A battery of null models is used to test for the presence of six aspects of community structure in the pattern of plant species distributed in patches around focal or ''nurse'' plants at a site in the Bolson de Mapimi, Chihuahua Desert, Mexico. Significant community structure was found in five of the six types of test. There were significantly more positive associations between species than expected for a random assemblage. Examined pairwise, negative associations were almost all with one of the two main focal plants: Larrea tridentata or Opuntia rastrera. The other species were generally positively associated with each other, because of their rarity in small patches. Species-poor patches contained nonrandom subsets of the flora of species-richer patches (i.e., the species/patch matrix was significantly nested). This is interpreted as indicating the existence of species whose establishment is dependent on conditions that can be provided by other species. There was also checkerboarding in the species/patch matrix, indicating the possibility of some competitive exclusion. All species tended to occur more often on species-rich patches, but relative to other species Larrea occurred significantly more often on small patches. Therefore we suggest that the presence of this shrub may be required before other species, such as Opuntia rastrera, Jatropha dioica, and Opuntia schottii/grahamii, which tended to occur on two-species patches, may colonize a patch. Some species, most of them cacti, showed a significant tendency to occur in the largest patches, suggesting that they were able to invade a patch only after preconditions for dispersal or establishment had been satisfied by habitat changes induced by other species. The two main foci, Larrea and O. rastrera, differed in the overall species composition of their associated flora. Many species showed a significant tendency to occur in adjacent patches, indicating the importance of dispersal, probably by rodents, in determining distribution. We conjecture that these patterns result from colonization processes in which facilitation, inhibition, and tolerance all play a role.
