Background/Question/Methods It is well known that many species show strong temporal variation in diet. Long-term dietary trends may be important in assessing the effects of ecological change such as global warming, land use change, or introductions of invasive species. Short-term variation in food sources or prey selection may be crucial for understanding population dynamics in poorly understood species. The Barton Springs Salamander (Eurycea sosorum) is an endangered species endemic to four small spring outflows in downtown Austin, Texas. This species remains aquatic throughout life and inhabits benthic rocky substrate. While little is known about its foraging ecology, E. sosorum is assumed to be a generalist predator with the abundant amphipod Hyalella azteca as its primary food source. The salamander's secretive behavior and obscure microhabitat make direct foraging observations impossible. Thus, stable isotope applications may be the only feasible means of estimating diet in these animals. Measurements of δ¹³C and δ¹⁵N for wild-caught Eurycea sosorum and a suite of potential invertebrate prey (amphipods, chironomids and planarians) were taken in November 2007, February 2008 and May 2008. Quantitative invertebrate censuses track seasonal changes in invertebrate relative abundance. A multi-source mixing model is applied to field isotope data, and a distribution of likely prey item contributions to salamander diet is obtained for each season.

Results/Conclusions Measurement of δ¹³C and δ¹⁵N from captive-bred E. sosorum raised on a constant diet indicate minimal fractionation of δ¹³C (<1‰) and approximately 2.3‰ enrichment in δ¹⁵N per trophic level. Mixing model results show that salamanders feed nearly equally on planarians (53-57% of diet) and amphipods (43-47% of diet) during the course of the study, and that chironomids do not contribute to the diet of E. sororum. The relative isotopic relationship between the three prey items and salamander tissue remains constant over the course of the study. Furthermore, individual variation in salamander diet is large in all seasons (range of 6-8‰ of δ¹³C values for individual salamanders), potentially reflecting the highly clumped prey item distributions or individual specialization on different food resources. Stable isotope techniques allow investigation of how diet composition responds to temporal changes in prey availability in field populations and the degree to which intrapopulation diet variation influences overall population diet estimates. This work provides conservation biologists at Barton Springs with essential information on the diet and ecology of E. sosorum, which is being used to improve species management plans that will be applicable to other spring systems.