Many pool-breeding amphibians have life history strategies that favor persistence under conditions of environmental uncertainty. In effect, adults of some species can act as a “seed bank” weathering harsh conditions and emerging under more favorable conditions to lay many eggs, thereby contributing to population persistence. If this is the case, than why are so many amphibian populations experiencing decline? Although there are many hypotheses for why amphibian populations are going extinct, few generalities have emerged. The intent here was to evaluate the impact of environmental stressors and increased environmental variability on extinction probability in pool-breeding amphibians in light of specific life history characteristics. I constructed a density-dependent matrix population model based on life history traits of the narrow mouth toad (Gastrophryne carolinensis). Survival and reproductive rates of this species are similar to other pool breeding amphibians.
Results/Conclusions
Under baseline conditions, I assumed a drought probability (hence zero reproduction) of 0.15 which resulted in an extinction risk of 0.008. Increasing the probability of drought corroborates results of other studies showing that some amphibian life history strategies are favored under certain forms of environmental variability or stress. For example, not until the probability of drought increased to 0.35 did extinction risk increase significantly to 0.10. Alternatively, a 0.15 probability of 0.50 lower yearly survival in the terrestrial phase increased extinction risk to 0.10 from 0.008. I also simulated the effects of environmental stressors that directly reduce reproduction or survival. Contrary to intuition, decreasing fecundity increased population size and decreased the probability of extinction via relief from aquatic phase density dependent effects. However, at very low fecundity, the simulated population became extremely sensitive to small changes; for example decreasing the number of eggs produced by two increased the probability of extinction from 0.11 to 0.36. Unlike for fecundity, the toad populations are very sensitive to changes in the survival of terrestrial stages. A 0.10 decrease in terrestrial phase survival increased the probability of extinction by a factor of six. Taken as a whole the results suggest that some amphibians may be relatively resistant to environmental uncertainty or stressors when effects are focused on the aquatic phase but only up to a certain threshold. Effects occurring in the terrestrial stage generally have strong negative consequences for extinction risk. The results are discussed with respect to factors, such as chytridiomycosis, known to be driving some amphibian population declines.
