Temperature and dung availability drive intraspecific dung beetle body size across grassland grazing regimes

Abstract Animal body size is an important trait with implications for an organism's ecological roles. Larger dung beetles can bury more dung, increasing soil nitrogen retention, reducing greenhouse gas emissions, and reducing livestock parasites. We studied two focal dung beetle species across shortgrass prairie sites to examine the influence of four key drivers (local temperature, plant nutrient availability, presence and density of large mammalian grazers and insecticide use) on intraspecific beetle body size and trap catch. Dung beetles were collected using pitfall traps for one growing season. We collected over 17,000 dung beetles and measured body size parameters on over 4600 individuals of two common species in northeastern Montana, USA: Canthon pilularius , a large native species, and Onthophagus nuchicornis , a smaller non‐native species, across 24 field sites. Bayesian linear models were used to estimate body sizes and trap catches, and to assess responses to grazing regimes and habitat characteristics. The lowest trap catches for both species were in sites treated with insecticides and ungrazed sites. Compared with other treatments, C. pilularius was most numerous in sites grazed by cattle, and O. nuchicornis was most numerous in untreated prairie dog sites. C. pilularius tended to have smaller body sizes in sites with insecticide use and bison and cattle units, but was larger in areas with more browser dung (deer [ Odocoileus spp.] and pronghorn antelope [ Antilocapra americana ]). O. nuchicornis had smaller body sizes in prairie dog sites. Hotter temperatures within the 20 days prior to capture resulted in smaller individuals, especially of the native species C. pilularius, while increased browser dung 20 days prior to capture was associated with larger body sizes in both beetle species. Dung beetle species varied in their responses to large herbivores and dung availability, with browser dung being a potentially overlooked key resource for the most common, native dung roller in our system. Lag effects had measurable impacts on dung beetle body size, suggesting environmental conditions during dung beetle development can affect the ecosystem services dung beetles provide. Warming temperatures due to climate change may reduce body sizes of dung beetles, negatively affecting their ability to provide ecosystem services related to animal waste removal and nutrient cycling.