A Single Mechanism to Account for Regular, Random and Aggregated Populations

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Abstract
It is possible for a single mechanism of population growth to account for regular, random and aggregated populations. An aggregated population may result even when germinating seeds are distributed at random in a homogeneous habitat, provided that the seeds arrive sequentially in time and that each plant''s growth is limited by the space left available to it by earlier colonizers. Thus intraspecific competition results in the occurrence of small individuals in the gaps among the larger ones, and hence in aggregation of the whole population. The construction and testing of synthetic models of such populations are described. When intraspecific competition causes clumping in this way, the distance (center to center) between each individual and its nearest neighbor must be positively correlated with the sum of some measure of the sizes (e.g., trunk circumferences in the case of trees) of the two plants between which distance was measured. Such correlation was found to occur in a natural, aggregated population of Pinus ponderosa. It therefore follows that the individual trees were competing with one another for space, and that this competition was responsible, at least in part, for the aggregation of the population.