Scramble in behaviour and ecology

Nicholson's distinction between 'scramble' and 'contest' modes of competiti on has received widespread attention in ecology and in behaviour, though th e emphasis has been different between the two disciplines. In ecology the f ocus has been on the effects on population; in behavioural ecology the focu s has been on the consequences at the individual level. This paper reviews and develops a theory of scramble competition at the individual level, deri ving a general evolutionarily stable strategy (ESS) for individual scramble expenditure in a patchy habitat in which individuals compete in local grou ps for available resources, and examines two population consequences. The c ritical parameter determining the relationship between individual scramble expenditure and the number of competitors in a patch is the expected resour ce per capita. If resource input, R, to a patch is constant and independent of the number of competitors, n, then as the number of competitors increas es, the per-capita resources declines as R/n, and the ESS scramble level de clines (in proportion to (n-1)/n(2)). However, if the resource input to a p atch is positively related to the number of competitors in the patch, scram ble expenditure may increase with the number of competitors. In the case wh ere the per-capita resource input stays constant (i.e. R(n) = Rn), the scra mble level increases with competitor number tin proportion to (n-1)/n). The re are plausible ecological reasons why either of these extreme limits may be approached in nature, making it important to ascertain the relationship between R and n before predicting individual scramble expenditure. For exam ple, resource input may be constant when groups of competitors are constrai ned to remain together in given patches, and constant per-capita resources may be approached when ideal-free foraging rules apply. However, in the lat ter case, scramble expenditure must be accounted for in determining the ide al-free distribution. An analysis shows that this leads to 'undermatching', i.e. the ratio of numbers of competitors for good/bad patches becomes prog ressively less than the ratio of input rates for good/bad patches as the di fference between the good and bad patches increases. A second population co nsequence of the scramble ESS relates to the fact that scrambles may dramat ically affect fitness. The per-capita gain in energy can be reduced by a fa ctor of up to 1/n as a result of scramble expenditure, potentially reducing realized population size to as little as the square root of the maximum po tential carrying capacity, though reasons are given why such large reductio ns are unlikely.