Nested subset analyses of colonization-dominated communities: Metazoan ectoparasites of marine fishes

Communities have a nested subset structure if the species comprising depauperate assemblages represent non-random subsets of progressively richer communities. Nestedness results from differential extinction probabilities, differential dispersal and colonization probabilities, nested environmental tolerances, or facilitated succession. Previous research suggests that the frequency of significant nestedness, and the degree of nestedness as measured by a standardized index, C, should be greater for extinction-dominated communities than colonization-dominated communities. To test these hypotheses, we conducted nestedness analyses on 38 communities of metazoan gill and head ectoparasites infecting different species of marine fishes. Extinction, environmental tolerances and niche space, and interspecific interactions do not significantly influence the structure of these ectoparasite communities. As such, they provide an appropriate system in which to examine the frequency and degree of nestedness in colonization-dominated assemblages relative to extinction-structured communities; Using two common analyses, we found that nestedness in these ectoparasite communities was rare (1 of 38 communities after a conservative Bonferroni correction for multiple comparisons). However, the mean standardized nestedness score (C = 0.483 +/- 0.243) was not significantly different from the mean nestedness score of extinction-structured communities reported in the literature. Therefore, although these communities do seem less nested than extinction-dominated communities (from the frequency analysis), the C statistic is unable to distinguish these groups. Also, neither the frequency of significant nestedness nor mean standardized C scores were related to mean water temperature, host habitat (benthic, pelagic, or benthopelagic), host trophic level (herbivore, planktivore, predator, or omnivore), or whether or not the host is a schooling species. These results confirm previous conclusions stating that these communities are largely random, unstructured assemblages.