Construction and analysis of a giant gartersnake (Thamnophis gigas) population projection model

Open-File Report 2017-1164
Prepared in cooperation with the California Department of Water Resources
By: , and 



The giant gartersnake (Thamnophis gigas) is a state and federally threatened species precinctive to California. The range of the giant gartersnake has contracted in the last century because its wetland habitat has been drained for agriculture and development. As a result of this habitat alteration, giant gartersnakes now largely persist in and near rice agriculture in the Sacramento Valley, because the system of canals that conveys water for rice growing approximates historical wetland habitat. Many aspects of the demography of giant gartersnakes are unknown, including how individuals grow throughout their life, how size influences reproduction, and how survival varies over time and among populations. We studied giant gartersnakes throughout the Sacramento Valley of California from 1995 to 2016 using capture-mark-recapture to study the growth, reproduction, and survival of this threatened species. We then use these data to construct an Integral Projection Model, and analyze this demographic model to understand which vital rates contribute most to the growth rate of giant gartersnake populations. We find that giant gartersnakes exhibit indeterminate growth; growth slows as individuals’ age. Fecundity, probability of reproduction, and survival all increase with size, although survival may decline for the largest female giant gartersnakes. The population growth rate of giant gartersnakes is most influenced by the survival and growth of large adult females, and the size at which 1 year old recruits enter the population. Our results indicate that management actions benefitting these influential demographic parameters will have the greatest positive effect on giant gartersnake population growth rates, and therefore population persistence. This study informs the conservation and management of giant gartersnakes and their habitat, and illustrates the effectiveness of hierarchical Bayesian models for the study of rare and elusive species.

Suggested Citation

Rose, J.P., Ersan, J.S.M., Wylie, G.D., Casazza, M.L., and Halstead, B.J., 2018, Construction and analysis of a giant gartersnake (Thamnophis gigas) population projection model: U.S. Geological Survey Open-File Report 2017–1164, 98 p.,

ISSN: 2331-1258 (online)

Table of Contents

  • Acknowledgments
  • Abstract
  • Background
  • Purpose and Scope
  • Giant Gartersnake Biology
  • Study Area
  • Goals and Objectives
  • Section 1: Fitting a von Bertalanffy Growth Model for Giant Gartersnakes
  • Section 2: Reproductive Frequency and Size-Dependence of Fecundity in the Giant Gartersnake
  • Section 3: Integrating Growth and Capture-Mark-Recapture Models to Estimate Size-Dependent Survival in Giant Gartersnakes
  • Section 4: Development and Elasticity Analysis of an Integral Projection Model for the Giant Gartersnake
  • Summary and Conclusions
  • References Cited
  • Glossary
Publication type Report
Publication Subtype USGS Numbered Series
Title Construction and analysis of a giant gartersnake (Thamnophis gigas) population projection model
Series title Open-File Report
Series number 2017-1164
DOI 10.3133/ofr20171164
Year Published 2018
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Ecological Research Center
Description viii, 98 p.
Online Only (Y/N) Y
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