Multiple stressors and complex life cycles: Insights from a population‐level assessment of breeding site contamination and terrestrial habitat loss in an amphibian

Abstract

Understanding the effects of chemical contaminants on natural populations is challenging, as multiple anthropogenic and natural stressors may individually and interactively influence responses. Population models can be used to evaluate the impacts of multiple stressors and to provide insight into population-level effects and/or data gaps. For amphibians with complex life cycles, population models may be useful in understanding impacts of stressors that are unique to the habitat type (aquatic, terrestrial) and that operate at different times in the life cycle. We investigated the population-level effects of aquatic contaminants (coal combustion residues, CCR) and terrestrial habitat loss on the eastern narrowmouth toad, Gastrophryne carolinensis, using existing empirical data that demonstrated negative reproductive and developmental effects of CCR and a series of population models that incorporated density dependence and environmental stochasticity. Results of deterministic models indicated that when terrestrial habitat was abundant, CCR-exposed toads had a larger population size compared to the reference population as a result of reduced density-dependent effects on larval survival. However, when stochasticity in the form of catastrophic reproductive failure was included, CCR-exposed toads were more susceptible to decline and extinction compared to toads from the reference populations. The results highlight the complexities involved in assessing the effects of anthropogenic factors on natural populations, especially for species that are exposed to multiple biotic and abiotic stressors during different periods in the life cycle.

Effects of metal and predator stressors in larval southern toads (Anaxyrus terrestris)

“…We show here that parental and larval exposure to metal contamination increases larval period length and reduces size at metamorphosis, both of which have the potential to decrease survival to adulthood and reduce lifetime reproductive output. Our results suggest that multistressor studies that examine interactive effects provide more environmental realism than single stressor studies-given the importance of the early months post-metamorphosis to juvenile survival in amphibian population models (sensu Taylor and Scott 1997;Salice et al 2011;Willson et al 2012), similar studies that examine the potential carryover effects of larval stressors to the terrestrial stage will be an important research direction in order to better predict the effects of environmental contaminants.…”

Making leaps in amphibian ecotoxicology: translating individual‐level effects of contaminants to population viability

“…For example, many studies evaluating the detrimental effects of environmental contaminants have relied on standard toxicological methods that measure effects on individual organisms (Cairns and Pratt 1993, Clements and Kiffney 1994, Stark 2005. Long-standing recognition of the limitations of this approach (Cairns 1984, Kimball and Levin 1985 prompted several studies that used integrative methods to bridge the gap between individual-level effects of environmental contaminants and population-level processes (e.g., Forbes and Calow 2002, Tanaka 2003, Miller and Ankley 2004, Lin et al 2005, but this approach has seldom been applied to terrestrial vertebrates (but see Nakamaru et al 2002, 2003, Karraker et al 2008, Salice et al 2011.…”

“…However, theoretical frameworks for extrapolating the individual-level contaminant effects observed in experimental studies to amphibian population viability remain limited. For example, prior models for the effects of contaminants on amphibians have not considered ways in which sublethal effects of contaminants (e.g., reduced body size) may influence populations and few have developed methods to incorporate variation in recruitment due to precipitation or other forms of environmental stochasticity (Salice et al 2011).…”

Integrative demographic modeling reveals population level impacts of PCB toxicity to juvenile snapping turtles

“…While population-level assessment can provide insights into ecological effects of contaminant exposure, they are also more data intensive compared to other risk estimators, especially hazard or risk quotients (Forbes et al, 2008;Salice et al, 2011b). At a minimum, life history data on the schedules of survival, growth and reproduction are needed to parameterize population models.…”