II.4 Metapopulations and Spatial Population Processes

Abstract

Most landscapes are complex mosaics of many kinds of habitat. From the viewpoint of a particular species, only some habitat types, often called ‘‘suitable habitat,’’ provide the necessary resources for population growth. The remaining landscape, often called the (landscape) matrix, can only be traversed by dispersing individuals. Often the suitable habitat occurs in discrete patches, an example of which is a woodland in the midst of cultivated fields—for forest species, the woodland is like an island in the sea. The woodland may be occupied by a local population of a forest species, but many such patches are likely to be temporarily unoccupied because the population became extinct in the past and a new one has not yet been established. At the landscape level, woodlands and other comparable habitat patches comprise networks in which local populations living in individual patches are connected to each other by dispersing individuals. A set of local populations inhabiting a patch network is called a metapopulation. In other cases, the habitat does not consist of discrete patches, but even then, habitat quality is likely to vary from one place to another. Habitat heterogeneity tends to be reflected in a more or less fragmented population structure, and such spatially structured populations may be called metapopulations. Metapopulation biology addresses the ecological, genetic, and evolutionary processes that occur in metapopulations. For instance, in a highly fragmented landscape, all local populations may be so small that they all have a high risk of extinction, yet the metapopulation may persist if new local populations are established by dispersing individuals fast enough to compensate for extinctions. Metapopulation structure and the extinction–colonization dynamics may greatly influence the maintenance of genetic diversity and the course of evolutionary changes. Metapopulation processes play a role in the dynamics of most species because most landscapes are spatially more or less heterogeneous, and many comprise networks of discrete habitat patches. Human land use tends to increase fragmentation of natural habitats, and hence, metapopulation processes are particularly consequential in many human-dominated landscapes.