I examined patterns of distribution and diversity of species and form in columnar cacti across Mexico. In particular, I wanted to find out what is the relative importance of phylogeny vs. environment in structuring plant associations and determining taxon distributions. As a starting point, phylogenetic and spatial relationships were plotted with values for morphological characters and environmental parameters on a topographic base map. In the case of columnar cacti it is important to distinguish and characterize geographic areas where this life-form can be considered dominant from those areas where its distribution is patchy--where populations of relatively small geographic extent are restricted to “benign islands” surrounded by “hostile” environments. I have defined four biogeographic regions for columnar cacti north of the Isthmus of Tehuantepec, Mexico, each with regional levels of endemism ranging from 50 to 90%. The percentage of shared taxa between biogeographic regions is related to environmental similarity, rather than geographic proximity. In general, biogeographic regions at lower latitudes have greater species diversity, although the number of co-occurring taxa within any particular habitat appears to be fairly constant. Moreover, taxa involved in one association are often close relatives of taxa in another association in a different biogeographic region. Thus, across Mexico, species diversity of columnar cacti is, in part, the sum of repeated patterns of associated taxa. The radiation of these associations is hypothesized to be a result of speciation due to isolation with divergence driven by morphological and architectural adaptations to local environments. Hence, the similarity of patterns across biogeographic regions can be attributed to phylogenetic propinquity. However, relationships between taxa within associations are influenced and perhaps maintained by various developmental restrictions place on morphological and architectural variation, a process visualized here through mapping.

Key words: biogeography, climate, columnar cacti, Mexico, phylogenetics