GEOGRAPHICAL AND SEASONAL-VARIATION IN THE RICHNESS OF ANT-PLANT INTERACTIONS IN MEXICO

The richness and seasonal variation of ant-plant interactions were com pared in four habitats in Mexico: lowland tropical dry forest (La Manc ha, Veracruz), coastal sand dune matorra (San Benito, Yucatan), semiar id highland vegetation (Zapotitlan, Puebla), and lower montane humid f orest (Xalapa, Veracruz). The effects of temperature and precipitation on the seasonal distribution of the number of ant-plant interactions differed among habitats. The general linear models fitted to the ant-p lant interaction curves explained 78.8 percent of the variation for La Mancha, 80.1 percent for Zapotitlan, 18 percent for San Benito, and 2 9.5 percent for Xalapa. Even though rainfall is low in Zapotitlan and San Benito, minimum temperature was the most important factor accounti ng for the seasonal distribution and low number of interactions. At La Mancha, with milder minimum temperatures and higher water availabilit y, temperature alone did nor account for the seasonal distribution and number of interactions, whereas the effect of the precipitation X tem perature interaction was highly significant. Xalapa exhibits the lowes t temperatures and the highest precipitation, but the role of these fa ctors was only marginal. We suggest that the vegetation at Xalapa, a m ixture of tropical and temperate floristic elements, constrains ant-pl ant interactions due to a limited presence of nectaries. Also, ants ar e less abundant in cool and relatively aseasonal habitats. The other h abitats have tropical floristic elements that are abundant and frequen tly have nectar-producing structures. We report considerable variation among habitats in the number and seasonal distribution of ant-plane i nteractions, and suggest that it is due to the effect of variation in environmental parameters, the richness of plants with nectaries in the vegetation, and habitat heterogeneity.