what happens when the metapopulation colonization rate decreases but stays larger thenevtinction rate
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1. Multi-population systems
In recent times more than before, human populations expand and require larger areas for agriculture and other land-use goals, often at the expense of natural ecosystems. Many organisms have to face large-scale habitat destruction and fragmentation of larger populations. New approaches have been developed to deal with this problem theoretically and practically. These are based on the idea that empty fragments (patches) are colonized by the organism, and that populations persist for a while but eventually become extinct. Thus, metapopulations persist as long as colonization takes place despite extinctions. The notion of meta-populations (populations of populations) was not the first to address this question: Island Biogeography (MacArthur and Wilson 1967) already dealt with immigration of species into islands, from a mainland.
One of Island Biogeography's important contributions was the idea that each island has an equilibrium number of species on it, which is a function of local extinction rate and of colonization (or immigration) rate: extinction increases as more species colonize the island, while colonization decreases as more species already had colonized. Since immigration also depends on the distance between the mainland (the source of immigrants) and the island (the sink, in a broad sense), nearby coastal islands should have more species of animals and plants than remote oceanic islands. Furthermore, large islands should also have more species than smaller ones.
However, the presence of a large mainland was not realistic enough to deal with sets of more or less isolated populations. In fragmented habitats extinctions take place, but colonization is not automatic since there is no infinite reservoir of immigrants. According to the metapopulation approach, all immigrants have to come from other fragments. In its "classical" form (Levins 1969), a metapopulation consists of isolated patches (islands) within an unsuitable matrix (sea), without a mainland. Thus, all local populations can become extinct, as can the entire metapopulation (i.e., when the longest-lived population becomes extinct).
2. The metapopulation approach
Metapopulations persist due to the balance between extinction and colonization of local populations. In contrast to the IFD, each population in a patch has its own local dynamics, which are not dependent on the state of other patches as in structured populations, nor on immigration from other patches as sink-populations do. In "true" metapopulations, immigration is just sufficient to recolonize unoccupied patches (where there may have been a population before). We can view this as an intermediate case of a continuum of connections between patches (Fig. 1), including 1) continuous populations with approximately instantaneous immigration among its sub-populations, 2) structured populations, with sub-populations, whose dynamics depend on the others in the landscape, as in sink populations and populations behaving more or less according to the Ideal Free Distribution, 3) metapopulations whose populations receive some immigrants, including island - mainland systems, and 4) isolated populations too far apart for effective immigration..
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