Species richness tends to peak at intermediate altitudes over elevational gradients. However, different mechanisms may determine why species numbers decrease up and downwards from this peak. From the richest elevation, variation in beta diversity and the way species are lost may give rise to different patterns: species turnover (driven by interspecific competition or other interactions) or nestedness (driven by environmental filtering).
Here, we analyze the structure of Scarabaeinae dung beetle communities along an elevational gradient in Itatiaia National Park (Rio de Janeiro, Southeastern Brazil). We evaluate community distribution and beta diversity (species turnover and nestedness) at both sides of the point of the highest richness to assess the mechanisms governing the variation in composition and species richness.
We used the Quantitative Bipartite Modularity (QuaBiMo) algorithm based on species co-occurrence to assess community structure along the mountain. If species are distributed in several groups (modules) along the mountain, this may suggest a metacommunity structure. Furthermore, we used beta diversity and a nestedness metric based on overlap and decreasing fill (NODF) to assess which pattern is associated to the decrease in species richness variations at both sides of the peak of maximum richness.
Results/Conclusions
We identify four modules of species that tend to co-occur more often along the elevational gradient. Species turnover is larger among modules and nestedness within the modules. Each module presents a similar pattern of species richness peak at their intermediate elevations. This result suggests environmental filtering restricting groups of species in different parts of the altitudinal gradient, resulting in a metacommunity structure.
Species turnover contributes more strongly to overall beta diversity when considering the whole gradient. However, while compositional variations are characterized by species turnover below the peak of maximum richness, nestedness is most important at higher elevations. These results suggest that the lower elevations present better climatic conditions, with interspecific competition or other community-level processes structuring dung beetle assemblages. In contrast, the nestedness at higher altitudes reflects the limitations of the species to transpose their environmental constraints in progressively harsher climates. Strikingly, these patterns are similar within each module, evidencing that different groups of species react in a similar way to climate gradients. These results show that climate is the main driver structuring Neotropical dung beetle communities, so species with different tolerances to climate lead to arrangements with both species turnover and nestedness along the elevational gradient.