The presence of an individual animal in a place does not imply that it is its habitat, in which it can survive and reproduce, allowing for the positive rate of population increase. Otherwise, animals may occur both in part of the area where their rate of increase r > 0, as well as in the parts where r < 0. Since populations increase in the former parts of the area and they decrease in the latter ones, they are called sources and sinks, respectively. Note that r denotes intrinsic rate of increase, not the actual rate that can be altered by competition. If the actual rate of increase is negative due to high density it is called a pseudosink. The idea of source and sink can be also extended to include plants that disperse their seeds to the hostile habitats with no chance to reproduce.
A system of sources and sinks allows the regulation of population densities by an increase in sources, dispersal, and a decrease in sinks. Dispersal is known in almost all groups of organisms and an important question is what makes an individual move outside the favorable habitat, usually the one in which it was born, into another unknown one. The most important factor seems to be high density in the favorable habitats as well as temporary and spatial heterogeneity of natural environments. Other ultimate reasons for dispersal are the avoidance of inbreeding and of the competition with closely related individuals. Empirical evidence that the system of sources and sinks may regulate population densities comes from experimental enclosures in which densities are much higher than outside the enclosures. This phenomenon was shown both in the field and in the laboratories. Identification of the sources and sinks is of importance for nature conservation, since natural reserves should be established in sources not in sinks.
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