Watersheds Messenger Spring 2003 Vol. X, No. 1 PDF ISSUE
The Delicate Balance of Dragonflies
Adult dragonflies are masters of the air: fast-flying, extremely maneuverable and equipped with extraordinary senses. They are large, colorful, sometimes territorial, predatory insects with fascinating behavior, and they display most of the hunting characteristics of raptors (hawks and falcons). Their prey can be anything from smaller flying insects to other dragonflies.
More than 300 species of dragonflies are found in North America north of Mexico. About 15 percent of North American dragonflies are at risk of extinction.
Dragonflies go through three stages in their life cycle: egg, aquatic larvae and adult. It is during the first two of these stages that dragonflies often run into trouble.
Many species of darners, a dragonfly type, insert their eggs into emergent vegetation. When emergent vegetation around a body of water is removed, darners are also removed.
Some dragonflies will lay eggs in soil near bodies of water, which makes them susceptible to trampling and other such disturbances. " 'Cleaning up' a pond by mowing its borders and removing all the aquatic vegetation pretty much sterilizes it for most dragonflies, notes dragonfly expert Sidney Dunkle.
The larvae of all dragonfly species are aquatic and require water. Changes to a watershed that reduce the water-holding capacity of the land, resulting in alternating spring run-off floods and late season droughts, reduce the amount of suitable aquatic habitat.
Reservoirs are poor habitat because fluctuating water levels often preclude the establishment of aquatic vegetation. No aquatic vegetation equals no dragonfly production. If the aquatic habitat is free of fish, dragonfly larvae are usually the top aquatic predator. Waters that contain fish will produce significantly less dragonfly adults (80-90 percent less according to some studies).
The small seeps, springs and riparian zones on our arid public lands should be excellent dragonfly habitat, but humans have eliminated dragonflies from many of these habitats by allowing springs, seeps and small riparian zones to suffer livestock use so heavy that plants are eliminated.
Springs, seeps and small riparian zones are frequently "developed" into waters that dragonflies can't utilize at the expense of waters that they can. As a final insult, we allow livestock feces and urine to contaminate and eutrophy these habitats, resulting in dissolved oxygen levels below what these active aquatic predators can tolerate.
The loss of dragonfly larvae from these habitats is a two-edged sword: Not only do we lose an amazing creature from a local ecosystem but we also enhance populations of the equally amazing diptera (a.k.a. mosquitoes and flies). Many of the diptera breathe air, not water, and can be quite at home in waters that are actually quite foul. In my college days, I was often appalled at the quality of waters I would sample for larval mosquitoes.
As adults, dragonflies are powerful predators. Though they emerge fully-grown, It is as larvae that the bulk of their eating and growing is performed. As a child, I kept dragonfly larvae in a small aquarium and can vouch for the fact that they eat many aquatic insects and even small fish!
Just as the removal of fish from a body of water enhances dragonfly survival, removal of dragonflies enhances survival of other aquatic insects.
Dragonflies are an important part of the world around us. The adults you see may be present in habitats that wouldn't support the entire life cycle; the mobility of these creatures is part of their appeal.
Kent Fothergill is a professional biologist and WWP member from Buhl, Idaho.