The Spatial Structure of Populations

Environmental heterogeneity and the patchy distribution of organisms in space can also be considered in evaluating the risks to populations. 

Five general categories of spatial structure can be identified for the purposes of investigating the effects of toxicants upon populations (McLaughlin and Landis 2000). 

Isolated populations are a collection of habitats without migration or dispersal between them. These isolated populations act as if they are self-contained. Contaminants in one isolated patch do not affect dynamics in other patches. Conversely, once extinction has occurred in a patch, recolonization does not occur. 

Classic metapopulations result from low to intermediate migration between habitat patches. Not all potential habitats necessarily contain populations. Migration between patches affects the dynamics of local populations, even including recolonization following extinction. If sufficient dispersal between patches exists, then a rescue effect can prevent local extinctions. Persistence of a metapopulation requires migration rates between patches, which are sufficient enough to offset local extinction rates. 

Source-sink and mainland-island metapopulations result when one or more of the local populations differ in the probability of local extinction. In a source-sink structure, the source has excess organisms that migrate to other habitat patches. The other habitat patches, sinks, do not contain the resources to maintain a growing population. In contrast to a classical metapopulation, dispersal is not equal between patches but is from the source to the sinks. In a mainland-island metapopulation, the difference is principally size, and all patches can support viable populations. Since smaller populations run a greater risk of extinction, the mainland can often provide a source for recolonization and the establishment of a new population on that patch. Conversely, islands can also act as refugia in cases where the mainland population becomes extinct. 

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Thomas, C.D., Kunin, W.E. (1999). The spatial structure of populations. Journal of Animal Ecology 68,... 

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