Effects of land use and field margins on ant community composition in agroecosystems

Background/Question/Methods

The increased cultivation of arable land in North America is dramatically simplifying landscape composition, and agricultural intensification has decreased arthropod biodiversity. Increasing plant diversity within agroecosystems is purported to increase arthropod diversity, but is largely dependent on land use, habitat fragmentation, and agricultural management practices. This research was designed to test the hypothesis that ant communities and their prey items in soybeans shift with changes in land use and field margin composition. Ant (Hymenoptera: Formicidae) communities were used as indicators of ecosystem health and biodiversity since they react quickly to environmental changes, are long-lived within an ecosystem, provide ecosystem services, and are easily captured and identified. Ants were collected at GPS-tagged locations with pitfall traps, hand samples, and sweep nets from 25 soybean fields in Kentucky over three years. Land use, richness, and Simpson’s diversity around fields (5 km) were analyzed in relation to ant abundance, species richness, and Shannon-Weiner diversity using multivariate regression in SAS and geographically-weighted regression in GIS. The direct impacts of field margin composition on ant abundance and movements were analyzed using and Spatial Analysis by Distance IndicEs (SADIE). Indirect effects of margins on ants were measured by SADIE association tests between ant and prey communities.

Results/Conclusions

Increasing land use richness and corn presence had a negative impact on ant species richness but not on abundance or diversity. This could indicate potential negative effects of habitat fragmentation or loss due to agricultural intensification on the ant communities. We also found variable effects of specific land uses on ant abundance, with the presence of walnut groves and water bodies decreasing the abundance of ants while mixed forests and shrubs increased abundance. When margin distance was analyzed, those close to field sites positively influenced ant abundance, but the influence was reversed in habitats further afield. This reversal might be due to ant foraging habits, where field margins 10-20 meters from the crop system were too far distant an environment for ants to utilize. Field margin compositions at both distances impacted the abundance and movements of prey items, indicating indirect effects of field margins on ant movements as ant populations were significantly spatially and temporally associated with their prey. This study shows that land use has a direct effect while margins indirectly alter ant communities through arthropod prey, indicating that altering land use compositions at multiple spatial scales can be used to augment ant species richness.