Pacific Southwest

Title: Garter snake population dynamics from a 16-year study: considerations for ecological monitoring

Author: Lind, Amy J.; Welsh Jr, Hartwell H.; Tallmon, David A.

Date: 2005

Source: Ecological Applications, Vol. 15(1): 294-303

Description: Snakes have recently been proposed as model organisms for addressing both evolutionary and ecological questions. Because of their middle position in many food webs they may be useful indicators of trophic complexity and dynamics. However, reliable data on snake populations are rare due to the challenges of sampling these patchily distributed, cryptic, and often nocturnal species and also due to their underrepresentation in the ecological literature. Studying a diurnally active stream-associated population of garter snakes has allowed us to avoid some of these problems so that we could focus on issues of sampling design and its influence on resulting demographic models and estimates. From 1986 to 2001, we gathered data on a population of the Pacific coast aquatic garter snake (Thamnophis atratus) in northwestern California by conducting 3–5 surveys of the population annually. We derived estimates for sex-specific survival rates and time-dependent capture probabilities using population analysis software and examined the relationship between our calculated capture probabilities and variability in sampling effort. We also developed population size and density estimates and compared these estimates to simple count data (often used for wildlife population monitoring). Over the 16-yr period of our study, we marked 1730 snakes and had annual recapture rates ranging from 13% to 32%. The best approximating demographic model for our data demonstrated higher survival rates for females than males and showed low and annually variable capture probabilities for both. Annual population size estimates (converted to linear densities), ranged from 58 to 131 snakes/km. Mean annual field counts typically accounted for only 5–10% of the total population size estimated using capture–recapture models. We found no evidence for a changing population size throughout the study. We found a positive relationship between sampling effort and capture probabilities. We evaluate survey design options that would help us approach recommended levels of capture probabilities and thus increase the precision of our estimates, allowing derivation of more complex demographic models. Our results should be useful in the development of monitoring programs for snakes and other secretive wildlife species and provide target demographic rate values for restoration of related at-risk snake species.

Keywords: capture probability, long-term study, northwestern California USA, Pacific coast aquatic garter snake, population size, Program MARK, sampling effort, survival rate, Thamnophis atratus

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Citation

Lind, Amy J.; Welsh Jr, Hartwell H.; Tallmon, David A.  2005.  Garter snake population dynamics from a 16-year study: considerations for ecological monitoring.   Ecological Applications, Vol. 15(1): 294-303.