SREL Reprint #3177

Ecological and methodological factors affecting detectabillity and population estimation in elusive species

John D. Willson, Christopher T. Winne, and Brian D. Todd

Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802, USA

Abstract: Although mark-recapture methods are among the most powerful tools for monitoring wildlife populations, the secretive nature of some species requires a comprehensive understanding of the factors that affect capture probability to maximize accuracy and precision of population parameter estimates (e.g., population size and survivorship). Here, we used aquatic snakes as a case study in applying rigorous markrecapture methods to estimate population parameters for secretive species. Specifically, we used intensive field sampling and robust design markrecapture analyses in Program MARK to test specific hypotheses about ecological and methodological factors influencing detectability of two species of secretive aquatic snakes, the banded watersnake (Nerodia fasciata), and the black swamp snake (Seminatrix pygaea). We constructed a candidate set of a priori mark-recapture models incorporating various combinations of time- and sex-varying capture and recapture probabilities, behavioral responses to traps (i.e., trap-happiness or trap-shyness), and temporary emigration, and we ranked models for each species using Akaike’s Information Criterion. For both banded watersnakes and black swamp snakes we found strong support for time-varying capture and recapture probabilities and strong trap-happy responses, factors that can bias population estimation if not accommodated in the models. We also found evidence of sex-dependent temporary emigration in black swamp snakes. Our study is among the first comprehensive assessments of factors affecting detectability in snakes and provides a framework for studies aimed at monitoring populations of other secretive species.

Keywords: aquatic snakes, behavioral responses, detection probability, mark-recapture, minnow trap, Nerodia fasciata, population estimation, robust design, Seminatrix pygaea, temporary emigration

SREL Reprint #3177

Willson, J. D., C. T. Winne, and B. D. Todd. 2011. Ecological and methodological factors affecting detectabillity and population estimation in elusive species. Journal of Wildlife Management 75(1): 36-45.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).