Patchy landscapes and the spatial spread of populations

Background/Question/Methods

Two pressing ecological concerns are the spread of biological invasions and species range shifts with climate change. At the center of each dynamic is population spread across the landscapes. Although much of our understanding of how populations spread has come from models of spatially homogeneous environments, landscape heterogeneity is characteristic of most natural and anthropogenic habitats. We used simulation and analytical approximations to explore the effect of density dependence on the spread of annual plants moving through heterogeneous landscapes with gaps in suitable habitat. 

Results/Conclusions

We found that landscape patchiness and discrete population size interacted to generate a strong role for density dependence, a factor unimportant in continuously favorable landscapes. Density dependence was important because it regulated how rapidly populations could produce enough seeds to surpass habitat gaps.  Populations with continuously varying density showed no such effect of density dependence.  Finally, we developed a Markov chain approximation that reasonably predicts the spread velocity for populations with a discrete numbers of individuals moving through landscapes with large gaps.  Our theoretical work suggests that ecologists must consider reproduction at both low and high density when predicting which species will migrate with climate change and which introduced species are most likely to become successful.