Taxonomic and dimensional structure of benthic macroinvertebrate guilds in the Margherita di Savoia Salt Pans (Italy)

1 - The importance to community assembly of species selection mechanisms based on functional traits (i.e. niche filtering) is the object of intense debate. In this study, the influence of selection mechanisms was estimated in an ecosystem, the Margherita di Savoia Salt Pans, where the salinity gradient is expected to constitute a strong selective constraint. 2 - The spatial and seasonal variations of organisation of the macrobenthic communities was studied along a salinity gradient. At the taxonomic level, species richness, numerical abundance of individuals and diversity were found to vary with gradient levels while not directly with salinity. 3 - Under the most extreme conditions (the highest gradient level) benthic macroinvertebrate guilds became highly simplified, with Chironomus salinarius being dominant. 4 - In terms of dimensional structure, the individuals sampled ranged across five orders of magnitude, and were divided into 19 octaves of abundance. In general, size-abundance distributions indicated equal resource partitioning with body size; the distributional parameters showed significant variations with salinity. 5 - The relative importance of the main “niche oriented” and “body size oriented” organization mechanisms of benthic macroinvertebrates guilds along the salinity gradient was evaluated and discussed.