Abstract
All ecosystems on Earth are facing unprecedented levels of human-induced disturbances. Tropical forests, which support enormous diversity of species, currently suffer the most dramatic landscape changes. These forests are also characterized by elevated proportions of rare species, which are the first to become extinct under the increasing and cumulative impacts. Given this scenario, a precise quantification of the biotic responses to environmental changes has become urgent. Moreover, we need to develop predictive approaches capable of identifying the consequences of species extinction to the structure of communities and to ecosystem functioning. Biodiversity should thus be considered in its several facets. Assessing the diversity and distribution of functional traits within species assemblages (i.e., functional structure) is a promising perspective to investigate these changes in ecosystems. In this context, the present study focused on a vulnerable and species-rich group: Amazon stream fishes. Our main objectives included: 1) determining the mechanistic pathways through which land use affects the functional structure of stream fish assemblages in the human-modified mid-eastern Amazon; and 2) investigating the possible consequences of the extinction of rare species on the functional structure of stream fish assemblages. To achieve the first goal, we sampled fish in 94 streams, and characterized stream habitat conditions and key landscape variables, including density of road crossings (i.e., riverscape fragmentation), degree of deforestation, and agricultural land use intensification. 141 species were functionally characterized using ecomorphological traits describing feeding, locomotion, and habitat preferences. We found that multiple drivers operating at different spatial scales influence stream condition and the functional structure of the fish assemblages. Riparian deforestation increased submerged vegetation, which reduced the functional evenness of assemblages (i.e., domination of a few trait combinations). Fragmentation upstream from sampling sites and deforestation altered channel morphology and stream bottom, changing the assemblage functional identity. Fragmentation downstream from sites reduced functional richness, evenness and divergence, suggesting a reduction in the range of niches filled and a functional homogenization of local assemblages. To achieve the second goal of the study, we sampled 320 streams along the main tributaries of the Amazon Basin, and functionally characterized all 395 fish species found in the samples. We then built an integrative measure of species rarity (i.e., by combining local abundance, geographic range, and habitat breadth) and assessed the contribution of rare species to complementary facets of assemblage functional structure using realistic scenarios of species loss. To enhance the generality of our findings, we applied this framework to other two sets of tropical assemblages: trees from French Guiana, and birds from the Australian Wet Tropics. We show that rare species have the most extreme and unique combinations of traits for the three taxonomic groups, and detected disproportionate impacts of rare species potential extinction on the functional structure of the assemblages. These results justify the application of the precautionary principle for tropical biodiversity conservation, despite the expected buffering effects provided by functional redundancy in such species-rich systems. Overall, we believe that this study gives important insights to improving the management and conservation of tropical biodiversity.