Biotic Regulation versus Nutrient Limitation: Additional Considerations No. 2
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Biotic regulation and the concepts
of genetic adaptation and nutrient limitation

Additional considerations No. 2

The general pattern to be explained is as follows. One introduces an additional amount of some resource (e.g. certain food) into the ecosystem, and one observes that the population number of animals depending on this resource increases, which is interpreted as limitation.

The majority of animals keeps a constant average population density on the basis of their own species-specific genetic program of the control of individual territory (home range). However, in predators such a program can be weakened, because the stability of their population density can be ensured by the genetic program of their preys who do control their individual territories and keep their own population density stable. The population density of such predators should be limited by a sufficiently high population density of their preys, at which the predators are physically incapable of eliminating all their natural prey animals.

However, if one artificially feeds such predators introducing additional amounts of food into the ecosystem, the predators may increase their population density up to a level when they completely 'eat up' all their natural preys, who thus become extinct. Obviously, when the artificial feeding ceases, the predators become extinct as well.

A well-known example of such a situation are given by the anthropogenically supported urban populations of freely living (homeless) dogs and cats, who find their food near garbage cans in large cities. In those cities where the authorities do not control the population density of these animals (who may become a source of infectious diseases), the homeless dogs and cats completely destroy the local populations of some of their natural preys, e.g. birds who nest on the ground or low bushes.

There are other examples of such a situation. For example, the fishermen in some regions of the White Sea (Northern Russia) are used to throw away the smallest fishes caught in their nets. Huge amounts of fish become thus available to fogs living in forested areas near the sea. As a result of such practice, the population of fogs has increased dramatically and continues to grow.

According to the majority of current population dynamics models, the population of fogs will stabilise at some population number when, roughly speaking, there will be enough fogs to eat all the fish. This will correspond to a new stable state of the ecological community.

However, the question of environmental stability is completely neglected in such a consideration. An important fact is that in winter the numerous fogs migrate from the overpopulated areas to remote islands and often remain there for the whole summer. On these small islands they destroy nests of all seabirds, which imposes a serious threat to the very existence of their populations in the region.

According to the biotic regulation concept, each species performs some important work on environmental regulation, representing an indispensable part of the regulatory mechanism. Removal of any species from the community is equivalent to a partial degradation of the regulatory mechanism, which may undermine the long-term stability of the ecosystem's existence. Hence, the hypothetical ultimate new state with many fogs and no seabirds (or with many cats and dogs and no birds) is not equivalent to the initial non-perturbed stable state where there are as many fogs and seabirds as needed for the maximum efficient biotic regulation.

Therefore, as well as prolonged maintenance of an elevated concentration of some nutrient may lead to degradation of the ecosystem, the prolonged maintenance of elevated amounts of other resources may cause the same effect. On the other hand, if the fishermen stopped supplying fish to fogs, the latter would soon eat up all the artificially available fish. In the end, both the natural state of the environment (no dead fish on the coast) and the natural population number of fogs would be restored, in accordance with the predictions of the biotic regulation concept.

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Main page in English   6 October 2001.