Co-occurence of Odonata in the eastern United States

The structure and dynamics of inaiviaual ecological communities reflect the frequency and intensity of interactions on a regional scale among populations mixed by dispersal. We therefore characterize and compare the distributions of 37 broadly sympatric odonate species across 201 aquatic sites in the Eastern United States to investigate these regional interactions. Since the 37 species all co-occur at a relatively well-studied site (Bays Mountain Park, Sullivan County, Tennessee USA), we are able to relate life-history characteristics and within-site distribution across habitats to between-site habitat use. A series of statistical tests based on the chi-square statistic is used to assess the distributions of the species across broad habitat categories (stream, river, pond, lake) and the pairwise associations of species across all sites. In the 43 best-studied sites, 25.6 odonate species co-occurred on average, considerably more than would be expected if the species were distributed randomly across sites. Species occupying relatively many of the study sites tended to co-occur with fewer species per site and to have somewhat longer flight seasons than the less widespread species. Those species that were more uniformly distributed across site-habitat categories tended to occupy more total sites but did not appear to have longer flight seasons or lower co-occurrence frequencies. Negative associations of species pairs were relatively rare, perhaps because competitive exclusion is uncommon among odonates; some possible exceptions merit additional study, such as Enallagma civile vs Lestes vigilax. Niche complementarity is not apparent between species, but different degrees of specialization within vs between sites may be conspicuous in individual species like the local habitat specialists at Bays Mountain Park (Plathemis lydia, Ischnura posita and I. verticalis).