Seed removal in transformed habitats: Pogonomyrmex barbatus and five cactus species in a semiarid zone of central Mexico

Background/Question/Methods

Seed removal by ants have been broadly studied in deserts of North and South America, however very few is known for inter-tropical deserts such as the Tehuacán-Cuicatlán valley, a semiarid zone in central Mexico. At this place local people carried out a high diversity of activities such as agriculture, grazing, firewood and fruit collection, having as consequence the transformation of habitats. In this study the main question was: How transformation of habitat is affecting seed removal by ants? We studied the effect of such transformation on the seed removal of five cactus species by the harvester ant Pogonomyrmex barbatus. Seed removal was quantified by offering dry seeds of each species outside the entrance of five nests of P. barbatus at three habitats which have been under the effect of human activities: An abandoned field crop (FC), an open shrubland with evidence of human activities (OS), and a closed shrubland inside a botanical garden, without human activities (CS). We hypothesized that habitats under intense human activities would have low rates of seed removal because they offer harsh conditions for harvester ants reducing their foraging activity.

Results/Conclusions

Results suggest that human activities modify vegetation and surface soil characteristics of habitats affecting the removal rates of cactus seeds. The lowest seed removal rate was found at FC which is the most transformed habitat (17 seeds / hour). In contrast to our hypothesis the highest seed removal rate was found at OS (798 seeds / hour), the habitat with an intermediate level of transformation. We suggest this habitat still conserve some characteristics such as high vegetation cover, which allow intense foraging activity by P. barbatus. We also found that seed removal rate varied among different cactus species. Seeds of E. chiotilla had the highest removal rate (400 seed / hour), whereas O. decumbens had the lowest (0.3 seed / hour). These differences could be related to seed size, since there was a significant negative correlation between seed size and removal rate (R² = 0.75, F = 9.23, P = 0.05). These results suggest that habitat transformation could be affecting seed removal by harvester ants. Seed removal however could also be affected by the external morphology of seeds, particularly seed size.