Evaluating the effects of density-dependence and climate at multiple life-stages in an amphibian metapopulation

Background/Question/Methods

To understand the causes of population growth and decline, it is often necessary to investigate processes acting across multiple spatial scales. For example, population growth may be driven by local processes such as density-dependence, regional processes such as climatic changes affecting multiple populations, or a combination of these scales.  Further, the interaction between local and regional processes may be altered by dispersal among populations.  I investigated the role of local and regional processes, as well as dispersal, as part of an 8-year study of six populations of wood frogs (Rana sylvatica). Over the course of this study, I measured the key components of population growth (e.g., larval survival, postmetamorphic survival, fecundity) and tested how they were influenced by local density and climatic variation.

Results/Conclusions

Over the course of this study, no breeding site consistently acted as a source or a sink.  Instead, population growth rate at each breeding site changed over time, and was driven by both local density-dependence during the larval stage, and annual variation in weather conditions. Local density had a substantially stronger effect on population growth rate than climatic factors, and acted through multiple vital rates, including larval survival and post-metamorphic survival.  Thus, effects of high density during the larval stage acted through multiple life stages to impact overall population growth rate. While dispersal by female frogs was rare, dispersal by male frogs occurred at a much higher rate, and was influenced by conspecific density experienced as larvae. Thus, population dynamics in this system are largely driven by local density-dependent interactions, and are not synchronized by either regional climate or dispersal among populations. These results have important implications for selecting strategies for protecting amphibian populations in the face of habitat loss and climate change.