II.4 Metapopulations and Spatial Population Processes
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II.4 Metapopulations and Spatial Population Processes Ilkka Hanski OUTLINE 1. Metapopulation patterns and processes 2. Long-term viability of metapopulations 3. Metapopulations in changing environments 4. Evolution in metapopulations 5. Spatial dynamics in nonpatchy environments 6. Metapopulations, spatial population processes, and conservation 7. Conclusion 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. GLOSSARY connectivity. An individual habitat patch in a patch network and a local population in a metapopulation are linked to other local populations, if any exist, via dispersal of individuals. Connectivity measures the expected rate of dispersal to a particular patch or population from the surrounding populations. dispersal. Movement of individuals among local populations in a metapopulation is dispersal. Migration is often used as a synonym of dispersal. extinction–colonization dynamics. Local populations in a metapopulation may go extinct for many reasons, especially when the populations are small. New local populations may become established in currently unoccupied habitat patches. Local extinction and recolonization are called turnover events. extinction debt. If the environment becomes less favorable for the persistence of metapopulations through, e.g., habitat loss and fragmentation, species ’ metapopulations start to decline. For some metapopulations, the new environment may be below the extinction threshold. Extinction debt is defined as the number of species for which the extinction threshold is not met and that are therefore predicted to become extinct but have not yet had time to become extinct. extinction threshold. Classic metapopulation may persist in a habitat patch network in spite of local extinctions if the rate of recolonization is sufficiently high. Long-term persistence is less likely the smaller the habitat patches are (leads to high extinction rate) and the lower their connectivity (leads to low recolonization rate). Below extinction threshold, recolonizations do not occur fast enough to compensate for local extinctions, and the entire metapopulation becomes extinct even if some habitat patches exist in the landscape. local population. This is an assemblage of individuals sharing common environment, competing for the same resources, and reproducing with each other. In a fragmented landscape, a local population typically inhabits a discrete habitat patch. metapopulation. A classic metapopulation is an assemblage of local populations living in a network of habitat patches. More generally, spatially structured populations at landscape scales are often called metapopulations. metapopulation capacity. This is a measure of the size of the habitat patch network that takes into account the total amount of habitat as well as the influence of fragmentation on metapopulation viability . network of habitat patches. In a fragmented landscape, habitat occurs...