Transitional Waters Bulletin TWB, Transit. Waters Bull. 4 (2010), n. 2, 19-30 ISSN 1825-229X, DOI 10.1285/i1825229Xv4n2p19 http://siba-ese.unisalento.it © 2010 University of Salento - SIBA http://siba-ese.unisalento.it/ese Evaluating nestedness in a spatially structured detritus-based systems B. Bellisario* 1 , F. Cerfolli 1 , G. Nascetti 1 1 Department of Ecology and Biological Sciences, University of Tuscia, 01100 Viterbo, Italy. *Corresponding author: Phone: +39 761 357753; E-mail: bruno.bellisario@ unitus.it Abstract 1 - Evaluating patterns and mechanisms behind species aggregation represents a fundamental issue in community and conservation ecology. 2 - The extent to which species colonize or disappear from habitats and resources is mostly related to several different factors, such as random processes of birth, death and migration, or complementarity in species responses to environmental disturbances, habitat heterogeneity or spatial distributions. 3 - Patterns of species aggregation can be evaluated by considering the network structure of such assemblages, where the use of ad-hoc null-models could help our understanding of the mechanisms underlying these structures. 4 - Here we evaluated the possible mechanisms underlying the nested assemblage of macroinvertebrates on leaf detritus in different sites of the transitional ecosystem of Tarquinia saltern, by using quantitative networks based on the abundances and frequencies of sampled taxa. 5 - Our results show that the use of quantitative information is able to closely mimic the pattern of incidence observed in the real network, with important outcomes for ecosystem’s functionality and its conservation. Keywords: Detritus; macroinvertebrates; patches; network; nestedness. RESEARCH ARTICLE Introduction In small-fragmented areas, species experience environmental variations at small spatial and temporal scales, influencing the pattern of occurrence within patches (Wiens, 1989). Species may differ in their temporal occurrence, with some species occurring only few times and others more frequently, differing at the same time in terms of abundance, with species more locally abundant than others. However, real communities may represent a continuum of such aggregation, and a proper evaluation of different colonization mechanisms in promoting nestedness is crucial, also for ecosystem functioning. We have developed this theoretical and conceptual framework to evaluate the role of multiple determinants in the nested structure of macroinvertebrates on leaf detritus, under different and variable environmental conditions. To date, only few studies have attempted to study the nested structure in detritus-based systems (Yee and Yee, 2007; Bellisario et al ., 2010), revealing the importance of habitat quality/heterogeneity and detritus level for the observed community assemblage, and suggesting the