Cross-community scaling of benthic macroinvertebrate assembalges: a functional approach to community organisation in Mediterranean inland waters

Littoral and transitional macrozoobenthic communities are characterized by a broad diversity in size and behavior. Because metabolic and geometric constraints underpin body-size scaling relationships, metabolic theory offers a useful framework to predict the numeric abundances of macrozoobenthic species. Cross Community Scaling Relationships (CCSR) expresses the relationship between the average size of an individual in an assemblage and the total number of individuals. Where resource availability or space is constant, the average amount of resource (or space) used per individual directly determines the number of individuals that can be supported. However, subsidiary factors can modulate the realized expression of metabolic/geometric scaling rules by modifying the resources accessibility/availability across the individual body size spectra. The mechanistic relevance of individual body-size on coexistence relationships still requires field and laboratory tests and community level scaling-up. Here, the influence of different physical constrains on CCSR descriptors is investigated through the analysis of a large transitional water macrozoobenthic dataset. Results show that, while the pro capite individual body size has a major role in determining the realized individual density, subsidiary physical factors can remarkably increase the accuracy of our predictions. We discuss and interpret the implications of our observations for theoretical and applied ecology.