Detecting functional rarity in a hyperdiverse Amazonian dung beetle assemblage

The functional space of an ecological community is not evenly filled by its species. Instead, it is usually divided into areas of high species density, where species with common combinations of functional traits are grouped into species rich clusters, while lower density areas are filled with species poor clusters or isolated species with uncommon or unique combinations of traits. Species whose functional traits differ from those of the rest of the community are expected to play distinct ecological roles and over-contribute to ecosystem functionality. Sampling methods used to characterize arthropod assemblages usually target subsets of the community and filter species based on their functional traits, reducing the probability of detecting species with uncommon combinations of functional traits. Consideration of these species is particularly important for characterizing the functional structure and resilience of ecological communities inhabiting threatened ecosystems such as lowland Amazonian rainforests, in which dung beetles play multiple ecological roles and are commonly used as ecological indicators. In this study, we characterize the functional structure of a diverse Amazonian dung beetle assemblage using five different sampling methods (dung- and carrion-baited pitfalls, dung-baited aerial traps, flight interception traps, and visual transects) and 18 different functional traits. Our main objective is to assess the suitability of these methods for the detection of the functional volume of the assemblage and of species with uncommon or unique combinations of functional traits that are expected to support vulnerable ecosystem functions. We found that, although dung-baited pitfalls (DPF) detected the highest species richness and proportion of the functional volume, they did not detect 30 species out of 103 and over 20% of the functional volume of the assemblage. The additional sampling methods captured more species with uncommon combinations of functional traits than expected by chance, some of which were trophic specialists not attracted to DPFs. The use of complementary sampling methods resulted in a better characterization of the functional structure of dung beetle communities inhabiting both terra firme and floodplain Amazonian rainforests and revealed functional resilience and rare species playing ecological roles overlooked by DPFs. The consideration of these species is of great importance given the threats that tropical rainforests face worldwide.