The effect of urbanization and agriculture on predacious arthropod diversity in the Highveld grasslands

Abstract

Biodiversity provides vital ecosystem services and more diverse ecosystems are known to be more stable and resilient in the face of disturbance. Predacious arthropods provide a valuable ecosystem service to control pest numbers. We hypothesised that anthropological activities, which result in land-use change and habitat fragmentation, deplete this species pool. However, current knowledge is lacking regarding the diversity and abundance of arthropod predators in crop fields within the agricultural and urban environments of South Africa. We studied the diversity, abundance and species assemblages of predaceous arthropods of the Chrysopidae, Mantodea, Araneae and Coccinellidiae at different intensity levels of disturbances within urban and agricultural sites in the grasslands of South Africa. Study sites included agricultural (maize field-field margin-untransformed grassland) and urban (ruderal-fragmented-untransformed grassland) gradients. The optimal sweep net sampling time was also determined within these agricultural ecosystems. Sweep nets were used to sample fields and field margins during the following daily times (07:00, 12:00, 15:00 and 17:00). Sampling was replicated four times. Shannon diversity index values did not differ between the different sweep net sampling times. Predacious arthropods were however more abundant within maize fields in the mornings (07:00) and adjacent fields around midday (12:00). The results indicated there were no statistical differences despite the higher abundance of predators at the previously mentioned times. The study done on the agro-ecosystem zones did however indicate that the field margins were the most diverse and abundant in terms of predacious arthropod diversity along a maize field-field margin-untransformed grassland gradient. Despite this our results indicated that urban areas had a similar species richness of arthropods as agro-ecosystems, but predator species were more abundant in urban areas. With an increase in distance away from disturbance, in both the agricultural and urban environments, an increase was noticed in diversity and abundance. The maize fields were the least diverse and abundant for these predacious arthropods. Surrounding agricultural landscapes could therefore play an important role in the maintenance of predacious arthropod diversity as well as future integrated pest management strategies. This study generated baseline data to monitor the effects of anthropological activities on predator diversity and abundance in urban- and agro-ecosystems.