Environmental pollution is one of the greatest threats to biodiversity world-wide and is suspected to contribute to global amphibian declines.  This concern has prompted extensive experimental investigation of contaminant effects in a variety of amphibian taxa. Most early studies evaluated acute effects on amphibian embryos and larvae under controlled laboratory conditions, but recent studies have broadened the focus of amphibian ecotoxicology to include other ecologically relevant endpoints (e.g., sublethal effects) and have used mesocosms or artificial ponds to increase ecological realism. These studies have demonstrated that contaminants can have a broad array of adverse effects on larval amphibians, and these effects may interact with a variety of biotic (e.g., presences of predators, conspecific or heterospecific competitors) and abiotic (e.g., hydroperiod, temperature) factors. However, the links between individual-level effects observed in experiments and amphibian population dynamics have seldom been explicitly explored, complicating our ability to effectively conserve amphibian populations or species. We use our research on the effects of mercury (Hg) on toads as a model for bridging the gap between individual-level effects of environmental contaminants and amphibian population dynamics.

Modeling Approach

Previous research in our lab has examined the effects of environmental mercury (Hg) exposure through multiple pathways throughout the life cycle of a common and widespread amphibian, the American toad (Bufo americanus). Specifically, we have used a highly integrative combination of field surveys, and laboratory, mesocosm, and terrestrial enclosure experiments to examine the lethal and sublethal effects of maternal and dietary Hg on B. americanus embryos, larvae, and terrestrial juveniles, and on reproduction of adult females. We are now using a stage-based matrix population model to mechanistically evaluate the consequences of these effects on toad population dynamics in the context of natural population drivers including larval density-dependence and environmental stochasticity (variation in precipitation and catastrophic reproductive failure due to drying or flooding of floodplain breeding pools). Our study will be among the first to link individual-level effects of contaminants to amphibian population dynamics and will shed light on several novel endpoints, including how the timing of larval mortality interacts with larval density-dependence to affect recruitment and whether sublethal effects on maturation rate or female fecundity alter population dynamics.

Expected Results

Using simulation-based sensitivity analyses of our model, we will explore the sensitivity of B. americanus population dynamics to shifts in vital rates that may be affected by Hg, (including stage-specific survival, fecundity, and maturation rates) to evaluate how effects of contaminants on individual amphibians influence population size and extinction probability. We will then parameterize our model to comprehensively incorporate the specific Hg effects we have detected experimentally. We will simulate several realistic exposure scenarios to explicitly evaluate the effects of Hg on B. americanus populations within a contaminated river floodplain in Virginia. Based on previous studies of anuran population dynamics, we expect that effects of Hg on embryos and early-stage larvae will have minimal population-level consequences, due to density-dependent compensation in the larval stage.  However, we expect that the most severe effect of Hg we observed, a 50% reduction in survival of late-stage larvae exposed to both maternal and dietary Hg, will have a strong effect on extinction probability and adult population size.  Our results will shed light on the risks environmental Hg contamination poses for amphibian populations and will guide development of restoration/remediation strategies for amphibians.