Understanding what bioindicators are actually indicating: Linking disturbance responses to ecological traits of dung beetles and ants

Biological responses to a disturbance can vary among taxa, which challenges the use of bioindicators for representing biodiversity responses more broadly. Linking ecological traits to disturbance response helps clarify what different bioindicator groups are actually indicating, providing a mechanistic basis for predicting the responses of other taxa. Dung beetles and ants are ecologically contrasting but widely used bioindicator taxa. Here we link variation in dung beetle assemblages to body size and dung-feeding behavior, and variation in ant communities to habitat preferences, foraging behaviour and competitive dynamics, to clarify what dung-beetles and ants are indicating in terms of biodiversity responses to pastoral land management in a Brazilian savanna landscape. Dung beetles and ants were sampled at 24 pastoral sites that varied in grass cover, tree density, pasture age, cattle density, and frequency of parasiticide use. Community metrics (abundance, richness, composition, functional group composition) of both dung beetles and ants varied markedly with pastoral land management, but this variation was not correlated, or only weakly, between taxa. For dung beetles, abundance, biomass and richness were all best predicted by frequency of parasiticide use, whereas species composition was best predicted by cattle density. Tree density was the most important variable for explaining ant abundance (both total and of several functional groups) and was also significantly related to ant species composition. Dung beetles and ants provided different indications of the impacts of pastoral management on biodiversity. However, by linking responses to ecological traits we were able to clarify what they were actually indicating. Leaf-cutter ants showed a similar response to dung beetles; collectively they are indicating an impact of parasiticide use on dung-feeders. The abundance of small-sized roller beetles was negatively related to grass cover; along with ants they are indicating the importance of vegetation structure. Such a linking of disturbance responses to ecological traits allows for more-robust predictions of likely responses of other faunal groups. We believe that such an approach provides a substantially improved foundation for the use of bioindicators in land management.