Contrasting responses to climate reveal unexpected patterns of population vulnerability near the northern and southern range limits of a montane amphibian

Background/Question/Methods

Climate change is expected to create physical and biological conditions conducive to population growth and persistence at the poleward range limits, while abundance and persistence are expected to decline at equatorward limits. Range shifts have been observed or predicted for many species, but few studies have explicitly incorporated demographic data from multiple populations across species’ ranges to evaluate these general predictions of climate-driven extinction vulnerability. Here, we used stage-based demographic models and climate forecasts for two populations of a montane amphibian in the northern hemisphere, Cascades frog (Rana cascadae), at the northern (poleward) and southern (equatorward) extent of the species’ range to test the prediction that the southern population should experience higher, and the northern population lower, extinction risk due to expected changes in climate by the 2080’s. We evaluated how current climate affects aquatic stages by estimating larval mortality due to desiccation (pond drying) in each population, and for terrestrial stages using long-term population abundance data linked to a suite of climate variables (temperature, precipitation, snowpack dynamics, and desiccation). We used these relationships to model the stochastic population growth rate (λ_s) and 20-year extinction risk of each population for current (1980-2006) and future periods (2080’s, A1B emissions scenario).

Results/Conclusions

We found that currently, both populations are near stable, with the northern population increasing slightly (λ_s = 1.01), and the southern population decreasing slightly (λ_s =0.98). We found that current survival rates for aquatic stages in both populations are driven by changes in precipitation that lead to pond drying, but found divergent patterns for terrestrial stages, especially between survival and winter length (a top ranked variable in our survival analysis). By the 2080’s we predict a slight increase in λ_s for the southern Trinity Alps population (λ_s = 0.99), compared to a sharp 5% annual decline for the northern Olympic population (λ_s = 0.95). Our results highlight that climate change can influence local landscape processes in ways that lead to unexpected consequences for population stability (i.e. higher vulnerability for populations at the poleward range extent) for species with complex life cycles in the face of climate change.