Since 2005 I have been studying the behavioral ecology of the endangered Barton Springs Salamander (Eurycea sosorum) in order to inform conservation management plans for this species. These salamanders are fully aquatic and neotenic, meaning they do not metamorphose, and retain feathery-looking gills throughout life. This species as been found only at four spring locations in Zilker Park in Austin, Texas. Salamanders are primarily found around spring outflows with fast flowing water and live among benthic rocky substrate. Eurycea sosorum is impacted by water quality as the pollutants that enter the Edwards Aquifer must exit through salamander habitat. Also, development over the surface of the Edwards Aquifer prevents rainwater from re-entering and filling up the aquifer, reducing spring flow. Thus, E. sosorum is informally considered to be an indicator species of water quality for the highly popular Barton Springs Pool.
For my dissertation I conducted three major projects on these salamanders including a stable isotope analysis of diet, a time-series analysis of population dynamics and a sensory ecology study of predator detection. You can view a short slideshow from my dissertation research here, and you can see an archived video of my dissertation defense seminar here.
Feeding Ecology Using Stable Isotope Analysis
Feeding ecology is the study of feeding behavior, diet, variation in diet over time and responses to availability and size of prey items. Studying feeding ecology helps define resources necessary for sustenance of individuals of a species. Since survival and reproduction of individuals depend on successful feeding, understanding and quantifying feeding ecologies are essential to the understanding and conservation of species. Secretive or endangered species, like the Barton Springs Salamander, that are not present in large numbers or are difficult to observe directly in the field make it hard to assess feeding ecology.
I combined a survey of available invertebrate prey with carbon and nitrogen stable isotope analysis to study how feeding ecology of adult E. sosorum changes over time. Stable isotope analysis uses tiny tissue samples from the tails of salamanders to make inferences about dietary composition by comparing the isotope “signatures” of the salamanders with the isotopic “signatures” of the prey. This approach has several advantages over traditional methods of dietary analysis, because it allowed me to (1) non-lethally estimate diet in this endangered species, (2) detect diet composition in this secretive species without direct observation and (3) observe a long-term rather than “snap-shot” view of diet. Stable isotope analysis revealed a novel prey item that had been overlooked in the two decades since E. sosorum was listed as an endangered species: planarian flatworms in the genus Dugesia. My research also revealed that E. sosorum employs an opportunist feeding strategy and is able to switch its diet to include midge fly larvae (family Chironomidae) and amphipods (Hyallela azteca) when planarians are less abundant in Eliza Spring. Despite the great potential of stable isotope techniques to address questions about amphibian ecology, this is one of very few studies to apply these methods to amphibian conservation. Click here to see my poster from the 2009 Annual Meeting of the Ecological Society of America in Nature Proceedings.
Time-Series Analysis of Population Dynamics
Monitoring the size of populations of endangered species through time is often critical step in gauging the health of these populations and determining if populations are declining, increasing or stable. Monitoring also helps us to understand which biological (prey population size, predator population size) and environmental (climate, precipitation, temperature) factors may be the most important to keeping populations healthy. Unfortunately for many amphibian species, long-term datasets for population size are hard to come by. Fortunately for the Barton Springs Salamander, the City of Austin’s seven year time-series of monthly abundance of three size classes of E. sosorum from multiple populations is one of the best long-term datasets known for an amphibian population.
Though headwater springs are typically described as having relatively stable environmental conditions, results of my analysis indicate that E. sosorum abundance was strongly influenced by periodic extremes in rainfall. Particularly, it seems that cold winter rainfall which drives down water temperatures in Barton Springs may be particularly important for successful growth and development of juvenile salamanders. Results of this study also suggest that E. sosorum employs a “storage effect” type life history strategy in which in which a few long-lived females capable of sperm storage, high fecundity and prolonged survival in subterranean habitat during adverse surface conditions may be sufficient to sustain population sizes. As climate change threatens to increase climatic variability in central Texas, analysis of population trends as more data is collected will be crucial for determining how E. sosorum responds to such changes in the coming years. I recently presented this research at the 2011 Ecological Society of America meeting in Austin, TX and led an official ESA field trip to Barton Springs.
Sensory Ecology of Predator Detection
Predators affect prey populations both directly (through consumption) and indirectly, by inducing costly changes in prey characteristics. This includes changes in prey behavior such as reduced time spent foraging or seeking mates. These indirect predator effects are often stronger in aquatic habitats, as some cues used by prey to detect and avoid predators are more effectively, or only, transmitted in water. To date, the role of olfactory (smell) cues in anti-predator behavior by aquatic prey has received much more research attention than either visual or bioelectric cues (weak electric potentials generated by movement of aquatic predators). In addition, few studies have compared the relative use of these predatory cues to anti-predator behavior within a single species.
I tested whether the Barton Springs Salamander reacts to visual, bioelectric or olfactory cues from two potential predators: largemouth bass (Micropterus salmoides) and red crayfish (Procambarus clarkii). Eurycea sosorum reduced activity in response to visual and bioelectric cues from potential predators, but did not reduce activity in response to olfactory cues or a blank control. Responses observed in this study are consistent with the expectation that visual cues should be most effective in clear, shallow aquatic habitats inhabited by E. sosorum. This is the first study to demonstrate an anti-predator response by an amphibian mediated by bioelectric cues, and one of very few to observe this phenomenon among aquatic vertebrates.
I continue to work with the endangered and threatened Eurycea salamanders of central Texas through the EuryceAlliance, a working group I founded in 2010 that brings scientists together from a diversity of agencies in order to promote research and conservation of the Texas Eurycea. I also enjoy field trips throughout the Texas Hill Country to look for new populations of Eurycea salamanders, and to check up on known locations to see how they’re doing in the drought.
My dissertation research forms the basis for a successful ‘proof of concept’ in using stable isotopes for conservation research. I am now eager to broaden the scope of this research from a single species in a single spring to the suite of threatened species in the Eurycea complex, and to develop improved methods for sampling stable isotopes from amphibians, which will greatly advance conservation research for these taxa. My students and I are just beginning to explore the effects of anthropogenic nutrient pollution on headwater spring and stream food-webs and the top-predators in these habitats in Texas are often Eurycea salamanders.
In the lab, I am planning physiological ecology investigations into how water temperature affects larval Eurycea growth, survival and development to assess how climate change could affect the ability of these species to reproduce successfully in the wild. I am also planning a series of lab experiments to develop protocols for sampling stable isotopes from the mucus coating of amphibians. Because amphibians lack external tissues such as feathers, scales and fur, it can be difficult to obtain non-lethal or non-invasive tissue samples for isotope analysis. Developing mucus-sampling methods could revolutionize the study of amphibians by providing non-lethal and non-invasive methods for studying these increasingly threatened organisms