Comparing performance of multiple non-invasive genetic capture–recapture methods for abundance estimation: a case study with the Sonoran pronghorn Antilocapra americana sonoriensis SUSANNAH P. WOODRUFF,PAU L M. LUKAC S and L ISETTE P. WAI T S


Abstract Demographic monitoring is required in threa- tened species management, yet effective and efficient mon- itoring is challenging for species that are difficult to capture or susceptible to capture stress. One possible monitoring approach for such species is non-invasive genetic sampling with capture–recapturemethods (genetic capture–recapture). Weevaluated the performance of genetic capture–recapture in a challenging model system, monitoring the threatened Sonoran pronghorn Antilocapra americana sonoriensis.In an effort to determine the best (i.e. efficient, accurate, pre- cise, cost-effective) method for abundance estimation, we used simulations to examine the optimal genetic capture– recapture faecal sampling design for this population. We si- mulated encounter histories for 100–300 individuals, with 0.33–3.33 samples/individual/session, in 1–3 sampling ses- sions. We explored trade-offs between sample size, number of sessions and multi-session (MARK) versus single-session (capwire) closed capture–recapture abundance estimators, and an accurate and precise estimate. We also compared the cost between the genetic capture–recapture approaches and current aerial monitoring methods. Abundance was biased positively in capwire and negatively in MARK. Bias increased and precision decreased with fewer samples/ individual/session. Annual genetic capture–recapture mon- itoring cost was nearly twice the cost of aerial surveys, although genetic capture–recapture methods provided much higher precision.However at the current estimated abundance (c. 200), the same level of precision achievedwith aerialmeth- ods can be obtained by collecting 0.75 samples/individual in a single session, for an annual cost saving of.USD4,000.This approach of comparing estimator performance and cost can easily be applied to other systems and is a useful evaluation for managers to implement prior to designing capture–recapture studies.


Keywords Antilocapra americana sonoriensis,capture– recapture, non-invasive genetic sampling, simulations, Sonoran pronghorn, threatened species


Supplementary material for this article is available at https://doi.org/10.1017/S003060531800011X


Introduction


ment population growth and detect population declines. To make effective decisions, managers must obtain information on a population’s status within an appropriate time frame, and in small or Critically Endangered populations, early detection of population decline is critical to prevent extinc- tion. Capture–recapturemodelling is one of the most com- monly used approaches for population monitoring (Otis et al., 1978; Pollock et al., 1990; Williams et al., 2002). Traditionalmethods of physical capture inherently involve risk and the potential for injury or mortality for both ani- mals and researchers. Non-invasive genetic sampling com- bined with capture–recapture methods (genetic capture– recapture) eliminates much of that risk, provides reliable population estimates, and can be more cost efficient com- pared to other methods such as radio-collaring and aerial telemetry (Solberg et al., 2006; DeBarba et al., 2010b). Even with non-invasive approaches, obtaining adequate


P


SUSANNAH P. WOODRUFF* (Corresponding author) and LISETTE P. WAITS Department of Fish and Wildlife Sciences, University of Idaho, 875 Perimeter Drive, Moscow, Idaho, 83844, USA. E-mail susannah.woodruff@alaska.gov


PAUL M. LUKACS Department of Conservation and Ecosystem Sciences, College of Forestry and Conservation, University of Montana, Missoula, Montana, USA


*Current address: Alaska Department of Fish and Game, Division of Wildlife Conservation, Douglas, Alaska, USA


Received 12 September 2017. Revision requested 27 December 2017. Accepted 15 January 2018. First published online 15 August 2018.


sample sizes and sufficient recapture rates can be difficult or costly with species that occur in low numbers or in low densities. Abundance estimates generated from small sam- ple sizes are often biased and are subject to low precision (Robson & Regier, 1964), and thus conducting sampling at a time and location that maximizes the probability of cap- ture improves the chance of success. Accuracy and precision can be affected by multiple factors, including number of sampling sessions, sample size, and failure of poor-quality samples (Burnham, 1987; Lukacs & Burnham, 2005; Settlage et al., 2008; Laufenberg et al., 2013). By evaluating potential biases related to specific capture–recapture meth- ods and implementing a case-specific study design, re- searchers are more likely to attain the desired level of accuracy and precision.


Oryx, 2020, 54(3), 412–420 © 2018 Fauna & Flora International doi:10.1017/S003060531800011X


opulation monitoring of threatened species is necessary to verify recovery status, and allows managers to docu-


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