Diversity Patterns of Dung Beetles along a Mediterranean Elevational Gradient

Simple Summary Mountains are ideal natural laboratories to study how biodiversity is influenced by environmental characteristics because climate varies rapidly from lowlands to high elevations. Scientists have investigated how the number of species varies with elevation for the most disparate plant and animal groups worldwide. However, species richness is only one aspect of biodiversity that does not consider species abundances. The so-called Hill numbers are a unified family of mathematical indices that express biodiversity in terms of both richness and abundance. We used Hill numbers to investigate how dung beetle diversity varies along an elevational gradient in a Mediterranean mountain. We found that scarabaeids were the most abundant dung beetle group. These insects construct subterranean nests protecting their offspring from desiccation during warm and dry summer climatic conditions. Additionally, in accordance with their preference for open habitats, we found that dung beetles are more abundant and diversified in grasslands than in woodlands. In the woodlands, diversity increased with elevation because of tree thinning, whereas in the grasslands, diversity decreased because of increasingly harsher environmental conditions. This indicates a trade-off in the beetle response to elevation between the positive effects of increasing the availability of suitable habitats and the worsening of environmental conditions. Most studies of biodiversity-elevational patterns do not take species abundance into consideration. Hill numbers are a unified family of indices that use species abundance and allow a complete characterization of species assemblages through diversity profiles. Studies on dung beetle responses to elevation were essentially based on species richness and produced inconsistent results because of the non-distinction between different habitats and the use of gradients dispersed over wide areas. We analyzed dung beetle diversity in a Mediterranean mountain (central Italy) for different habitats (woodlands vs. grasslands) and taxonomic groups (scarabaeids and aphodiids). Scarabaeids were the most abundant. Since scarabaeids are able to construct subterranean nests, this indicates that the warm and dry summer climatic conditions of high elevations favor species capable of protecting their larvae from desiccation. Dung beetles were more abundant and diversified in grasslands than in woodlands, which is consistent with their preference for open habitats. In the woodlands, diversity increased with increasing elevation because of increasing tree thinning, whereas, in the grasslands, diversity decreased with elevation because of increasingly harsher environmental conditions. These results indicate a trade-off in the beetle response to elevation between the positive effects of increasing the availability of more suitable habitats and the decrease of optimal environmental conditions.