Geographical Patterns in Native and Exotic Weeds of Mexico1

Abstract

We compared distributions of native nonweed, native weed, and exotic weed species based on a weed checklist for Mexico and for each of the 32 federal states. The national checklist contains 2,197 native and 617 exotic weed species. We calculated floristic similarity between states for native and exotic weeds and grouped the state floristic similarities by cluster analysis. The exotic and native weed species show different patterns both for distribution size and for geographic groupings. About 64% of the exotic species occur in eight or less states, whereas 36% of the native weed species occur in this limited range. Only 5.3% of the exotic species but 8.5% of the natives are recorded from 25 or more states. Native nonweed species have even more limited distributions than exotic weeds. The cluster analysis shows that the groupings based on the native weed flora are mostly coherent with general phytogeographical classifications. The grouping of the states for exotic weeds was different, and we propose that it may be related to climatic factors (particularly extreme minimum temperatures) and to management. The data do not support the hypothesis that exotic species have a superior, more general-purpose genotype than members of a well-developed, native weed flora.Additional index words: Invasive plants, weed distribution, weed phytogeography.