Populations of many species like the Mountain Sheep (see figure) occupy patches of high quality habitat and use the intervening habitat only for movement from one patch to another. These species exist in a number of populations that are either isolated from one another or have limited exchange of individuals. Such a collection of interacting populations of the same species is called a metapopulation. Each distinct population in a metapopulation may be referred to as a subpopulation, a local population, or simply as a population.
Mountain Sheep (Ovis canadensis) Metapopulation in Southern California. Shaded areas indicate mountain ranges with resident populations, arrows indicate documented intermountain movements, the dotted lines show fenced highways (after Bleich et al. 1990).
Metapopulations both occur naturally and are created as a result of human actions. Many species naturally exist as metapopulations because the environmental factors necessary for their survival occur in patches. There are many examples of patchy distribution of habitats; you may think of ponds in a forest, islands in an archipelago, woods in an agricultural landscape, or mountaintops in a desert.
Habitat loss and fragmentation
Loss of habitat is probably the most important cause of species extinction in recent times. Habitat loss often results not only in an overall decrease in the amount of habitat, but also in discontinuities in the distribution of the remaining habitat. Discontinuities can be created by opening land to agriculture, construction of buildings, dams, roads, power lines and utility corridors. The result is the fragmentation of the original habitat which now exists in disjunct patches. Any population that inhabited the original habitat will now be reduced to a smaller total size that is divided into multiple populations. Further fragmentation results in a decrease in the average size of habitat patches, and makes them isolated.
Other effects of fragmentation are manifested through increased edge effects. When habitat patches decrease in size through fragmentation, the populations inhabiting them become more vulnerable to adverse environmental conditions that are prevalent at the edges of the habitat patch, but not in its interior. For a forest patch embedded in an agricultural, or a disturbed landscape, these environmental changes might include increased light and temperature or decreased humidity.
Spatial Structure and Geography
When a species lives in several patches, much depends on exactly where those patches are, i.e., on their spatial arrangement. This determines the distances between the patches, which is important for dispersal rates. It also determines how similar (or, correlated) the environmental conditions in the neighboring patches are. Both of these spatial factors (dispersal and correlation) are very important in determining the risk of extinction or decline of a species. Thus, the extinction risk of a species living in a metapopulation cannot be estimated from a single-population model, or from a collection of such models. To correctly simulate the dynamics of a metapopulation, all populations in the metapopulation must be modeled together, and their geography (or, locations) must also be incorporated. For more information on parameters for a metapopulation model, see description of RAMAS Metapop
Metapopulation models assume that some parts of the landscape are habitat patches (that are or at least can potentially be occupied by populations), and the remainder is unsuitable habitat. In some cases, the species in question has a specific habitat requirement which has sharp boundaries, making patch identification quite straightforward. Most examples of patchy habitats we discussed above (ponds in a forest, islands in an archipelago, woods in an agricultural landscape, or mountaintops in a desert) fit this category.
In other cases, habitat quality varies on a continuous scale and designation of areas as habitat and non-habitat may be somewhat arbitrary. Or, the boundaries may not be clearcut for human observers: what seems to us as a homogeneous landscape may be perceived as a patchy and fragmented habitat by the species living there. If the suitability of habitat for a species depends on more than one factor, and some of these factors are not easily observable, the habitat patchiness we observe may differ from the patchiness from a species' point of view.
The Role of Geographic Information Systems (GIS)
When the habitat requirements of a species includes several factors, the information about habitat requirements may be combined by computer maps of each required habitat characteristic, using geographic information systems. This allows us to see the habitat patches as perceived by the species. In various case studies, we used this approach to model metapopulations of threatened species. For more information, see Using GIS in Conservation of Threatened and Endangered Species.
Part of the above introduction to metapopulation dynamics is extracted from the textbook Applied Population Ecology.Bird Modeling Studies at Applied Biomathematics.
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