Cross-community scaling of benthic macro-invertebrate guilds: a functional approach to community organization in Mediterranean and Black sea lagoons

Searching for common patterns in community structure in nature has been considered a major approach to the understanding of community organization. Decoding patterns into underlying mechanism allows the highlighting drivers of community organization. Body-size is a fundamental individual trait as affects directly or indirectly individual energetics, individual perception of resource patchiness or density and space use behavior, individual life cycles, population and community structural properties. These latter have been described as size patterns of size-distribution relationships. Here, we focus on cross-community scaling relationships (CCSRs) describing at the community (or guild) level the relationship between average individual body-size and overall density. CCSRs, synthesizing community level energetics, allow to address underlying mechanisms of organization at high levels of ecological hierarchy; however, available evidences on CCSR patterns are scarce, so far. The aim of this study was to test CCSRs patterns in aquatic guilds, describe their shapes and decode patterns and shapes into drivers and mechanisms, if any. The study has been performed on benthic macro-invertebrate guilds of lagoon ecosystems in the Mediterranean and Black Sea, using already existing data from the TWPlatform. The original data were collected with a nested sampling design seasonally done on fifteen lagoons with different habitat types in each lagoon, two sampling stations per habitat type. Significant CCSRs have been observed in this study: The realized exponent of CCSRs was higher than expected from stochastic energetic arguments, but close to the exponents observed for the other size-abundance distributions. Dispersion of real macroinvertebrate guilds from CCSR expectations was explained by a small set of abiotic drivers, including anthropogenic pressures as chemical pollution and eutrophication. Response to pressures showed body-size dependency, which prevented a full reduction of inner data dependency from the dataset. Results suggest asymmetric influence of key abiotic drivers along the body-size axis, targeting large and dominant taxa.