Land use change affects the abundance, species richness, and the predatory activity of ground-dwelling ants

Background/Question/Methods

The loss of insect diversity within agricultural landscapes is of particular concern since many insect species provide ecosystem services of importance to farming. Ants, for instance, are important generalist predators and therefore can help to suppress pest outbreaks. However, little is known about how changes in the diversity and composition of ant assemblages, following the transformation of natural habitats into crops, may affect the role of ants as predators. Here we provide comparative information about the structure of ant assemblages in adjacent crop (soybean plantations) and non-crop (woodland savannas) habitats in southeastern Brazil. We also estimated rates of ant predation in crop and non-crop habitats and evaluated how rates of ant predation change as function of variations in the density of ant species. This allowed us to test the hypothesis that the predatory effects of ants on prey assemblages are enhanced when multiple ant species are present. Ants foraging above or belowground were collected in six different sites at varying distances from the border between crop and non-crop habitats. Rates of ant predation belowground were estimated using Tenebrio molitor larvae buried inside plastic containers that allowed ant access but prevented the larvae to escape. Aboveground predation rates were estimated using artificial caterpillars made with modeling clay.

Results/Conclusions

Distance from the non-crop habitat did not significantly affect the structure of ant assemblages or the rate of ant predation within crops. Aboveground predation rates did not differ between crop and non-crop habitats despite the fact that aboveground ant species density was much greater in the non-crop habitat. In contrast, rates of ant predation belowground were nearly two times greater in the non-crop habitat even though ant species density belowground did not differ significantly between crop and non-crop habitats. Ant abundance aboveground (all species combined) was only slightly greater in the non-crop habitat, whereas ant abundance belowground was more than two times greater in the non-crop than in the crop habitat. These results suggest that inter-habitat differences in predation rates are mediated mainly by changes in overall ant activity and not by changes in the number of ant species.