Ecosystem functioning is more strongly impaired by reducing dung beetle abundance than by reducing species richness

•Dung beetle species contribute unequally to dung removal.•Complementarity driven by assemblages containing a poorly performing species.•Loss of dung beetle abundance has a larger effect on dung removal than species loss.•Declining insect abundance in agroecosystems can impair ecosystem functioning. Intensive management practices have been widely shown to reduce the diversity and abundance of insects in agricultural landscapes. This loss has attracted considerable public and scientific interest, owing partially to the importance of insects in supporting ecosystem functions. The relative importance of diversity and abundance in underpinning ecosystem functioning, however, has not been widely explored. We examined the relative importance of diversity and abundance in ecosystem functioning using a model system of three widespread species of dung beetle (Coleoptera: Scarabaeoidea). We used a design that manipulated species richness, while also standardizing dung beetle abundance at two levels. We predicted that individual species would contribute unequally to ecosystem functioning, functioning in multi-species assemblages could be reliably predicted from single-species assemblages, and that loss of abundance would more strongly affect functioning than loss of diversity. Comparisons of functioning among three species showed that individual species contributed unequally to dung removal. In most cases multi-species assemblages provided higher levels of dung removal than predicted by single-species assemblages, demonstrating evidence of complementarity. The average effect of species richness loss had no significant effect on dung removal. In contrast a 33% loss of insect abundance corresponded to a 29% reduction in dung removal. Our work provides empirical evidence that loss of insect abundance, a widely occurring response to agricultural intensification, can have stronger consequences for ecosystem functioning than reductions in species richness. Further efforts should confirm whether this relationship is consistent across other ecosystem functions. Should this be observed, ecosystem functioning arguments could be useful in motivating agricultural producers to participate in practices such as agri-environment schemes which have potential to simultaneously conserve the diversity and abundance of insects in agroecosystems.