Estimating relationships between size and fecundity in the threatened giant garter snake in seminatural and agricultural wetlands

• Published in: Population ecology Vol. 61; no. 2; pp. 141 - 149
• Main Authors: Scherer, Rick D., Hansen, Eric C., Joseph, Max, Wack, Raymund F.
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 01-04-2019; Blackwell Publishing Ltd
• Subjects: Agricultural management; Bayesian analysis; Body size; Breeding; Data processing; Decision making; Demographics; effects of agriculture; Endangered animals; Endangered plants; Fecundity; Females; Fetuses; Life history; Modelling; population ecology; Populations; Pregnancy; Probability theory; snake; Snakes; Wetland agriculture; Wetlands; wildlife management
• ISSN: 1438-3896; 1438-390X
• DOI: 10.1002/1438-390X.1015

Increasing numbers of populations of threatened and endangered plants and animals occur in environments altered by human activity. Life history traits may vary among populations in natural and human‐dominated environments, and therefore, management strategies may need to be tailored to different environments to promote persistence. The primary objectives of this project were to estimate the probability of being pregnant for females of the threatened giant garter snake (Thamnophis gigas) and test for differences in these demographic parameters between populations in seminatural wetlands and wetlands in areas dominated by rice agriculture. We also hypothesized that the probability of breeding and the number of fetuses would increase with increasing body size, a commonly reported relationship for snakes. We analysed data on T. gigas fecundity from 9 years using a Bayesian hurdle model and found no evidence of variation in the probability of being pregnant or in the number of fetuses given pregnancy for T. gigas in the two environments. The probability of being pregnant in a year was higher for females of larger size and ranged from <0.10 for snakes smaller than 631 mm snout‐to‐vent length (SVL) to >0.96 for the largest females. Similarly, given that a female was pregnant, the number of fetuses produced was higher for larger females. Pregnant females of average SVL were expected to produce 14 fetuses, and the largest females were expected to produce 34 fetuses. The inferences from our analyses are needed for subsequent population modeling, which will guide planning and decision‐making for T. gigas.
We used a Bayesian hurdle model to estimate the probability of being pregnant for females of the threatened, endemic giant gartersnake (Thamnophis gigas) in California, U.S.A., and to test for differences in these demographic parameters between populations in semi‐natural wetlands and wetlands in areas dominated by rice agriculture. We did not find evidence that the demographic parameters differed in the two environments. However, the probability of being pregnant and the number of fetuses produced in a year were greater for females of larger size.