populations

Notable Things to Understand Raven Predation on Tortoises

• Ravens as a Subsidized Predator • Tortoises as Raven Food • Human Activities

Notable Features of Ravens

• Northern Hemisphere (2 species) • Intelligent • Adaptable • Curious • Social • Subsidized Predators

Notable Features of Tortoises

• Long-lived • Delayed maturity – consequences for demography • Shelled, but soft for 5-7 years • Accosted on many fronts – “Death by a thousand cuts.” • Populations declining

Notable Features of Humans

• Ubiquitous • Growing, spreading • Linear Features • Point Features • Many activities impact tortoises and their habitat:

– Recreation, military training, vehicles, energy generation, urban developments, mining, poaching, etc.

Support for Conceptual Model

• Non-breeding Adults and Floaters – Anthropogenic Habitat Use

• EAFB, Fort Irwin, MCAGCC, BCCE • Landfills>>Sewage Ponds>Towns>Open Desert

– Movements – EAFB • Large circuit • Not well studied

Non-breeding Season/ Floaters

Mating

Nesting

Fledging

Human Influence

Tortoise Populations

Support for Conceptual Model

• Non-breeding Adults and Floaters – Anthropogenic Habitat Use

• EAFB, Fort Irwin, MCAGCC, BCCE • Landfills>>Sewage Ponds>Towns>Open Desert

Attraction Sites Differ

Land

fill

Sew

age

Pond

City

Gol

f Cou

rse

Con

trol 1

Con

trol 2

Boron

EAFB

Mojave

0

10

20

30

40

Number of Average

Ravens

R2=0.73

p<0.001

Boarman et al. 2005, USGS Publ.

Age Distribution at Landfill

Spring

Camp et al. 2013, West. N. Am. Natur.

Number Captured

Adults Juveniles

n=263

Support for Conceptual Model

• Non-breeding Adults and Floaters

– Movements – EAFB • Large circuit • Not well studied

Ravens Wandered Great Distances Particularly in the Summer

Movements of radio & wing-tagged birds caught at a landfill

n≈80

Boarman et al. 2005, USGS Publ.

Support for Conceptual Model

• Breeding – Nesting territory – Nest Habitat/Substrate

• Natural • Anthropogenic

– Foraging Distance – Tortoise shells at nests

Support for Conceptual Model

• Breeding – Nesting territory – Nest Habitat/Substrate

• Natural • Anthropogenic

Ravens Use Different Structures for Nests

Kristan & Boarman 2007, Ecol. Applic.

0

100

200

300

Trees Poles Bldgs Cliffs

Number of Nests

n=343

Support for Conceptual Model

• Breeding

– Foraging Distance

Non-breeding Season/ Floaters

Mating

Nesting

Fledging

Human Influence

Tortoise Populations

75% of Time Foraging within 600 m of Nest

Sherman 1993, MS Thesis

0

5

10

15

20

25

30

35

40

Rel

ativ

e Fr

eque

ncy

Distance from Nest (m)

Support for Conceptual Model

• Breeding

– Tortoise shells at nests

Nests of “Offending Ravens”

15% Boarman and Hamilton, unpubl.

Anthropogenic Effects on Reproductive Success

– Fecundity – Hatching success – Fledging success – Date/Temperature

Reproductive Success Improves Near Developments

– Fecundity – Hatching success – Fledging success

Fled

ging

Suc

cess

Least Developed Most Developed Kristan & Boarman 2007, Ecol. Applic.

Webb, Boarman, Rottenberry 2004, Condor

Anthropogenic Effects on Reproductive Success

– Date/Temperature

Surv

ival

to

Dep

artu

re (%

)

Tem

pera

ture

(°C

)

Departure Date Webb, Boarman, Rottenberry 2004, Condor

n=87

Anthropogenic Effects on Reproductive Success

• Juveniles – Survival – Movements/Dispersal

Anthropogenic Effects on Reproductive Success

• Juveniles – Survival

Surv

ival

(%

)

Months Since Departure

n=90

• Survival declined with distance of nest to nearest anthropogenic resource.

• Effect lasted for up to 9 months

Webb, Boarman, Rottenberry 2004, Condor

Anthropogenic Effects on Reproductive Success

• Juveniles

– Movements/Dispersal

• Followed 90 fledglings for up to 33 months • Dispersal distance Median 7.9 km Maximum 17.8 km • Primary destination factors Conspecifics Point sources of resources • None dispersed to open desert

Webb, Boarman, Rottenberry 2009, JWM

Annual Cycle in Abundance

• Annual Cycle in Abundance

0

0.5

1

1.5

Winter Spring Summer Fall

Mea

n N

o. R

aven

s pe

r Po

int

Chamblin & Boarman 2005, USGS Publ.

Consequences for Desert Tortoise Populations

• Prey Size • Predation Risk • DT Demography

Non-breeding Season/ Floaters

Mating

Nesting

Fledging

Human Influence

Tortoise Populations

Ravens Eat Juvenile Tortoises

0

20

40

60

80

30-39 40-49 50-59 60-69 70-79 80-89 90-99 100-109 110-119 120-130

Tortoise Carcass Carapace Length (mm)

Freq

uenc

y

Boarman & Hamilton unpubl.

• Prey Size

Consequences for Desert Tortoise Populations

• Predation Risk

Kristan & Boarman 2003, Ecology

• Predation risk was highest near: • Anthropogenic resources • Successful nests • Concentrations of

nonbreeding ravens

Determine Temporal & Spatial Patterns • Predation pressure

– 60 Motion-triggered cameras – 3D-printed tortoise shells

In collaboration with Tim Shields & AutoDesk, Inc. using Remake™

Consequences for Desert Tortoise Populations

• Predation Risk • Prey Size • DT Demography

Management and Effectiveness Monitoring

• Reduce Carrying Capacity - Resource Denial • Area Denial • Reduce Reproductive Success • Training • Anti-perch Devices • Lethal Removal

• Reduce Carrying Capacity - Resource Denial

Waste Management Changes Reduced Ravens

Boarman et al. 2005, USGS Publ. Treatment References

Agricultural Resources Pistachio Orchards

• Reduce Carrying Capacity - Resource Denial

Phenomenally Successful

• Area Denial

Headstarting Facilities

• Area Denial

Laser Gun

Removing Nests

• Reduce Reproductive Success

50% reduction

Boarman 2010, Unplul. Rpt.

Conditioned Taste/Scare Aversion

• Training

Perching

• Anti-perch Devices

Targeting Birds at Food Resource Sites

• Lethal Removal

Targeting Known Offenders

• Lethal Removal

Management

• Reduce Carrying Capacity - Resource Denial • Area Denial • Reduce Reproductive Success • Training • Anti-perch Devices • Lethal Removal

Thank You!

Conceptual Model

Non-breeding Season/ Floaters

Mating

Nesting

Fledging

Human Influence

Tortoise Populations