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Published on: October 11, 2016
Can modeling improve estimation of desert tortoise population densities?
Kenneth E Nussear1, C Richard Tracy
1U.S. Geological Survey, Western Ecological Research Center, Henderson, Nevada 89074, USA. knussear@usgs.gov
Summary
Estimating desert tortoise (Gopherus agassizii) populations is challenging due to unpredictable burrowing behavior. Current monitoring methods struggle with variable activity, impacting population density accuracy.
Area of Science:
- Wildlife ecology
- Conservation biology
- Population dynamics
Background:
- Federally listed desert tortoise (Gopherus agassizii) populations are monitored using distance sampling to estimate density.
- Distance sampling accuracy relies on quantifying the proportion of animals available for counting (g0).
- Desert tortoises' extensive burrow use creates highly variable availability, challenging the g0 assumption and impacting population estimates.
Purpose of the Study:
- To investigate factors influencing desert tortoise activity and improve population density estimation.
- To assess the predictability of tortoise activity for refining distance sampling correction factors (g0).
Main Methods:
- Utilized activity data from a large experimental population (N=150) to model activity based on environmental factors.
- Assessed the correlation of tortoise activity on consecutive days under similar environmental conditions to gauge behavioral predictability.
- Evaluated if environmental models improved precision of g0 estimates for focal populations.
Main Results:
- Models of tortoise activity based on environmental biophysical attributes did not enhance the precision of g0 estimates.
- Tortoise activity showed weak correlation on consecutive days, even with identical environmental conditions.
- This indicates that daily behavior is not consistently repeatable or predictable.
Conclusions:
- The unpredictable nature of desert tortoise activity significantly complicates accurate population density estimation using distance sampling.
- Current methods for estimating the availability correction factor (g0) are imprecise due to small sample sizes and high behavioral variability.
- Further research is needed to develop more robust methods for monitoring desert tortoise populations, accounting for their complex activity patterns.
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