Monitoring with benthic macroinvertebrates: advantages and disadvantages of body size descriptors

1. The search for simple and effective descriptors of biological ecosystem components is a major challenge of monitoring aquatic ecosystem health. 2. The relevance of body-size-related descriptors of benthic invertebrate guilds in monitoring the health of transitional aquatic ecosystems is discussed. The rationale is that macroinvertebrate body size relates body-size–abundance distributions to disturbance pressures through individual energetics, population dynamics, interspecific interactions and species coexistence responses. 3. Body size is generally easy to measure and amenable to intercalibration procedures, it is comparable across taxa, guilds and sites, and, as a community feature, it is expected to vary on disturbance gradients, according to energetic and ecological constraints. 4. The mechanistic relevance of individual body size as a community feature, through coexistence relationships, still requires field and laboratory tests; standard methods to analyse body-size–abundance distributions are not yet fully developed. 5. Field experiments on coastal lagoons and freshwater ecosystems of southern Italy, which were designed to test the relevance of body-size-related constraints on the organization of detritus-based benthic guilds, are reviewed. 6. Study cases emphasized a number of interesting features of body size and related descriptors, that support their relevance as benthic invertebrates descriptors of ecosystem health: (a) body-size–abundance distributions are consistently less variable than taxonomic composition; (b) the width of body-size–abundance distribution is mainly due to the interspecific component; (c) the descriptors of body-size–abundance distributions seem to respond on environmental gradients and generally covary with species density, richness and diversity, on which most of the monitoring programmes actually rely.