Species are expected to shift their ranges in response to temporally nonstationary environmental conditions, such as climate change. Range shifts are seen throughout the paleo record, and there is increasing evidence for contemporary shifts driven by anthropogenic change. Ultimately, range shifts are the result of population dynamics in the presence of spatial and temporal environmental change, and they feed back to dynamics by determining relationships between organisms and the environment. Most theoretical treatments of the population consequences of shifting environmental conditions investigate persistence conditions in models with binary habitat quality. We take a more general approach, examining how environmental nonstationarity in space and time affects the stationarity of population dynamics. We ask: when are range shifts successful in insulating populations from nonstationary environmental change?
We simulate populations experiencing environmental conditions that are nonstationary in space and time, yielding shifts in the locations of favorable habitat and in habitat quality. We investigate the stationarity and relative stationarity of three outcomes: regional population size, the experienced environment, and the distribution of environmental conditions. These outcomes capture changes to the environment, both locally and regionally, and the resulting dynamics and distributions of the population.
Results/Conclusions
When spatial environmental gradients are linear, the locations of favorable environmental conditions shift in space, but the amount of favorable habitat remains stationary. In this case, populations can follow environmental conditions with little change in either overall dynamics or experienced environmental conditions provided demographic and dispersal rates are high enough. With lower demographic and dispersal rates, population distributions lag behind favorable environmental conditions, dependent on the rate of temporal change. In contrast, nonlinear spatial gradients, e.g. more rapid change in conditions through space as occurs moving up streams or mountainsides, cause the amount of favorable habitat to change along with location. In this case, the regional population does not remain stationary, but reflects the amount of favorable habitat. Still, the experienced environment may remain stationary provided dispersal is not too long.
Our results show how spatial shifts can maintain stationary population dynamics and experienced environmental conditions in the face of temporal environmental change. Moreover, experienced environmental conditions may remain stationary despite changing population sizes until very near extinction. When these conditions are not met, a population may move about the landscape, but these range shifts will be unable to maintain stationary dynamics.