From forecasting vulnerabilities to assessing recovery: the utility of demographic models in addressing population declines

Curbing species’ decline driven by anthropogenic modifications to natural systems requires a deep understanding of how specific changes to biotic and abiotic processes affect populations. Individual life history stages may differ in their response to such changes, consequently buffering or accelerating population declines. I explore the concept of demographic compensation among life stages using stage-structured demographic models to improve predictions for two conservation challenges; 1) forecasting climate change impacts to amphibian populations in montane ecosystems, and 2) identifying the most effective life history targets for recovering declining amphibian populations. In Chapters 2 and 3, I use demographic data for the Cascades frog (Rana cascadae) at northern and southern range boundaries to parameterize stochastic matrix population models under current and future environmental conditions to evaluate how climate change affects population stability. I demonstrate that R. cascadae populations at the northern range boundary are stable, but that compounding negative effects of climate on early and late life history stages creates a demographic tipping point by the 2080’s. I find that counter to range shift predictions, the population growth rate for the southern population will change little in the face of climate change, and differences in population stability between northern and southern range limits are driven by contrasting responses to climate. Equally important to forecasting population vulnerability, is preventing extinction of declining populations. In Chapter 4, I use demographic models to elucidate recovery potential for declining populations of Oregon spotted frogs (Rana pretiosa) by evaluating the effectiveness of population supplementation at multiple life stages. I compare two supplementation strategies, head-starting early life stages and captive breeding, and find captive breeding up to two orders of magnitude more effective at reducing extinction probabilities than head-starting. In Chapter 5, I extend the utility of such models using formal decision analysis to evaluate tradeoffs between the effectiveness of conservation actions and their economic costs. I reveal that the supplementation of wild populations with captive bred larvae results in the largest reduction in extinction risk per dollar invested. In this thesis, I use demographic models to improve our predictions of species’ responses to climate change before declines occur, and conversely, advance the quantitative framework for recovering declining populations.