Source–sink dynamics
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 a very low quality habitat that, on its own, would not be able to support a population. However, if the excess of
Theory development
Although the seeds of a source–sink model had been planted earlier,[1] Pulliam[2] 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[3] 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.[4] 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[3] caution about identifying pseudo-sinks was followed by Dias,[5] 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[6] 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 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.,
One of the more confusing issues involves identifying sources and sinks in the field.[9] Runge et al.[9] 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.[9] showed how to integrate the theory of source–sink dynamics with population projection matrices[10] and ecological statistics[11] in order to differentiate sources and sinks.
| Source–sink | Source–pseudosink | Ecological trap | ||
|---|---|---|---|---|
| Source patch (high quality habitat) |
Stable or growing Attractive Net exporter |
Stable or growing Attractive Net exporter |
Stable or growing Avoided (or equal) Net exporter | |
| Sink, pseudo-sink, or trap patch (low quality habitat) |
Declines to extinction Avoided Net importer |
Declines to stable size Either Net importer |
Declines to extinction Attractive (or equal) Net importer | |
| ||||
Modes of dispersal
Why would individuals ever leave high quality source habitat for a low quality sink habitat? This question is central to source–sink theory. Ultimately, it depends on the organisms and the way they move and distribute themselves between habitat patches. For example, plants disperse passively, relying on other agents such as wind or water currents to move seeds to another patch. Passive dispersal can result in source–sink dynamics whenever the seeds land in a patch that cannot support the plant's growth or reproduction. Winds may continually deposit seeds there, maintaining a population even though the plants themselves do not successfully reproduce.[12] Another good example for this case are soil protists. Soil protists also disperse passively, relying mainly on wind to colonize other sites.[13] As a result, source–sink dynamics can arise simply because external agents dispersed protist propagules (e.g., cysts, spores), forcing individuals to grow in a poor habitat.[14]
In contrast, many organisms that disperse actively should have no reason to remain in a sink patch,
An alternative to the ideal free distribution and balanced dispersal models is when fitness can vary among potential breeding sites within habitat patches and individuals must select the best available site. This alternative has been called the "ideal preemptive distribution", because a breeding site can be preempted if it has already been occupied.[17] For example, the dominant, older individuals in a population may occupy all of the best territories in the source so that the next best territory available may be in the sink. As the subordinate, younger individuals age, they may be able to take over territories in the source, but new subordinate juveniles from the source will have to move to the sink. Pulliam[2] argued that such a pattern of dispersal can maintain a large sink population indefinitely. Furthermore, if good breeding sites in the source are rare and poor breeding sites in the sink are common, it is even possible that the majority of the population resides in the sink.
Importance in ecology
The source–sink model of population dynamics has made contributions to many areas in ecology. For example, a species' niche was originally described as the environmental factors required by a species to carry out its life history, and a species was expected to be found only in areas that met these niche requirements.[18] This concept of a niche was later termed the "fundamental niche", and described as all of the places a species could successfully occupy. In contrast, the "realized niche", was described as all of the places a species actually did occupy, and was expected to be less than the extent of the fundamental niche as a result of competition with other species.[19] However, the source–sink model demonstrated that the majority of a population could occupy a sink which, by definition, did not meet the niche requirements of the species,[2] and was therefore outside the fundamental niche (see Figure 2). In this case, the realized niche was actually larger than the fundamental niche, and ideas about how to define a species' niche had to change.
Source–sink dynamics has also been incorporated into studies of
Conservation
Land managers and conservationists have become increasingly interested in preserving and restoring high quality habitat, particularly where rare, threatened, or endangered species are concerned. As a result, it is important to understand how to identify or create high quality habitat, and how populations respond to habitat loss or change. Because a large proportion of a species' population could exist in sink habitat,[26] conservation efforts may misinterpret the species' habitat requirements. Similarly, without considering the presence of a trap, conservationists might mistakenly preserve trap habitat under the assumption that an organism's preferred habitat was also good quality habitat. Simultaneously, source habitat may be ignored or even destroyed if only a small proportion of the population resides there. Degradation or destruction of the source habitat will, in turn, impact the sink or trap populations, potentially over large distances.[8] Finally, efforts to restore degraded habitat may unintentionally create an ecological trap by giving a site the appearance of quality habitat, but which has not yet developed all of the functional elements necessary for an organism's survival and reproduction. For an already threatened species, such mistakes might result in a rapid population decline toward extinction.
In considering where to place
See also
- Conservation biology
- Ecological trap
- Ecology
- Landscape ecology
- List of ecology topics
- Metapopulation
- Perceptual trap
- Population dynamics
- Population ecology
- Population viability analysis
- Refuge (ecology)
References
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- ^ Foissner W (1987). "Soil protozoa: fundamental problems, ecological significance, adaptations in ciliates and testaceans, bioindicators, and guide to the literature". Progress in Protistology. 2: 69–212.
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- ^ a b c Roberts CM. "Sources, sinks, and the design of marine reserve networks". Fisheries. 23: 16–19.
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Further reading
- Battin J (December 2004). "When good animals love bad habitats: ecological traps and the conservation of animal populations". Conservation Biology. 18 (6): 1482–91. S2CID 2383356.
- Delibes M, Gaona P, Ferreras P (September 2001). "Effects of an attractive sink leading into maladaptive habitat selection". The American Naturalist. 158 (3): 277–85. S2CID 1345605.
- Dwernychuk LW, Boag DA (May 1972). "Ducks nesting in association with gulls—an ecological trap?". Canadian Journal of Zoology. 50 (5): 559–63. doi:10.1139/z72-076.
- Misenhelter MD, Rotenberry JT (October 2000). "Choices and consequences of habitat occupancy and nest site selection in Sage Sparrows". Ecology. 81 (10): 2892–901. .
- Purcell KL, Verner J (April 1998). "Density and reproductive success of California Towhees". Conservation Biology. 12 (2): 442–50. S2CID 85652768.
- Schlaepfer MA, Runge MC, Sherman PW (October 2002). "Ecological and evolutionary traps". Trends in Ecology & Evolution. 17 (10): 474–80. .
- Weldon AJ, Haddad NM (June 2005). "The effects of patch shape on Indigo Buntings: evidence for an ecological trap". Ecology. 86 (6): 1422–31. doi:10.1890/04-0913.