Relationship Between Lotic Odonate Communities, Waterway Width, and Habitat Integrity in the Mountains and Piedmont of South Carolina, USA

Dragonflies (Insecta: Odonata) are sensitive to anthropogenic disturbances that affect water quality, siltation, and riparian canopy cover; but these characteristics also vary with waterway size. We used the visual scan method to sample adult odonates in a 100 x 10 m plot at 20 sites (sampled six times each) on 12 waterways of different size (1-50 m wide) in forested habitats in northwestern South Carolina, USA. We used AICc to compare 'best-subset' linear models evaluating the effects of waterway width and nine other habitat characteristics, considered independently and as a combined Habitat Integrity Index (HII), on odonate communities. Mean adult abundance (log(10) transformed) and species richness (improved Chao estimated) were only predicted in linear models by waterway width, and were significantly greater in large waterways (> 10 m) than small (< 10 m; t-tests). Simpson's diversity (Chao estimated) was only predicted by the HII and was only positively correlated with the index when waterway width was included. The species composition of odonate communities inhabiting large waterways was also significantly different than in small waterways (NMDS and PERMANOVA analyses). Communities on small waterways were significantly nested subsets of species in communities on larger waterways, whether waterways were ordered by waterway width or grouped in 'large' and 'small' categories (NODF nestedness analyses). Forest specialists needing shady habitats used all waterways, but large waterways also supported habitat generalists preferring sunny areas and river specialists. Because large waterways already have open canopies, differences in waterway width may complicate the effects of anthropogenic disturbance.