Integrative approaches to exploring functional roles of clandestine species
Willson, John David
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Understanding the functional roles of species is critical to making informed conservation and management decisions. Although the roles of many common species have been thoroughly studied, species that are infrequently encountered are often assumed to be rare and of minor functional significance. Snakes are among the most secretive vertebrates and consequently, we know little about snake population ecology, predator-prey relationships, or the roles that snakes play within ecosystems. Prompted by this lack of knowledge, I used novel and integrative methods to investigate the roles of these inconspicuous and difficult-to-sample predators. Specifically, I (1) used mark-recapture, stable isotope analyses, and laboratory experiments to estimate the predatory role of aquatic snakes within an isolated fishless wetland, (2) investigated ways in which prey communities may mediate snake’s roles, and (3) theoretically evaluated ways in which snakes, as ectotherms, may differ from comparable endothermic predators in their interactions with prey populations. Using advanced mark-recapture analyses to develop a thorough understanding of factors that affect detectability in aquatic snakes, I estimated snake population density at Ellenton Bay, an isolated, 6 ha freshwater wetland in South Carolina, USA. I found that snakes exist at extremely high densities (>170 snakes ha-1) within this productive habitat. Using data on snake density, growth rate, diet, and mass conversion efficiency, I estimated that snakes consumed over 200 kg of amphibian prey annually at Ellenton Bay, translating into >150,000 kJ ha-1 of energy flow from secondary to tertiary consumers within the food web. Additionally, I demonstrated that aquatic salamanders, which are characteristic of fishless wetlands, represent a higher quality prey for watersnakes than sunfish, which are characteristic of permanent wetlands. Watersnakes consumed more and larger salamanders than sunfish with less impairment to post-prandial locomotor performance. Thus, although snakes can be extremely important predators within some aquatic ecosystems, snake’s roles may be mediated by prey community composition. Finally, using a mathematical predator-prey model, I demonstrated that shifting predator numerical responses from those characteristic of endotherms to more linear curves characteristic of ectotherms was generally stabilizing to predator-prey dynamics, indicating that snakes may exhibit more stable predator-prey interactions than those of endothermic predators.