An organism's adult body size impacts their ecological function and the ecosystem services they can provide. In dung beetles, the amount of dung buried increases with body size, benefitting soil nitrogen retention, greenhouse gas reduction, and parasite depletion. We examined drivers of body size in two species of dung beetle. We collected 18,068 specimens and measured 4,646 individuals of two common species of dung beetles found in northeastern Montana, USA: Canthon pilularius, a large native species, and Onthophagus nuchicornis, a smaller non-native species. Both species were the least common in ungrazed and insecticide treated areas. O. nuchicornis was more abundant on prairie dog towns. C. pilularius tended to have smaller body sizes with insecticide use, but non-significantly. Hotter temperatures prior to capture resulted in smaller individuals, especially of the native species, C. pilularius. Surprisingly, C. pilularius was smaller in bison and cattle units and in areas with more dung patties. Similarly, O. nuchicornis was smaller on prairie dog towns and in areas with more prairie dog dung. In contrast, increases in browser dung (from deer and antelopes) paralleled increases in the body size of C. pilularius. Decreases in dung beetle body sizes with rising temperatures could reduce the ecosystem services beetles provide. Dung beetle species varied in their responses to large herbivores with browser dung being a potentially overlooked resource. Understanding drivers of dung beetle populations and traits in rangelands informs conservation actions for these keystone decomposers.
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