The role of density dependence in populations spreading through patchy landscapes

Background/Question/Methods

Predicting how an invasive species will spread through a new landscape or how a native species will expand its range with climate change requires understanding the speed at which species move across landscapes. Much of our current understanding of spread comes from theoretical models that treat landscapes as uniformly favorable environments. However, real landscapes are mosaics of suitable and unsuitable habitat. Recent models predict that gaps between suitable habitats can fundamentally change how invasions advance. In contrast to continuous landscapes, where rare individuals at the front drive populations forward, spread in patchy landscapes is suggested to be driven by individuals growing at high density. Here we combine empirical work with theoretical models to more closely examine the role and mechanisms by which density dependence drives spread in patchy landscapes.

Results/Conclusions

We use a model system of the annual plant Arabidopsis thaliana (thale cress), to investigate the relationship between plant density and dispersal. We show that the dispersal kernel is highly influenced by plant density, such that dense populations have shorter mean dispersal distances compared to solitary plants. With integro-difference equation models of population spread, we then show how different forms of density dependence, including its influence on the dispersal kernel, affect spread velocity. We found that spread velocity is slowed in a fragmented landscape, and increasingly so by the effect of density on the mean dispersal distance. Our findings have consequences for predicting invasion speed in fragmented landscapes, and importantly do so by bridging the gap between theoretical and empirical approaches.