Influence of leaf-cutting ants (Atta mexicana) on performance and dispersion patterns of perennial desert shrubs in an inter-tropical region of Central Mexico

The influence of defoliation by leaf-cutting ants (Atta mexicana) on the dispersion patterns of xerophytic shrubs in the semi-arid valley of Zapotitlan, Mexico was analysed. All shrubs and ant nest entrances were mapped in a 65 x 50 m stand, and the vigour of each individual shrub was estimated. A regression analysis between the distance to the nearest nest and the vigour of plants showed a significant relationship for Acacia constricta, Castela tortuosa, Echinopteryx eglandulosa and Mimosa luisana, revealing higher vigour as distance to the nest entrances increases. A point pattern analysis showed a clear clumped pattern for the latter three species, plus Prosopis laevigata; whereas Acacia constricta and Cercidium praecox, as well as the ant nest entrances, did not show any pattern significantly different from chance. A bivariate point pattern analysis did not show the expected repulsion pattern among shrubs and nest entrances. According to an auto-correlation analysis, individuals of Mimosa luisana forming dense clusters away from the ant nests entrances showed higher vigour values than those plants with higher inter-individual distances closer to the nests entrances. Although the unveiling of the ecological implications of ant-shrub and shrub-shrub interactions demands a detailed experimental approach, our results reveal the importance of herbivory in the structuring of natural communities. (C) 2000 Academic Press.