Résumé
There is broad consensus that the diversity of functional traits within
species assemblages drives several ecological processes. It is also widely
recognized that rare species are the first to go extinct following
human-induced disturbances. Surprisingly, however, the functional
importance of rare species is still poorly understood, particularly in
tropical species-rich assemblages where the majority of species are rare
and the rate of species extinction can be high. Here we investigated the
consequences of local and regional extinctions on the functional structure
of species assemblages. We used three extensive datasets (stream fish from
the Brazilian Amazon, rainforest trees from French Guiana, and birds from
the Australian Wet Tropics) and built an integrative measure of species
rarity vs. commonness, combining local abundance, geographic range, and
habitat breadth. Using different scenarios of species loss, we found a
disproportionate impact of rare species extinction for the three groups,
with significant reductions in levels of functional richness,
specialization and originality of assemblages, which may severely
undermine the integrity of ecological processes. The whole breadth of
functional abilities within species assemblages, which is
disproportionately supported by rare species, is certainly critical in
maintaining ecosystems particularly under the ongoing rapid environmental
transitions.