The effects of experimental drought, grazing, and seasonality on ant and spider diversity in an arid region of South Africa

Abstract

Coupled with irreversible shifts, climate-driven disturbance of ecosystems will probably be the largest impact of climate change on humans. With higher rainfall variability and more intense events, separated by extended dry periods, semi-arid rangelands will be severely altered. Adding to these climatic events, livestock grazing is one of the most common land use practices globally, which will increase as human populations grow. The magnitude of these synergies between drought and grazing could also vary with season. Our understanding of faunal response to the interaction of both drought and grazing is limited. This study investigated the response of ants and spiders, two dominant components of epigeal invertebrate assemblages, to a long-term drought and grazing experiment. The experiment consisted of four 40 x 40 m blocks, each with nine 10 x 10 m plots. Drought was simulated using rainout shelters, whilst grazing was manipulated by excluding livestock from the plots using ~ 1 m high fence around the plots. Ants and spiders were collected seasonally in a blocked and two-way crossed experiment using a total of 96 pitfall traps. Both ant and spider size were assessed using community weighted mean. Grazing treatment had the bigger impact on ants than drought, as they got smaller, functionally less diverse and less active with grazing. The interaction between grazing and drought encouraged the increase in activity of large ants. There was an interesting interaction between grazing and drought, plots with grazing had smaller ants than plots without grazing, but only in plots for which there had been drought. This could be attributed to antlion abundance and how they interact with grazing on the specific plots. Assemblage level analysis confirmed the importance of size in structuring these communities, with larger species of ants associated with open and dry habitats. In contrast, spiders were more responsive to drought than grazing treatments. The spider response to the treatments was in conjunction with seasonality, with spider richness decreasing with cold, dry season (winter) and hot, dry season (spring) respectively. Spiders also became less active during the dry seasons. Interestingly, smaller spider species were more active during cold, dry season, and larger species were more active during the hot, dry season where there is no grazing. At assembly level, grazing and no drought interaction encouraged mobile species that were more of active hunters, as open habitats are ideal hunting ground. Our findings clearly suggest that both long-term drought and grazing have an impact on ants and spiders. How these taxa respond to long-term effects on drought and livestock grazing in semi-arid rangelands in the face of climate change is likely to shed light also on how other invertebrates are likely to be affected, and what it could mean for the ecosystem as whole. Further studying is needed to uncover other underlying changes, how that affects and shapes the semi-arid rangeland ecosystems in the long-term.