Density Dependence

The growth of a population is dependent upon the average survival, migration, and reproductive rates of

Box 1 Coral bleaching: the edge of resistance

Copyright Peter Harrison

Coral bleaching is a phenomenon affecting coral reefs worldwide whereby symbiotic algae may eject from coral tissue causing loss of pigment, loss of energy balance, and sometimes death. Coral taxa have existed for millennia in stable temperatures, and were apparently well buffered to variations in water temperature. This resistance was partly due to suitable conditions for symbiotic algae (zooxanthellae) that may drive coral metabolism. However, currently many corals exist at the edge of their thermal maximum, usually around 30 °C. The rapidity of increased temperatures over the last decades may prevent possible acclimatization. Sea warming of only a few degrees around coral reefs has led to mass bleaching events since 1998, and the extent of coral mass mortality is expected to increase as sea temperatures rise. Indirect effects include loss of habitat for live reef-dwelling fishes, such as damselfishes and gobies, but the algal overgrowth of dead corals resulting from bleaching may lead to herbivorous species prospering.

its members. These averaged rates, however, are rarely constant in time and in fact are most often dependent upon the density of individuals (number per unit area) currently in the population. Survival rates, for instance, often vary in a density-dependent manner being greater at low than at high densities. This effect is due to either an accelerating functional response by predators or increased competition for resources such as food or shelter which may be in short supply at higher densities. For example, it has been demonstrated for coral reef fish that high densities can cause increased competition for shelter space with the losers experiencing much higher mortality from surrounding predators as they are pushed to the peripheral and much less protected area of the reef. Density dependence in reproductive output is also common where increased densities lead to a lack of resources required to produce offspring (food, nesting sites). Song sparrows (Melospiza melodia) on Mandarte Island, British Colombia, Canada, produce up to four times fewer young per female at high densities than at low densities due to food scarcity. While this is the typical nature of density-dependent reproductive output (increased reproduction at low densities) in some organisms the reduced ability to locate a mate can actually lead to reduced reproduction. This 'Allee effect' as it is known, can lead to an unstoppable decline toward population extinction. Of course many organisms can and do move in response to these negative conditions created by high densities. Juvenile male barnacle geese (Branta leucopsis) increasingly disperse from their natal breeding colonies on islands in the Baltic Sea to other colonies as the natal colony density increases. Density-dependent migration can serve to temper the effects of a disturbance quite quickly by relocating individuals in a manner that leads to maximum possible survival and reproduction. In the case of a dramatically reduced abundance due to disturbance, the immigration of sexually mature individuals in response to the reduced density will lead to a much faster population recovery than the production of new offspring which must first reach sexual maturity before they can contribute.

In combination these factors make the population growth rate dependent upon the density of individuals and thereby give the population an ability to regulate its own numbers. At low densities, growth rates are high, while at high densities, growth rates are low. Generally this type of density-dependent population growth is known as logistic; however, there are a variety of ways of varying levels of complexity with which it can be described. Regardless of how it is described, the relevant factor for understanding the buffering capacity of the population is the strength of these density-dependent relationships. If a population is only mildly density dependent, responses to disturbance will be slow. If the disturbance is not severe, the population may still eventually return to pre-disturbance densities; however, if the disturbance is either lengthy or intense, the population may well go extinct or be reduced to densities at which Allee effects begin to occur. If a population is strongly density dependent, its growth rate response to changes in density due to disturbance will be rapid and its ability to buffer itself from disturbance high.

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