Dragonfly abundance and emergence behavior before and after bank stabilization on the Connecticut River in Gill, Massachusetts

Fred Morrison, David McLain & Laurie Sanders

A Natural Focus

Abstract:

While stabilization has become an important tool for reducing excessive riverbank erosion, the impacts on emerging dragonflies are unknown. To investigate the effects of bank stabilization, we surveyed a 1200-ft. stretch of eroding bank on the Connecticut River in Gill, Massachusetts for emerging dragonfly species before (2001) and after (2002-2003) bank stabilization. The site was stabilized in Fall 2001 by grading the slope, planting with native vegetation, and adding a rock footing at the average water line. We collected exuviae from the entire site at least weekly from early June to late July each year. We also observed the behavior of nymphs in the process of emerging from the river. In 2003, we added 4 reference sites for comparison between stabilized and natural habitat. Several of the 15 species showed marked increases in abundance following stabilization. The most dramatic change was with Cobra Clubtail (Gomphus vastus), which increased from 357 in 2001 to 12,270 in 2003. Spine-crowned Clubtail (Gomphus abbreviatus), Arrow Clubtail (Stylurus spiniceps), Riverine Clubtail (S. amnicola) and Black-shouldered Spinyleg (Dromogomphus spinosus) were more abundant after stabilization, but declined in the third year of the study. Stygian Shadowdragon (Neurocordulia yamaskanensis) and Illinois River Cruiser (Macromia illinoisensis) declined in the second year and were most abundant in the third year. Skillet Clubtail (Gomphus ventricosus) was only common in the third year, while Brotherly Clubtail (G. fraternus) was absent following stabilization. The changes in abundance between years could not be differentiated between cause-and-effect and natural fluctuations. However, notable changes occurred in the behavior of emerging nymphs. After stabilization, G. abbreviatus, S. spiniceps, S. amnicola, and D. spinosus eclosed close to the water line when the river level was low on the riprap. This behavior made them susceptible to being splashed by boat waves and submerged by rapidly rising water level. These species were much more abundant at the natural reference sites than at the stabilized sites. Nymphs of S. spiniceps crawled a significantly (a = 0.05) shorter distance on the riprap (0.9 ft.) than on natural banks (11.2 ft.). Mortality of G. abbreviatus from boat waves and rising water was as high as 33% in 2002. While the impact of riprap on dragonfly populations is unknown, the observed mortality indicates that standard-sized riprap does not provide a favorable substrate for dragonfly emergence. Alternative stabilization methods should be explored that incorporate dragonfly conservation.

Biography: Fred Morrison has a B.S. in Geology and M.Ed. from the University of Massachusetts . He recently retired following 35 years of teaching science in the Northampton Public School system. His interests include butterflies, odonates, tiger beetles, reptiles, amphibians, and freshwater mussels. Fred has worked on biological inventories for MassWildlife, the MNH&ESP, and private clients. He also helps produce “Field Notes,” a natural history series on WFCR 88.5 FM and Connecticut public radio. 

Dave McLain has a B.S. in wildlife management from the University of Maine , and has completed an M.S. and is now working on a Ph.D. at the University of Massachusetts studying the federally endangered Dwarf Wedgemussel. For the past 19 years, Dave has been active in surveys for birds, mussels, snails, odonates, butterflies, fish, herps, and plants, including international research. He is the conservation coordinator at Massachusetts Audubon’s Arcadia Wildlife Sanctuary in Easthampton .

Contact Info: Fred Morrison, A Natural Focus, Montague Road , Westhampton , MA 01027 . Email: anaturalfocus@crocker.com

David McLain, 123 Combs Road , Easthampton , MA 01027 . Email: Stylurus@aol.com