Terrestrial habitat availability alters the effect of aquatic contaminants on amphibian populations

Background/Question/Methods   Widespread declines of amphibian populations have been attributed to many factors that often act simultaneously. Although the individual-level effects of aquatic contaminants (e.g., pesticides) on larval amphibian survivorship are well studied, these are rarely extrapolated to the dynamics of populations. Further, ecotoxicology studies rarely consider these effects in the broader landscapes in which amphibian populations are embedded. A population is most susceptible to decline when a stressor alters survivorship in the life stage that has the largest contribution (high elasticity) to the population growth rate (λ). For example, long-lived amphibians have a low elasticity to the aquatic (larval) stage and are thus expected to be less susceptible to aquatic chemical contamination. When an organism uses different habitats throughout their life, habitat availability influences the elasticity of the life-stage in each habitat. Many amphibians utilize terrestrial and aquatic habitats, and thus changing the aquatic to terrestrial ratio (ATR) can alter the relative elasticities, and the resulting influence of aquatic stressors. Here, we use simulations to determine the influence of an aquatic stressor on λ of two long-lived and two short-lived amphibians, and then we altered the ATR to explore the variation in those results.

Results/Conclusions   We simulated the effects of carbaryl, a common aquatic contaminant used as a pesticide, on two long-lived amphibian species. Our results showed a slight decrease in the population growth rate (λ). Alternatively, carbaryl caused dramatic decreases in the λ of the two short-lived amphibians. We simulated the destruction of terrestrial habitat for each species by lowering the survivorship rate of adults and juveniles. As terrestrial habitat abundance was reduced, the effect of carbaryl on λ of the short-lived species was unchanged, whereas the population level effects of carbaryl on λ of the long-lived species was intensified. Our results indicate that although it is important to model the effects of chemical contaminants at a population level, the overall influence of aquatic chemical contaminants on amphibian populations will be strongly influenced by the landscape (e.g., ATR) in which the amphibian populations are embedded