Sourcesink metapopulations and the rescue effect

The concept of source-sink metapopulations was put into the literature by Pulliam (1988), and is based on the fact that habitats are not uniform but differ in quality. High-quality patches produce large populations with positive growth rates, and are likely to be a source of emigrants. These high-quality areas, where r > 0, are known as source patches. Other habitat patches are of low quality, have small populations, and consistently have negative growth rates. That is, populations in sink patches have a negative r in the absence of immigration (Hanski 1999). If and when migrants from source populations arrive at sink patches, they become either founders of new populations, or new members of established populations. The rescue effect is based on the idea that emigrants from source areas regularly supplement these small, extinction-prone populations. If the expected size of the small population is increased through this supplementation it becomes less prone to, or is rescued from, extinction (Brown and Kodric-Brown 1977). In a true sink habitat a population would decline to extinction if cut off from its source population. A pseudo-sink population is one in which the population would decline to a lower equilibrium, but not go extinct, if cut off from its source population (Watkinson and Sutherland 1995). In practice it is difficult to distinguish between a true sink and a pseudo-sink population in the field.

The MacArthur and Wilson (1967) island biogeography theory, a mainland-island metapopulation, has much in common with source-sink metapopulations in that mainlands are large compared to islands and, in theory, not prone to extinction, while islands are small and there is always a finite probability of extinction. The mainland is the source, while the islands are, collectively, sink populations. While MacArthur and Wilson (1967) emphasized that extinction is a normal process in a community, they envisioned the mainland as a more or less uniform patch when compared to the island populations, and that the mainland would function as a source indefinitely. Moreover, while extinction was assumed to occur on islands, the island populations were also assumed to have positive growth rates in a normal year (Elmhagen and Angerbjorn 2001).

One of the best illustrations of the source-sink concept comes from the study by Hubbell and Foster (1986a, 1986b) of a 50-hectare plot of tropical moist forest on Barro Colorado Island in Panama. They mapped over 238,000 individual trees and shrubs of 314 species over a 13-year period. They found that at least one-third of the rare species were not self-maintaining populations. They were not reproducing effectively and their presence in the plot was a result of immigration from outside of the 50-hectare plot.

Sink populations can be important to the long-term survival of a source population. Assume that the source population is prone to chaotic behavior, or shows great fluctuations in size because of disease or sensitivity to environmental fluctuations such as drought or fire. In this case the source population itself could be rescued if connected to a sink (Gyllenberg et al. 1993).

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