Understanding invasions in patchy habitats through metapopulation theory

Abstract

Many invasive species experience a world that is heterogeneous and patchy, and experience it at multiple scales. In this chapter we will show how metapopulation theory can be used to explore general mechanisms which determine the initial spread and long term establishment of invasive species in novel environments. Metapopulation ecology is a branch of ecology that is focused on how species persist in fragmented or 'subdivided' habitats (Hanski 1999, Hanski and Gaggiotti 2004). Although real landscapes are often highly complex, with admixtures of smooth gradients, fractal edges, and juxtaposed habitats varying in quality, for many species it is natural to construe their world as consisting of sets of suitable habitat patches, arrayed within a largely inhospitable matrix separating those patches. Some such species inhabit naturally patchy habitats, such as islands or ponds (Fig. 1). Some metapopulations appear in artificially constructed habitat networks, such as the remaining fragments of a rain forest surrounded by cattle pastures, or concrete reefs in marine systems. Patch networks can also be formed in more subtle ways, but still make functional metapopulations. The patchy distribution of a host species for a specialist herbivore, parasite, parasitoid, or commensal (or indeed any vital and spatially delimited resource) can lead to metapopulation dynamics for the specialist. Biological nonlinearities can transform smooth environmental gradients into patchy distributions. For instance, strong Allee effects (positive density dependence at low numbers) can amplify patchiness in the environment. When dispersal is limited, permanent range limits can arise in patchy environments even without overall environmental gradients in the landscape (Keitt et al, 2001). Even without Allee effects, if the density of suitable patches declines along an environmental gradient, a species range limit may emerge, when there is no decline in the quality of suitable habitat patches (Holt and Keitt 2000). © 2006 Springer. All Rights Reserved.