Studies of ecological communities often make implicit assumptions that the species have nonrandom patterns organized through biotic and abiotic factors. Although such assumptions are generally not tested, the analyses and conclusions derived depend on nonrandom patterns' being present. Several "null" or "neutral" models have been proposed to test for these patterns. We contrast two of the more prevalent models and develop two new models, subsequently evaluating them with five sets of fish community data. Three of the null models provide similar results, from which it is concluded that fish communities from five regions of Ontario are nonrandomly structured. These three models evaluate pairs of species according to departures from null or random co-occurrence expectations, A Monte Carlo model based on the procedure proposed by E. F. Connor and D. Simberloff supports random community organization, but we attribute this discrepancy to the conservative pooling of species-pair information in that model. We recommend a hybrid model combining Monte Carlo and log-linear methods for future studies, although the log-linear model of M. E. Gilpin and J. M. Diamond provides a reasonable approximation. On the basis of species associations derived from the various models, we attribute much of the nonrandom structure to common habitat requirements among co-occurring species. A predominance of positively associated species generally involves pairs of species with similar ecological characteristics. Strong negative associations typically involve predator-prey species. Although competition is often identified as a significant feature in community ecology, we do not believe that competition is a major force structuring these fish communities.
