Integrating growth and capture–mark–recapture models reveals size‐dependent survival in an elusive species

Abstract

Citation: Rose, J. P., G. D. Wylie, M. L. Casazza, and B. J. Halstead. 2018. Integrating growth and capturemark-recapture models reveals size-dependent survival in an elusive species. Ecosphere 9(8):Abstract. Survival is a key vital rate for projecting the viability of wild populations. Estimating survival is difficult for many rare or elusive species because recapture rates of marked individuals are low, and the ultimate fate of individuals is unknown. Low recapture rates for many species have made it difficult to accurately estimate survival, and to evaluate the importance of individual and environmental covariates for survival. Individual covariates such as size are particularly difficult to include in capture-mark-recapture models for elusive species because the state of the individual is unknown during periods when it is not captured. Here, we integrate a von Bertalanffy growth model with a multi-state robust-design Cormack-Jolly-Seber model to test for a relationship between body size and survival in the elusive, threatened giant gartersnake, Thamnophis gigas. We take a Bayesian approach to model the size of an individual during periods when it was not captured and measured, which fully propagates uncertainty in this unobserved covariate. We found strong support for a positive relationship between snake size and annual survival, with survival increasing with size up to a peak for adult snakes, after which survival either declines slightly or plateaus for the largest individuals. Few captures of very small and very large individuals led to high uncertainty in the survival rates of these sizes. Survival of giant gartersnakes was also positively related to the amount of precipitation and the cover of emergent and floating vegetation at a site. To our knowledge, our study is the first to estimate a sizesurvival relationship in a snake while fully accounting for uncertainty in the size of unobserved individuals. Our results have implications for the management of this threatened species and illustrate the utility of integrating hierarchical Bayesian models to the study of survival in elusive species.

Demographic factors affecting population growth in giant gartersnakes

“…We searched the literature for studies that estimated neonate survival in gartersnakes or close relatives (Supporting Information). The range of values in our literature search (0.16–0.37) is similar to the uncertainty interval for the survival of neonate giant gartersnakes from capture‐mark‐recapture models (Rose et al ). Consequently, we built IPMs with 4 values for the probability of recruitment: 0.1, 0.2, 0.3, and 0.4.…”

“…Because of low capture probability for giant gartersnakes, estimated size–vital rate relationships, particularly survival, have substantial uncertainty (Rose et al , b, c ). In an earlier study comparing the 3 size–survival functions, the quadratic size–survival relationship exhibited the best fit to empirical data, followed closely by the asymptotic relationship, with a linear size–survival relationship having little support (Rose et al ). In addition, our apparent survival functions are likely biased low because they confound true survival and site fidelity.…”

“…Correlations between vital rate parameters can affect the conclusions from elasticity analyses (Doak et al ). We tested for covariance between survival and growth using data from 8 sites sampled from 2011–2016 (Rose et al ), but we found no evidence of covariance between annual growth and survival. We did not include covariance between fecundity and other vital rates because many of the fecundity data were not collected concurrently with survival and growth data.…”

“…The survival kernel, , defines the probability that a snake survives from year t to year t + 1, which depends on the snake's SVL ( x ) in year t . There is uncertainty about the form of the size–survival relationship in giant gartersnakes (Hansen et al , Rose et al ). Therefore, we constructed IPMs with 3 size–survival relationships (Fig.…”

“…Unknown vital rates and uncertainty in size–vital rate relationships exist for the giant gartersnake ( Thamnophis gigas ), a threatened semi‐aquatic snake endemic to California, USA. Recaptures of marked neonate giant gartersnakes are rare and estimates of survival for this life stage are imprecise (Rose et al ). In addition, support exists for 3 relationships between body size and survival in giant gartersnakes: linear, quadratic (unimodal), or asymptotic, where survival initially increases linearly with size but plateaus for large adults (Hansen et al , Rose et al ).…”