Ants drive invertebrate community response to cattle grazing

Livestock grazing acts as a disturbance that alters ground-level habitat structure and can shape faunal communities. Invertebrate fauna have complex responses to grazing, driven by grazing-induced changes to vegetation structure, and may also be influenced by changes to predation pressure and prey availability. However, there is a dearth of knowledge about the effects of grazing on invertebrates, and research is rarely conducted across multiple orders, or at multiple trophic levels. We examined the ordinal-level response of a tropical savanna invertebrate assemblage to four replicated grazing treatments in two vegetation types at a long-term grazing trial in northern Australia. We used overall measures of abundance, order richness, and the assemblage composition (using the 17 orders we observed) to compare invertebrate communities among grazing treatments and between vegetation types. Responses of individual orders were modelled in relation to microhabitat variables. At the end of the dry season in October 2014 and 2015, we sampled 25,785 invertebrates from 17 orders. Ants accounted for over 80% of all individuals captured. We found that, overall, invertebrate abundance was higher in the most heavily grazed treatments, driven by the response of ants, but assemblage composition was not significantly different among grazing treatments. Araneae, Orthoptera, Scolopendromorpha and Scorpiones responded to vegetation type and were associated with % cover of Carissa ovata, bare ground, and grass. Ants, while responsive to grazing treatment, was not associated with any of the microhabitat variables, suggesting habitat structure in combination with other factors may be driving this response. Our results confirm the importance of ants as a sensitive biological indicator of disturbance. Considering their role in ecosystem function, invertebrates, and their responses to livestock grazing, need to be considered when making land management decisions on rangelands, and maintaining invertebrate diversity should be a priority for land managers.