Lasting decrease in functionality and richness: Effects of ivermectin use on dung beetle communities

Abstract

Use of ivermectin in agro-ecosystems negatively affects non-target dung beetles, which provide keystone ecosystem services for pasture functioning. While the direct effects of ivermectin on dung beetles are at the center of scientific attention, its long-lasting effects on routinely treated pastures have not yet been studied on real communities and under field conditions. We focused on basic characteristics (species richness, abundance, biomass) and functionality (dung removal ability represented by guild composition) of dung beetle communities in response to ivermectin treatment. We selected 15 sites with routine ivermectin treatment and 11 without any treatment, both for at least five consecutive years prior to our sampling. We covered wide range of climatic conditions in the Czech Republic, seasonal variability, and sampled the dung of four main herbivore dung types. Additionally, we separated the samples from ivermectin treated sites into two categories, i) recently treated representing immediate effects (treated <8 weeks prior to sampling, expected lethal or sublethal levels of residues), and ii) long-ago treated representing lasting effects (treated >8 weeks, expected sublethal or inconsequential levels of residues). Ivermectin treated sites had ca. 35% lower species richness and 44% lower abundance per pat. Per pat biomass did not change. However, from a functional perspective, ivermectin use significantly decreased the biomass of beetles with high contribution to dung removal (relocating and dwelling beetles), while the biomass of beetles with low contribution (dung visiting adults with saprophagous larvae) was not affected. This was accomplished with decrease in per pat functionality at treated sites, i.e., the dung removal ability of the community. Our results highlight that the effects of ivermectin use can last far beyond its physical presence in the grazing system. Via its effects on dung beetles, routine antiparasitic treatment by ivermectin has negative consequences for both pasture biodiversity and ecosystem functioning. Therefore, all alternatives should be carefully considered to ensure that the positives of any antiparasitic treatment outweigh the damages. In areas where biodiversity is of concern, as well as in organic farming systems, all precautions should be taken to avoid excretion of toxic residues on-site.