Random Patch Formation and Weak Competition: Coexistence in an Epiphytic Chironomid Community

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Abstract
(1) Simulation models of random colonization and field colonization experiments were employed for the analyses of spatial distribution and coexistence in an epiphytic chironomid community in a small river in eastern England. (2) Amongst 140 pairs of species examined through the year, spatial segregation of eighty pairs was significant on the basis of a model in which individuals were randomly reassigned to habitat units. However, only four of these had observed segregation significantly smaller than would be expected from a model in which `patches' of individuals were randomized. (3) These results suggest that `random patch formation' whereby species form patches of aggregation independently of each other, has in effect contributed to spatial segregation of species in this community. (4) Colonization experiments were performed on four different occasions in a year. No significant differences were observed between species in their capacity to colonize new habitats. Time to reach 95% of the naturally occurring density was mostly in the range of 5.6-13.2 days. (5) Random patch formation as well as the absence of strong interspecific competition are considered to facilitate the coexistence of species in this community. (6) It is necessary to take a balanced view of the importance of stochastic processes as well as biological interactions in the determination of species diversity in ecological communities.

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