Population viability analyses (PVAs) based on detailed demographic data allow forecasting of future population trends under different management scenarios. Integral projection models (IPMs) that model demographic rates as functions of continuous variation in size/age rather than as point estimates of transition rates between different age/stage classes have proved to be a valuable new tool in the demographer’s toolbox, but have yet to be applied to amphibians. Here we create the first amphibian IPM based on two decade-long mark-recapture studies of the endangered California tiger salamander (Ambystoma californiense; CTS). We then incorporate this IPM into a PVA by identifying the primary sources of environmental stochasticity and modeling their impacts on the CTS population. Using historic climate records, we are able to validate model predictions by comparing them to observed population dynamics. We then incorporate data on terrestrial density distributions of the different age classes to identify the minimum dynamic area needed for a viable population.
Results/Conclusions
Our model indicates that CTS populations are naturally resilient to negative perturbations, with a strikingly high population growth rate (λ = 3.97) when at low density. We also find an additional stabilizing mechanism resulting from delayed life history effects, which leads the number of individuals in more advanced age classes to be less variable than in earlier age classes, even within the same cohort. Model validation based on comparisons to nine years of mark-recapture records found that the model can account for 91% of the variance in observed recruitment, with less than 5% error in total number of recruits. We find that each CTS breeding pond needs to be surrounded by a minimum buffer of 800 m of intact terrestrial habitat to have long-term viability. This result is surprisingly robust to variation in either carrying capacity or the degree of environmental stochasticity. We expect this model to be a valuable tool for assessing the viability of recovery units proposed for downlisting CTS from threatened/endangered status.