(1) A simulation model is presented of competition on a divided and ephemeral resource. (2) Coexistence between two species can be extended by dividing the resource into more and smaller breeding sites. (3) Aggregation of the superior competitor also promotes coexistence, and can lead to an equilibrium between the two species if contagion is strong enough. (4) If the degree of aggregation is allowed to vary with density in a realistic way equilibrium is nearly always obtained. (5) These results may explain the high species diversity commonly observed on divided resources.

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The emergences of domestic Drosophila species from fruits and vegetables in a market are analysed by partitioning the species diversity. Confidence limits are determined for the components of diversity.Despite some coexistence of different species within the same breeding site, ecological differences between them are revealed. Partitioning of the breeding sites is more important than partitioning of the season. D. melanogaster, D. simulans, and D. subobscura are specialised on fruits and D. busckii on vegetables. D. immigrans and D. hydei are less specialised. The coexistence is explained by reference to ecological theory.

(1) Models of competition in divided or heterogeneous environments are reviewed. (2) A model of competition in a divided environment is presented. The competition coefficients, a, are reduced by a term, 0, which measures the amount of overlap between species. (3) Actual values of 0 are presented for two Drosophila communities. (4) 0 depends on the degree of aggregation of the competing species and on their densities. The outcome of competition in a divided environment must, then, be density dependent. (5) Low values of 0 improve the stability of a multi-species community and allow more species to coexist.

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