Nestedness, anti-nestedness, and the relationship between prevalence and intensity in ectoparasite assemblages of marine fish: a spatial model of species coexistence

Nested species subset patterns consist in a hierarchical structure of speci es composition in related assemblages, with the species found in depauperat e assemblages representing non-random subsets of progressively richer ones. This pattern has been found at the infracommunity level in about a third o f the fish ectoparasite assemblages studied to date. Here we present eviden ce for another non-random structural pattern in assemblages of fish ectopar asites, anti-nestedness, which corresponds to situations in which parasite species are always absent from infracommunities richer than the most depaup erate one in which they occur. We show that this pattern is exactly as comm on as nestedness, and that anti-nested assemblages are characterised by sig nificantly lower prevalence and mean intensities of parasites than nested a ssemblages. in addition, we found a positive relationship between the preva lence and the mean intensity of parasites across the different assemblages. We propose a link between the nestedness/anti-nestedness continuum and the prevalence-intensity relationship that may involve colonisation-extinction processes. The results presented here suggest that, although nestedness ma y not be common in parasite communities, other departures from random speci es assembly are possible, and that some form of structure may be present in many communities. The continuum between nestedness and anti-nestedness als o has implications for recent models of species coexistence in communities. (C) 2000 Australian Society for Parasitology Inc. Published by Elsevier Sc ience Ltd. All rights reserved.