DIET AND PREY PREFERENCE OF GIANT GARTERSNAKES (THAMNOPHIS GIGAS) IN THE SACRAMENTO VALLEY OF CALIFORNIA A University Thesis Presented to the Faculty of California State University, East Bay In Partial Fulfillment of the Requirements for the Degree Master of Science in Biological Sciences By Julia Samiye Martins Ersan December, 2015 i Copyright © 2015 by Julia Samiye Martins Ersan ii Abstract The introduction of exotic species into an environment can introduce great change in the trophic dynamics of native species. This is of even greater concern if the native species are threatened or endangered. The giant gartersnake (Thamnophis gigas), an endemic predator of the Central Valley of California, is listed as threatened, its decline because of the conversion of its once vast wetland habitat to agriculture. Another anthropogenic factor contributing to this snake’s changing ecology and potentially its decline is the introduction of non-native prey species into their habitats. These introductions have resulted in a prey community that is almost completely composed of exotics. Exotic prey can be detrimental to predators, potentially causing injury, parasitic infection, and intraguild predation. A first step toward understanding the effect(s) of exotic prey on giant gartersnakes is to determine whether and the degree to which these snakes have an affinity toward and will actually consume these species. I examined dietary choices in the giant gartersnake using laboratory and field studies to determine whether these snakes are selecting their prey or if they are simply consuming what is abundant. The laboratory component of my investigation was comprised of two sets of behavioral trials in which naïve neonates of wild-caught adult females from three different areas served as the focal animals. I examined: 1) neonate prey preference in response to olfactory cues of prepared prey extracts and 2) what neonates actually consumed when provided with a simultaneous choice of different live prey items. The field studies involved an analysis of prey selection where I: 1) quantified the available prey species in several known giant gartersnake habitats and 2) examined stomach contents to reveal the composition of the iii snakes’ actual diet. Results from these studies revealed that giant gartersnakes both preferred and selected native Sierran treefrogs. These results will contribute to a further understanding of the giant gartersnake’s diet, its role as a predator and the degree of impact, if any, exotic species might be having on them. This, in turn, can direct management and conservation efforts. iv Brian Halstead 12 / 2 / 15 v Acknowledgments I would like to thank Dr. Erica Wildy for serving as my major advisor and advocate in all matters related to the department. Thank you for your positivity, support and kindness. I am most grateful and indebted to Dr. Brian Halstead. Thank you for your brilliant insight and guidance through developing the experimental design and the manuscript preparation. I am particularly appreciative for all of the direction and instruction in regard to the statistical analyses. I thank Dr. Brian Perry for serving on my committee and for comments for revision on my thesis. I would like to thank my professors, Dr. Ellen Woodard and Dr. Christopher Kitting, and colleagues Kristy Howe and Brianne Brussee for their opinions on specific sections or chapters. I would like to thank Dr. Glenn Wylie and Mr. Mike Casazza for their advice with regard to presentations, general support of my project and my continued employment at USGS. A big thank you to all the landowners and land managers for allowing USGS to conduct research and capture snakes on their properties. Another big thanks to the many employees and volunteers that contributed to the capture and care of the many snakes used in the study, including but not limited to: Justin Demianew, Nicole Dotson, Allie Essert, Kristin Fouts, Anna Jordan, Dan Knapp, Brianna Larsen, Desmond Mackell, Sam McNally, Kristin Ober, and Shannon Skalos. vi To those in the Shaffer lab, particularly, Dr. Christopher Searcy for instilling in me a foundation to pursue herpetological study. Thanks to my friends and family, above all my mom, a most excellent role model and cheerleader who provided me with opportunities that eventually allowed me to pursue a career in the field for which I am most passionate. Also, my dad and aunt for caring about my success and happiness. I thank my sisters Ari, Ayşe, Morgan, Juliana, Zeka, Anna Joy, and Katie for emotional support, long phone conversations, and sleepovers. I would like to thank my love, Andrew Sutherland. Thank you for challenging me, for the in-depth discussion and edits on my thesis, and for inspiring and sharing with me a love for nature and fauna. I dedicate this thesis to the snakes. vii Table of Contents Abstract .............................................................................................................................. iii Acknowledgments.............................................................................................................. vi List of Figures ..................................................................................................................... x List of Tables .................................................................................................................... xii Chapter 1 Thesis introduction ............................................................................................. 1 Statement of Thesis ......................................................................................................... 1 Biology of the Giant Gartersnake.................................................................................... 2 Taxonomy and Description ......................................................................................... 2 Natural History ............................................................................................................ 3 Distribution .................................................................................................................. 3 Threats and Conservation ............................................................................................ 5 Factors Potentially Affecting Diet Selection in the Giant Gartersnake .......................... 5 History of Methodology .................................................................................................. 8 Significance of the Methodology ................................................................................ 9 Thesis Organization....................................................................................................... 10 Chapter 2 Prey Preference and Selection of Neonate Giant Gartersnakes (Thamnophis gigas) from the Sacramento Valley of California ............................................................. 11 Introduction ................................................................................................................... 11 Materials and Methods .................................................................................................. 13 Experiment 1 – Neonate Responses to Olfactory Chemical Cues of Potential Prey (Day 15 to Day 20) .................................................................................................... 14 viii Experiment 2 – Neonate Prey Selection and Consumption (Multiple Aquatic Prey Species; Day 21) ........................................................................................................ 17 Experiment 3 – Neonate Feeding Behavior (Single Terrestrial Prey Species; Day 21) ................................................................................................................................... 21 Data Analysis ............................................................................................................. 22 Results ........................................................................................................................... 24 Prey Preference Trials ............................................................................................... 24 Prey Selection/ Consumption Trials .......................................................................... 28 Discussion ..................................................................................................................... 33 Chapter 3 Giant gartersnake (Thamnophis gigas) prey selection ..................................... 42 Introduction ................................................................................................................... 42 Materials and Methods .................................................................................................. 43 Field Methods ............................................................................................................ 43 Analytical Methods.................................................................................................... 50 Results ........................................................................................................................... 51 Discussion ..................................................................................................................... 54 Chapter 4 Thesis Discussion ............................................................................................. 58 References ......................................................................................................................... 63 Appendix