Dung beetles don’t play dirty: Bare ground prevent dung beetle attraction to feces

The surface area dedicated to grazing activity decreases worldwide, while the biomass of livestock increases, resulting in a high density of herbivores in enclosed pastures with direct impacts on the biodiversity, and on the physical and chemical properties of the soil. Overstocking pastures lead to a decrease in vegetation height and plant diversity, an increase of soil compaction, and an increase of accidental mortality of soil fauna. Dung beetles provide important ecosystem services to animal production by removing feces. While they rely on herbivores dung, they might be victims of high herbivore density. Thus, we define two contradictory hypotheses: either the resource abundance, or the impact of the herbivores on their environment, will limit the abundance of these insects. In this study, a dung beetle community was assessed within zoo enclosures housing different densities of African herbivores fed a standard diet. We set up 66 non-lethal pitfall traps during spring 2023 and 2024. We characterized the habitat at the enclosure scale (vegetation cover and soil compaction), and in a 10-meter buffer surrounding the traps (vegetation cover and feces abundance). We captured 830 dung beetles, belonging to 12 species. Onthophagus vacca (n= 269) and Onthophagus ruficapillus (n= 223) represented more than half of the abundance. In high herbivore density enclosures the surface area of bare ground increased while the vegetation height decreased. The surface area of bare ground at the trap scale had a significant negative effect on abundance and diversity of dung beetles. Despite being resource dependent, we demonstrated that at high densities of herbivores, the dung beetles were impacted by the compaction of soil caused by both trampling and agricultural machinery.