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Source–sink dynamics is a theoretical model used by ecologists to describe how variation in habitat quality may affect the population growth or decline of organisms. Since quality is likely to vary among patches of habitat, it is important to consider how a low quality patch might affect a population. In this model, organisms occupy two patches of habitat. One patch, the source, is a high quality habitat that on average allows the population to increase. The second patch, the sink, is very low quality habitat that, on its own, would not be able to support a population. However, if the excess of individuals produced in the source frequently moves to the sink, the sink population can persist indefinitely. Organisms are generally assumed to be able to distinguish between high and low quality habitat, and to prefer high quality habitat. However, ecological trap theory describes the reasons why organisms may actually prefer sink patches over source patches. Finally, the source-sink model implies that some habitat patches may be more important to the long-term survival of the population, and considering the presence of source-sink dynamics will help inform conservation decisions. ==Theory development== Although the seeds of a source-sink model had been planted earlier,〔Holt, R. D. 1985. Population-Dynamics in 2-Patch Environments - Some Anomalous Consequences of an Optimal Habitat Distribution. Theoretical Population Biology 28:181-208.〕 Pulliam〔Pulliam, H. R. 1988. Sources, sinks, and population regulation. American Naturalist 132:652-661.〕 is often recognized as the first to present a fully developed source-sink model. He defined source and sink patches in terms of their demographic parameters, or BIDE rates (birth, immigration, death, and emigration rates). In the source patch, birth rates were greater than death rates, causing the population to grow. The excess individuals were expected to leave the patch, so that emigration rates were greater than immigration rates. In other words, sources were a net exporter of individuals. In contrast, in a sink patch, death rates were greater than birth rates, resulting in a population decline toward extinction unless enough individuals emigrated from the source patch. Immigration rates were expected to be greater than emigration rates, so that sinks were a net importer of individuals. As a result, there would be a net flow of individuals from the source to the sink (see Table 1). Pulliam’s work was followed by many others who developed and tested the source-sink model. Watkinson and Sutherland〔Watkinson, A. R., and W. J. Sutherland. 1995. Sources, sinks and pseudo-sinks. Journal of Animal Ecology 64:126-130.〕 presented a phenomenon in which high immigration rates could cause a patch to appear to be a sink by raising the patch’s population above its carrying capacity (the number of individuals it can support). However, in the absence of immigration, the patches are able to support a smaller population. Since true sinks cannot support any population, the authors called these patches “pseudo-sinks.” Definitively distinguishing between true sinks and pseudo-sinks requires cutting off immigration to the patch in question and determining whether the patch is still able to maintain a population. Thomas et al.〔Thomas, C. D., M. C. Singer, and D. A. Boughton. 1996. Catastrophic extinction of population sources in a butterfly metapopulation. American Naturalist 148:957-975.〕 were able to do just that, taking advantage of an unseasonable frost that killed off the host plants for a source population of Edith’s checkerspot butterfly (''Euphydryas editha''). Without the host plants, the supply of immigrants to other nearby patches was cut off. Although these patches had appeared to be sinks, they did not become extinct without the constant supply of immigrants. They were capable of sustaining a smaller population, suggesting that they were in fact pseudo-sinks. Watkinson and Sutherland's〔 caution about identifying pseudo-sinks was followed by Dias,〔Dias, P. C. 1996. Sources and sinks in population biology. Trends in Ecology and Evolution 11:326-330.〕 who argued that differentiating between sources and sinks themselves may be difficult. She asserted that a long-term study of the demographic parameters of the populations in each patch is necessary. Otherwise, temporary variations in those parameters, perhaps due to climate fluctuations or natural disasters, may result in a misclassification of the patches. For example, Johnson〔Johnson, D. M. 2004. Source-sink dynamics in a temporally, heterogeneous environment. Ecology 85:2037-2045.〕 described periodic flooding of a river in Costa Rica which completely inundated patches of the host plant for a rolled-leaf beetle (''Cephaloleia fenestrata''). During the floods, these patches became sinks, but at other times they were no different from other patches. If researchers had not considered what happened during the floods, they would not have understood the full complexity of the system. Dias〔 also argued that an inversion between source and sink habitat is possible so that the sinks may actually become the sources. Because reproduction in source patches is much higher than in sink patches, natural selection is generally expected to favor adaptations to the source habitat. However, if the proportion of source to sink habitat changes so that sink habitat becomes much more available, organisms may begin to adapt to it instead. Once adapted, the sink may become a source habitat. This is believed to have occurred for the blue tit (''Parus caeruleus'') 7500 years ago as forest composition on Corsica changed, but few modern examples are known. Boughton〔Boughton, D. A. 1999. Empirical Evidence for Complex Source-Sink Dynamics with Alternative States in a Butterfly Metapopulation. Ecology 80:2727-2739.〕 described a source—pseudo-sink inversion in butterfly populations of ''E. editha''.〔 Following the frost, the butterflies had difficulty recolonizing the former source patches. Boughton found that the host plants in the former sources senesced much earlier than in the former pseudo-sink patches. As a result, immigrants regularly arrived too late to successfully reproduce. He found that the former pseudo-sinks had become sources, and the former sources had become true sinks. One of the most recent additions to the source-sink literature is by Tittler et al.,〔Tittler, R., L. Fahrig, and M. A. Villard. 2006. Evidence of large-scale source-sink dynamics and long-distance dispersal among wood thrush populations. Ecology 87:3029-3036.〕 who examined wood thrush (''Hylocichla mustelina'') survey data for evidence of source and sink populations on a large scale. The authors reasoned that emigrants from sources would likely be the juveniles produced in one year dispersing to reproduce in sinks in the next year, producing a one-year time lag between population changes in the source and in the sink. Using data from the Breeding Bird Survey, an annual survey of North American birds, they looked for relationships between survey sites showing such a one-year time lag. They found several pairs of sites showing significant relationships 60–80 km apart. Several appeared to be sources to more than one sink, and several sinks appeared to receive individuals from more than one source. In addition, some sites appeared to be a sink to one site and a source to another (see Figure 1). The authors concluded that source-sink dynamics may occur on continental scales. One of the more confusing issues involves identifying sources and sinks in the field.〔Runge, J. P., M. C. Runge and J. D. Nichols. 2006. The role of local populations within a landscape context:defining and classifying sources and sinks. American Naturalist 167:925-938.〕 Runge et al.〔 point out that in general researchers need to estimate per capita reproduction, probability of survival, and probability of emigration to differentiate source and sink habitats. If emigration is ignored, then individuals that emigrate may be treated as mortalities, thus causing sources to be classified as sinks. This issue is important if the source-sink concept is viewed in terms of habitat quality (as it is in Table 1) because classifying high-quality habitat as low-quality may lead to mistakes in ecological management. Runge et al.〔 showed how to integrate the theory of source-sink dynamics with population projection matrices〔Caswell, H. 2001. Matrix population models: Construction, analysis, and interpretation. 2nd edition. Sinauer. Sunderland, Mass., USA.〕 and ecological statistics〔Williams, B. K., J. D. Nichols, and M. J. Conroy. 2001. Analysis and management of animal populations. Academic Press. San Diego, USA.〕 in order to differentiate sources and sinks. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Source–sink dynamics」の詳細全文を読む スポンサード リンク
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