Struttura della comunità macrobentonica in torrenti alpini: una "naturale" risposta ecologica al riscaldamento climatico

The predominant influence of physico-chemical conditions on community structure in terms of abundance, species richness and diversity of macroinvertebrate assemblages in headwater streams is well documented in literature, but its implications on body size distribution has been not yet considered. Organism size is one of the key determinants of community structure, and how biomass is partitioned among the biota may determine the efficiency of relevant ecological processes. Understanding how the distribution of biomass is linked to abiotic conditions we could predict the effects of environmental changes on aquatic ecosystem processes, particularly in the case of a community with a relative poor composition and with simplified food web structure, like it is present in alpine freshwater systems. For these reasons, we compared the taxonomic structure and individual size distribution of macroinvertebrate assemblages of three Alpine streams presented similar geomorphology, water chemistry, and food chain structure, but different water thermal regimes, mainly influenced by different types of water source. Therefore, temperature was the only significant independent factor that influenced each scenario, and the relation of this variable to biological responses could be measured directly. Our results show that hydromorfological parameters, physico-chemical characteristics and biotic factors explain much of the variability of macroinvertebrate taxonomical structural attributes, but they do not influence body size where the low temperatures have been detected as the main limiting factor. Temperature acts as a key-driver controlling taxonomical structure and invertebrate body size distribution. In agreement with previous findings, higher values of abundance and species richness correspond to warmer temperatures, as well as shifts of biomass distribution towards smaller size classes. Observed patterns underline the relevance of possible implications of global warming on the robustness and functioning (e.g. increase of productivity, functional shift in the food-webs) of high-altitude and high-latitude sensitive ecosystems, such as Alpine headwater streams.