Natal habitat use by dragonflies along landscape gradients in Rhode Island

Urban environments often support depauperate insect faunas, although they can serve as suitable habitats for some taxa. The potential value of urban wetlands as habitat for regional dragonfly populations has not been well studied. Landscape patterns of natal habitat use by lentic dragonflies were studied at small wetlands in Rhode Island, U.S.A. during three field seasons (2004, 2005, 2006). Dragonfly exuviae were collected along defined perimeter paths on six (2004) or five (2005, 2006) site visits per season (May–October). The exuviae were identified in the laboratory, and the number collected per species/hour was tallied for each field season. Three landscape gradients (urbanization, distance from coast, and wetland size) were measured and assessed for each wetland, and the dragonfly and landscape data were analyzed in three different ways. First, natal habitat use by dragonflies was assessed on an urban to rural land-use gradient at a set of 21 wetlands, during two emergence seasons (2004, 2005). The wetlands were characterized for urbanization level by using the first factor from a principal components analysis combining chloride concentration in the wetland and percent forest in the surrounding buffer zone. Species diversity measurements and its components (species richness and evenness) were analyzed and compared along the urbanization gradient, as were distributions of individual species. Dragonfly diversity, species richness, and evenness did not change along the urbanization gradient, so urban wetlands served as natal habitat for numerous dragonfly species. However, several individual species had strong relationships to the gradient and most were more commonly found at urban sites, and at sites with fish. In contrast, rare species occurrences were predominantly on the rural end of the gradient. These results suggest that urban wetlands can play important roles as dragonfly habitat and in dragonfly conservation efforts, but that conservation of natural, rural wetlands is also important for some dragonfly species. Dragonfly species richness was assessed in relation to four environmental variables: chloride concentration, surrounding forest cover, wetland size and wetland distance from the maritime coast. The effect of fish presence on dragonfly diversity patterns was also evaluated. Dragonfly landscape distribution patterns based on data collected in 2004 and 2005 was compared with patterns from newly-selected sites in 2006. Species richness increased with wetland area, but no strong patterns emerged with chloride concentration, forest cover, or distance from the coast. However, some individual species showed strong trends along each of these gradients. Fish presence/absence had strong effects on some species, but did not result in different diversity patterns along the gradients in this study. Species that showed greater abundance at sites with high chloride concentration and little forest cover (urban sites) tended to be commonly collected species, while rarely collected species were more common at rural sites. Species that showed trends along the coastal-inland gradient tended to be more common inland. Some species were more common in wetlands with fish and some at sites without fish, but most showed no clear difference in abundance based on fish presence. Because individual lentic dragonfly species vary in their use of sites along these gradients, diverse wetlands at various points along these landscape gradients, including both urban and natural sites, have conservation value for the dragonfly fauna of southern New England. Knowledge of the persistence of exuviae on various substrates is necessary to accurately interpret exuvial surveys, so in 2006 I recorded exuvial persistence at defined areas in several of the study wetlands. Exuviae were field-identified, labeled with small daubs of nail polish, and observed every three weeks from June through September. Overall, exuvial persistence displayed exponential decline, disappearing rapidly during the first few weeks, and more slowly thereafter. The initial rate of decline was similar for most species, but differed in some taxa. There was no significant difference in exuvial retention on emergent vegetation vs. rock substrate. In conclusion, small urban wetlands can serve as natal habitat for numerous dragonfly species, so they can play a role in conservation of odonates. Small wetlands in rural areas should also be protected because they provide additional value by supporting different, and often rare, species