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.
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.
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).
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.
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).
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 found that the host plants in the former sources senesced much earlier than in the former pseudo-sink patches.
One of the most recent additions to the source–sink literature is by Tittler et al.,[8] who examined wood thrush (Hylocichla mustelina) survey data for evidence of source and sink populations on a large scale.
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.
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.
[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.
[16] When there are patches of varying quality available, the ideal free distribution predicts a pattern of "balanced dispersal".
[15] In this case, individuals should not remain in sink habitat for very long, where the carrying capacity is zero and the probability of leaving is therefore very high.
This alternative has been called the "ideal preemptive distribution", because a breeding site can be preempted if it has already been occupied.
[25] Land managers and conservationists have become increasingly interested in preserving and restoring high quality habitat, particularly where rare, threatened, or endangered species are concerned.
Degradation or destruction of the source habitat will, in turn, impact the sink or trap populations, potentially over large distances.
In considering where to place reserves, protecting source habitat is often assumed to be the goal, although if the cause of a sink is human activity, simply designating an area as a reserve has the potential to convert current sink patches to source patches (e.g. no-take zones).
Finally, areas that are sources or sinks currently may not be in the future as habitats are continually altered by human activity or climate change.