Linking demography to range dynamics with population projection models
Species geographic ranges reflect the combined result of many processes – e.g. demography, dispersal, biotic interactions, behavior, historical biogeography – that interact to produce observed spatial and temporal population patterns. While it is challenging to disentangle the relative roles of these processes across large geographic extents, we discuss scenarios in which demography may dominate spatial patterns, enabling useful predictive power at biogeographic scales from population projection models. Such dynamic, process-based models are critical for forecasting species response to global change and understanding nonequilibrium distributions. Because rich environmentally stratified demographic data is uncommon, we emphasize tools to handle sparse data and propagate uncertainty through all stages of the model.
We illustrate methods and insights via case studies with overstory shrubs in South African fynbos, invasive herbs and shrubs in New England, and canopy trees in the US. Success in accurately predicting well-studied species whose distributions are near equilibrium suggests that our approach can reliably forecast species ranges, at least in some cases. We highlight insights on (1) the ability to develop targeted demographic data collection strategies; (2) the geographic variation in the vulnerability of different life stages; (3) the ability to forecast population dynamics under environmental change or disturbance; (4) comparative demography. We conclude with strengths and weaknesses of our demographic range modeling approach in its application to other species with different types of available data, life history characteristics, or under the influence of other ecological processes.