Using ants for rangeland monitoring: Global patterns in the responses of ant communities to grazing

Ants are a prominent invertebrate group used to assess ecological change in response to disturbance. Their application as a bioindicator group has been particularly widespread in Australia, and a recent comprehensive review of their responses to environmental disturbance identified a range of consistent and predictable patterns. Here I conduct a literature review of the responses of ants to grazing globally, and specifically test whether key patterns identified in the review of ant responses to disturbance in Australia apply globally. The patterns tested were (1) soil and vegetation type are primary determinants of ant community composition, and often have a far greater effect on ant community composition than disturbance, (2) disturbance induces species compositional change, but does not necessarily affect overall species richness or abundance, (3) a species' response is not necessarily consistent across habitats because of variation in inherent habitat suitability, and (4) approximately one quarter to one half of species that are common enough for statistical analysis have significant responses to disturbance. All these patterns were found to hold true for grazing studies worldwide. All but three studies sampling multiple soils/vegetation types found the influence of these variables to override grazing effects. Community composition changed consistently, yet the responses of total ant abundance and species richness were highly inconsistent. All studies that analysed species-level data on multiple soils/vegetation types, showed mixed responses to grazing across habitats. On average, 33% of tested species had statistically significant differences across treatments. This is the first such formulation of global patterns for any terrestrial invertebrate group for their use in bioindication, and provides valuable support to the use of ants as indicators of ecological disturbance. The challenge now is to provide a predictive understanding of this context dependency, as well as to improve the precision of the predictive responses. The confirmation of global patterns to grazing presented here represents a first step in developing the valuable contribution that ants can provide to rangeland monitoring systems.