Friday, August 10, 2007 - 9:00 AM

COS 162-4: Micro-geographic variation in isotopic composition of a rattlesnake prey base: Implications for studies using stable isotopes as dietary indicators

Melissa A. Pilgrim1, Terence M. Farrell2, and Christopher S. Romanek1. (1) Savannah River Ecology Laboratory, (2) Stetson University

Variation in stable isotope ratios has been used to quantify trophic relationships in many communities.  Our long-term research goal is to use stable isotopes to investigate variation in diet composition among three pigmy rattlesnake (Sistrurus miliarius) populations located less than 4km apart in Central Florida.  To use stable isotopes as dietary indicators, prey species must have distinct isotope profiles.  In order to evaluate isotopic variation in the rattlesnake prey base, we used drift fence arrays to collect prey items from each population.  We collected 992 prey items, representing 10 amphibian, 8 reptile and 7 mammal species.  Prey isotopic compositions were highly variable; δ13C values ranged from -31.1 to -14.5‰ and δ15N values ranged from 0.3 to 7.0‰.  We determined the average δ13C and δ15N for each species.  Taxonomy was not the best predictor of isotopic similarity among prey species; Hyla squirrella (Squirrel treefrogs) had the most depleted δ13C values, while its congener Hyla cinerea (Green treefrogs) had among the most enriched δ13C values.  Generating site-wide averages of prey isotopic composition masked an interesting finding.  Isotopic compositions for some prey species that occurred in all three populations depended on capture location.  For example, Rana sphenocephala (Southern leopard frogs) were a major prey item available in each population and their δ13C values showed significant among-population variation.  In addition, ontogenetic shifts in R. sphenocephala isotopic composition varied by isotope and population.  Assessment of rattlesnake diets using stable isotopes requires generating population-specific and size-specific prey profile plots that incorporate ontogenetic shifts in isotope ratios.  

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