Invasion of the Bays Mountain Lake Dragonfly Assemblage by Dromogomphus spinosus (Odonata:Gomphidae)

Abstract

Dromogomphus spinosus (Odonata: Gomphidae) larvae are "burrowing" dragonflies which usually inhabit sand deposits in lotic systems. This species has recently invaded Bays Mountain Lake (Sullivan County, Tennessee, USA), where larvae now occupy widespread submersed macrophyte and allochthonous detritus habitats. It is now one of the more abundant species in a relatively stable odonate assemblage. During the Dromogomphus invasion, there was a gradual decline in abundance of Epitheca (Tetragoneuria) cynosura (Odonata:Corduliidae), the dominant anisopteran in that assemblage. Analyses of Dromogomphus and Epitheca numbers from 12 years (1977-1989) of monthly sweep-net samples at fixed stations within each of five types of littoral zone habitat document trends in population size, suggest that both species have partially semivoltine life-histories with comparable phenology, and show overlap in habitat use between species as well as negative correlations of within-sample abundances for some habitats. Gut contents of Dromogomphus collected in these sweep-net samples showed no change in diet over the 12-yr period. Predominant prey were midge larvae (56%), especially "burrowing" midges, and oligochaetes (34%). Analyses of fecal pellets produced by specimens of both Dromogomphus and Epitheca collected during 1988-1989 show that diets differed for both "small" and "large" instars. For both size categories, Epitheca tended to eat more microcrustaceans, but small larvae also ate more "miscellaneous arthropods" (including odonate larvae). Comparing 1988-1989 Epitheca diets with those reported previously from 1978-1979 suggests that large Epitheca exhibited a dietary niche shift during the Dromogomphus invasion. Contemporary large Epitheca eat fewer "burrowing" midges and more oligochaetes and microcrustaceans than they did previously.