Seed removal in transformed habitats: Pogonomyrmex barbatus (Hymenoptera: Formicidae) and five cacti species in central Mexico

Abstract

One of the main consequences of human activities in semiarid zones is the transformation of habitats. In this work we studied the effect of this transformation on seed removal of five cacti species by the harvester ant Pogonomyrmex barbatus in the Tehuacan-Cuicatlan valley, a semiarid zone in central Mexico. Seed removal was quantified at three sites which have been under the effect of human activities: an abandoned crop field (CCA), a site with evidence of current human activities (TAH), and a site inside a botanic garden (JB). We hypothesized that sites which have been under intense human activities would have low rates of seed removal because they offer harsh conditions for harvester ants, reducing their foraging activity. Results showed that vegetation and surface soil characteristics of the sites studied are affecting the rates of seed removal of the five cacti species studied. The lowest seed removal rate was found at CCA, the most transformed site. In contrast with our hypothesis the highest seed removal was found at TAH, the site which represents the intermediate transformation condition, because this site still conserves some characteristics which permit intense foraging activity by harvester ants. We also found that the seed removal rate varied among the different cacti species studied. Seed of E. chiotilla had the highest removal rate, whereas 0. decumbens had the lowest. Differences in seed removal rate could be associated with the high heterogeneity found in sites with intermediate levels of transformation. Another factor that must be considered is the external morphology of seeds since smaller seeds presented highest removal rates.