Models and mechanisms: understanding multiple stressor effects on an amphibian population

a. Objectives: This proposal aims to further our understanding of the effects of multiple environmental stressors, both natural and anthropogenic, on a wildlife population of the salamander, Ambystoma barbouri. Our objectives are to: 1) build, implement and evaluate a spatiaUy-explici~ individual-based population model, 2) conduct experiments to measure the effects of multiple stressors on parameters that enter into the model (e.g. survivaJ, fecundity, growth, extinction rates). and 3) measure endocri,ne and physiological variables to investigate potential mechanisms underlying effects of multiple stressors on the parameters that enter into the model. A. barbouri was selected due to increasing concern over the apparent global decline of amphibians and because amphibian life history characteristics make them particularly vulnerable to multiple stressors. To produce a realistic, situationspecific model, the combinatorial effects of multiple stressors likely to be encountered by this population will be used to model population demography (growth, survival, fecundity, extinction rates). These stressors. include habitat ephemerality (desiccation), predator-prey interactions (chemoreception, locomotion, drift) and anthropogenic pollutants (atrazine), In addition, physiological endpoints (hormone balance,. enzyme induction) will be studied to elucidate mechanisms underlying amphibian response to multiple stressors. Parameters for the model will be derived from these and other empirical data; sensitivity analysis will identify the influence of each parameter, and the model will be validated against an existing data set. lb. Approach: Salamander eggs will be collected from a. wildlife population, divided into multiple treatment groups and exposed to combinations of atrazine (a widely used herbicide), density stress, desiccation and predation throughout embryonic and larval development. Larvae will be assessed for growth, developmental rate, time to and size at metamorphosis, locomotor ability, activity levels, use of refugia, predator avoidance and drift rates. Following metamorphosis, juveniles will be evaluated for growth, time to maturity and survival. Adult reproductive capacity will be examined by studying male and femal.e gonadal histopathology. To understand mechanisms of action of multiple stressors, physiologically relevant parameters (estradiol-Up, testosterone, corticosterone, prolactin, thyroid hormones, vitel/agenin and steroid metabolizing enzymes) will be quantified at four different stages (early and late metamerphs, juveniles and adults). c. Expected results and benefits: The proposed research will help elucidate the effects of realworld risks posed by multiple stressors on a model species representative of a group of particular concern (amphibians). This research will result in a spatially-explicit individual-based model of population demography (growth, survival, fecundity and extinction rates) that discerns the effects of stressors, singly and in combination, and facilitates assessment of population sustain ability. The resulting life history data will relate stressor-response relationships to demography. In addition, this research will enhance our mechanistic understanding of stress responses, significantiy improving the predictive capability of the model. This model should prove useful to wildlife managers and risk assessors due to its incorporation of multiple stressors and its simple conceptual basis, which makes it readily modifiable for use with other species and other stressers. This work will generate new knowledge on the sensitivity of this little studied group (amphibians) to multiple environmental stressors and should increase our understanding of current amphibian declines worldwide.