Habitat and Seasonality As Niche Axes in an Odonate Community

This study focuses on the coexistence of the dominant populations among 46 odonate species found in Bays Mountain Park, Sullivan County, Tennessee, USA. By monthly sampling of six aquatic habitats within the park, we established the larval biomass distributions of the 12 dominant populations across habitats and seasons and from these derive estimates of distributions along corresponding niche axes. We use an index proposed by Hurlbert (1978) to estimate niche specialization (breadth— ¹) and overlap on habitat and seasonal axes, both separately and simultaneously (two—dimensional analysis). This enables us to evaluate two possible mechanisms of coexistence in this community: resource partitioning (i.e., differing time—average patterns of resource utilization among competitors) and ecological shift (i.e., nonevolutionary change in resource utilization in response to competition). Three results support the resource—partitioning mechanism: 1) Competition coefficients, obtained by dividing niche overlap values by the appropriate niche specialization values, are all less than one; this indicates that each population should inhibit its own access to a limited food supply more than it inhibits the access of the other populations. 2) Though little or no complementarity between the habitat and seasonality axes is apparent when single—axis index values are examined, the two—dimensional competition coefficients are slightly smaller on average than the product of single—axis means, suggesting two—dimensional complementarity. 3) There are about the same number of consistent 3—yr trends in specialization and overlap as would be expected by chance, suggesting a relatively persistent arrangement of odonate niches in niche space. But the ecological shift mechanism may be operating within or between those populations that do exhibit consistent 3—yr trends in specialization or overlap; more of these values consistently decrease than would be expected by chance. The six populations that comprise the detritus—submersed macrophyte guild account for most of the larval biomass, and the dominant population within the guild (and the community as a whole) is the semiovoltine anisopteran Tetragoneuria cynosura. A particularly intense interaction, as indicated by the competition coefficients, is between the abundant zygopterans Enallagma traviatum and E. signantum; unusually high specializations and overlap were observed for the rush—dwelling zygopterans Ischnura verticalis and I. posita.