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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