Dung beetle traitsA conceptual, experimental and biogeographical approach

Abstract

Although functional ecology has been considerably developed in plant ecology, animal ecology still needs to cover some gaps and caveats, namely, the assumption of trait functionality. Thus, whether traits are valid proxies for ecological functions and reflect either the response of organisms to environmental variation (response traits) or its effect on ecosystem functions (effect traits) requires further reseach. Within the taxa in which traits are used lacking a clear functional meaning, dung beetles are not an exception. An in-depth search of literature led us to conclude that functional ecology of dung beetles is still not well developed (chap1). Often, traits are not measured and, when they are, there is a lack of standard methodology. On the one hand, most of the studies focusing on responses of traits to environmental variability actually analyze patterns of trait diversity, without a functional interpretation. On the other hand, studies focusing on the effect of traits do not identify individual effect traits, but rather use trait diversity within the context of Biodiversity~Ecosystem Functioning (BEF). The most commonly used trait is still the way dung beetles utilize the excrement to feed and nest, which leads to an excessive use of the classification in telecoprids, paracoprids, endocoprids and kleptocoprids. Besides this major trait, body size has also been used to further characterize communities, their efficiency in removing dung from pasturelands and in some laboratory experiments. In general, for both effect and response traits, physiology, reproduction and dispersal traits remain largely unexplored. Similarly, only functions such as dung removal and secondary seed dispersal have been fairly studied. Finally, the response of traits to important environmental variables such as dung availability, soil, humidity and temperature is totally unknown. Trying to find traits that have an effect in the delivery of three ecosystem functions (dung removal, dung burial and secondary seed dispersal), we set up a mesocosm laboratory experiment with dung beetle sexual pairs provided with sheep dung containing a controlled set of seeds (chap2). We found out that the morphological traits body size, prothorax size and protibia area successfully explained the performance of dung beetles in the delivery of two important functions: dung removal and dung burial. Surprisingly, behavior (i.e. dung relocation strategy) did not show a significant effect on performance in either dung removal or burial, nor did species identity. We also found a negative effect of some traits in burrowing efficiency: elongation of fore and hind tibiae (related to a telecoprid morphology) and a relatively longer body shape (typical of endocoprids). Finally, we found that there is a positive effect of some shallow-burying species on seedling emergence. To be able to identify traits responding to environmental constraints or biotic interactions, we searched and sampled a steep aridity gradient that would impose a differential stress on dung beetles, shaping taxonomic composition of communities and the functional and phylogenetic structure. In chap3 we thoroughly analyzed the taxonomic patterns in space (along the aridity gradient) and time (between years and within years) and found two completely different taxonomic compositions in the communities before and after the rainy season, but nearly identical communities in consecutive years. After the rainy season, we found that aridity drives a species turnover (supported by a progressive but mild phylogenetic replacement) and that communities in the most arid areas are not a subset of species present in milder aridity conditions, but rather a unique set of species. These unique sets of species proved to have specific features responding to the severe environmental filter imposed by aridity (chap4). Indeed, the average single-trait values of the local communities along the aridity gradient clearly showed a pattern of replacement of the mainly coprophagous paracoprid and small-winged dung beetles in semiarid communities by small-bodied facultatively saprophagous endocoprid species with significantly larger wings compared to body size. This is probably a response to the sparse distribution of trophic resource (typically consistent of small droppings), extreme climatic conditions (high temperature and desiccation) and hard soils typical of arid areas. We conclude that aridity is a strong driver of community assembly via environmental filtering of the regional pool of dung beetles into local communities at the temporal and spatial dimensions.