Scramble and Contest Competition

The terms 'scramble' and 'contest' competition were introduced by A. J. Nicholson in 1954, but they became more precisely defined and explained later. Scramble is a sort of competition in which resources are more equally allocated among individuals, so that with increasing population size individual resource shares are not sufficient for many individuals to survive. Contest competition is based on monopolization of resources by some individuals, so that they survive, irrespective of the population size before competition. Equal resource partitioning and resource monopolization result in ideal scramble and ideal contest competition, respectively (Figure 1). The ideal scramble and ideal contest help to understand the mechanism of these two kinds of competition, but in the real world, most populations lie between these two ideal states (Figure 2). Many authors relate scramble and contest competitions to exploitation and interference, respectively; however, such a simple relationship does not seem justified. One can imagine interference among individuals with equal competitive

Ideal scramble

Ideal contest

Figure 1 Population size N2 after the competition as a function of its size N1 before competition. At low population size, in relation to available resources, neither scramble nor contest competition takes place and the density N2 is linearly related to N1. In pure scramble competition all individuals take the same resource share, and if this share is insufficient to support an individual, all of them die, and the population size after the competition falls to zero. In pure contest competition, some individuals are able to monopolize resources for themselves in such a way that the encroachment of others on these resources is impossible. It implies that population size after competition does not depend on this size before competition and it is determined by the amount of the available resources.

Scramble

Contest

Figure 2 Population size N2 after the competition as a function of its size N1 before competition. At low-population size, in relation to available resources, neither scramble nor contest competition takes place and the density N2 is linearly related to N1. Assuming unequal resource partitioning but without any monopolization, with increasing population size, the average individual share of resources decreases, so that only some individuals survive. If the largest individual resource share is smaller than required for the survival, all individuals die and population size falls to zero. This is a more realistic description of scramble competition that does not require identical resource share of each individual. Assuming unequal resource partitioning and ability to monopolize resources by some individuals, an increase of population size before the competition may diminish individual resource share and individual survival but only to a certain point. Some individuals are able to monopolize the resources or to find themselves in such places in which the encroachment of others on these resources is impossible. This is a more realistic description of contest competition that allows the reduction of population size due to the competition but at the same time prevents population extinction due to resource depletion.

abilities and similar resource shares that leads to scramble competition or a population with a very variable consumption rate, but with no monopolization by the interference but by their faster exploitation, leading to contest competition. Scramble and contest competition can be considered either within the lifetime of a cohort or between two generations. In the latter case, it includes the effect of competition not only on survival but also on reproduction. For generations in discrete time units, the type of competition may determine the shape of the function that relates population size to this size one generation earlier. This functional relation determines population stability and persistence. The ideal scramble competition with realistic set ofparameters leads to population instability and frequent population extinctions. On the other hand, the ideal contest competition assures stability and persistence, as long as environmental conditions are favorable enough. Such a phenomenon is clearly seen in natural populations of terrestrial plants competing for light.

V. C. Wynne-Edwards, who in 1962 postulated the self-regulation of population size brought about by group selection, justified this approach by a possibility of population extinctions due to resource depletion, resulting from high population density. Such extinction due to high population densities is known in laboratory populations only. Natural populations may become extinct for many reasons: due to deterioration of their habitats, action of predators, or interspecific competitors. However, there are no reliable empirical data on population extinction in the field, due to its high density followed by total resource depletion. Natural environments are usually heterogeneous in space, so that at the time of high population density and resource shortage some individuals are able to survive. It makes competition contest, but rarely ideal contest, and prevents total population extinction due to resource depletion.

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