Temperature effects on dispersal: Implications for warming-induced range shifts
Dispersal is a crucial component of species' responses to climate warming. Most organisms engage in temporary local scale movement as a means of avoiding excessively warm temperatures; they also engage in permanent long-distance dispersal that leads to changes in species' distributions across latitudes. Predicting how dispersal mediates species' responses to climate warming is therefore a crucial research priority. I present a mathematical model of density-dependent population dynamics and dispersal, which incorporates mechanistic descriptions of the temperature responses of reproduction, mortality and movement based on how temperature affects the underlying biochemical and physiological processes. The novelty of my approach is that all components of the dispersal process --- emigration, mortality during transfer from one habitat to another, and immigration --- are explicitly temperature dependent. Because temperature responses of life history traits are mechanistically derived from first principles of thermodynamics, this framework can accurately predict the magnitude of range shifts in ectotherm species for a wide range of climate change scenarios.
I report two key findings. First, the temperature-dependence of individual movement leads, at the population level, to a net movement of individuals from warmer to cooler habitats, with the potential to extend a species' range at its northern (high-latitude) limit and to prevent the contraction of the species' range at its southern (low-latitude) limit. When movement occurs down a temperature gradient (or equivalently, up a latitudinal gradient), temperature-dependent dispersal mortality can limit or even prevent the northward expansion of a species' range without appreciably affecting its southern range limit. In contrast, when movement occurs up a temperature gradient (or down a latitudinal gradient), temperature-dependent dispersal mortality causes a contraction of a species' southern range limit without substantially affecting the expansion of its northern range limit. Second, the exact magnitude by which an ectotherm species' range expands, contracts or shifts can be accurately predicted based on knowledge of how temperature affects the underlying life history traits (reproduction, survivorship, dispersal).