Replacing native mammal assemblages with livestock in African savannahs, impacts dung beetle diversity and reduces body size

Abstract

Replacement of complex assemblages of native mammal fauna with livestock species reduces niche heterogeneity. This negatively affects taxa such as dung beetles, which depend on them and can, in turn, affect ecosystem functioning. We assessed the response of dung beetle diversity, biomass and body condition to cattle-farming pressures in protected areas (PAs) and adjacent farms in the savannah biome of northern Namibia. We hypothesized that rich native mammal assemblages are essential to maintain dung beetle diversity and overall biomass. We further explored whether the low-quality dung resources on farms would lead to a decrease in individual body size for species found on both farms and PAs. Overall dung beetle richness increased with higher mammal richness but not relative abundance, demonstrating that a rich native vertebrate assemblage promotes dung beetle richness. Overall dung beetle biomass was lower on farms, but abundance showed no difference. Large-bodied dung specialist species, which are known to be functionally superior, were absent on farms while smaller generalist species proliferated. Furthermore, a large proportion of individuals of species common to both farms and PAs were physically smaller on farms, indicating reduced body condition. Our results show that a high diversity of native mammals maintains dung beetle diversity and higher biomass on PAs. A decrease in native dung on farms leads to reduced body mass, likely negatively impacting ecological function. These results suggest that the conservation of dung beetles and their functions in the expansive African savannah ecosystems is dependent on a complex mix of native mammal species.