Community structure, abundance, and morphology

The role of interspecific competition in structuring communities has been a highly debated issue for the last two decades. The nonrandom nature of mor phological patterns within communities has been at the center of this contr oversy. Null models addressing community-wide dispersions in morphology hav e produced equivocal results and may be based on assumptions that are too r estrictive (e.g., competitive exclusion or displacement). If morphological distinctiveness allows species to escape competitive pressures and exhibit higher densities, then a positive relationship should exist between morphol ogical dissimilarity and abundance. We develop a suite of models that evalu ates patterns in abundance that are associated with the morphological proxi mity of a species to other competitors. We evaluated the relationship betwe en morphological distance and abundance from a variety of morphological per spectives, from those representing strictly diffuse interactions to those r epresenting only interactions between a species and its nearest neighbor in morphological space. These models were sufficiently powerful to detect pos itive associations between abundance and morphological differences in a noc turnal desert rodent guild for which the effects of competition on structur e are well established. Models such as these may be more useful than tradit ional models evaluating morphological dispersions for many reasons. They do not require that communities reach equilibrium before competitive interact ions give rise to deterministic structure. They do not suffer from limitati ons of potentially inaccurate faunal pools or from phylogenetic constraints . Lastly, they may be used as a diagnostic tool in comparative studies to d etermine the degree to which competitive interactions structure communities .