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In Search of the Emerald EyesAn Endangered Dragonfly in an Illinois Wetlandby Ken Mierzwa |
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Walk in slowly across a sedge meadow, careful not to turn an ankle on the uneven tussocks. Enter the edge of a marsh and stop where a small rivulet, perhaps a foot across and a few inches deep, meanders through the seven-foot tall cattails. The water is cool, originating from nearby seeps and springs, and flows very slowly. In the heat of mid-summer, the marsh is otherwise dry despite recent evening thunderstorms.Curiously, while no field guide ever mentions this, it's more the shape and posture which sets the Hine's Emerald apart, especially at a distance. No other dragonfly on our study sites has this characteristic form and posture: A long abdomen heavier toward the back, constricted at the base, and carried in a gentle curve. For such a strong flier, it seems almost delicate, elegant in some sort of strangely primordial way. A second Hine's Emerald Dragonfly takes up a patrol along the same rivulet, a short distance in the other direction. My colleagues are having trouble getting photographs, the shutter lag on their digital cameras can't keep pace with the restless energy of the dragonflies. I have an old film rangefinder camera with me, brought along to take habitat photos with a wide-angle lens. The dragonflies are so near that I set the lens at closest focus, about two feet, stop way down for maximum depth of field, and am able to get several good shots as the dragonflies pass in front of me. Sometimes they are so close that I need to lean back to maintain focus. The Hine's Emerald Dragonfly, Somatochlora hineana, was described from Ohio specimens in 1931. By the 1950s those populations were gone, and the species was thought to be extinct. In 1983 a dragonfly was collected in Illinois, but could not be readily identified. In 1987 Tim Vogt saw the specimen, and realized that it was a Hine's Emerald Dragonfly. Vogt, along with Tim Cashatt of the Illinois State Museum, began to survey potential habitat. Within a few years they had identified populations at a number of sites in Will, DuPage, and Cook Counties Illinois, and in Door County Wisconsin. Other workers located additional populations in those same areas, and in the Upper Peninsula of Michigan and the Missouri Ozarks. In all of those places, Hine's Emerald Dragonflies are associated with seepage-fed wetlands on shallow soils over dolomite bedrock. No one is really sure why dolomite is such a consistent feature, but the effect of the cool, spring fed breeding habitat is clear enough. By breeding in such cool water, usually about 55-57 degrees near the source even in mid-summer, warming into the 60s a little downstream, Hine's Emerald Dragonflies avoid competition from other dragonfly species which require the warmer water of ponds and marshes. The trade off is a three to five year larval period. Hine's Emerald Dragonflies spend most of their lives in the aquatic larval stage, living in organic detritus along the rivulets or, in drier seasons, in crayfish burrows. Then they emerge as adults for a few weeks of feeding and breeding. On three sites in Will County Illinois we have monitored adult Hine's Emerald Dragonfly populations every summer since 1994. It's tempting to go to the best places, the places where there are almost always a few dragonflies to be seen, as we've done today. But normally, we walk randomly located transects. This is harder work, but it tells us a great deal about the distribution of the species within each site, and correlations to various habitat types. Randomizing avoids the bias so characteristic of human beings, and forces us to go into places we might not otherwise see. Perhaps most importantly, randomizing allows statistical comparisons from year to year and from site to site. The method that we use, known as distance sampling, allows us to estimate dragonfly density. On two of our sites, we now have 13 consecutive years of data. We've learned that populations fluctuate considerably. In the best year, 1999, at our best site, we estimated that on an average day a little over six adult dragonflies were present on each hectare of suitable habitat; and of course, for every adult, there are several age classes of larvae present as well. In 2005, during one of the worst droughts on record for this region, density fell to about 0.20 adult dragonflies per hectare. Two years later, the population appears to be in the early stages of rebounding. With a long larval period, it will be a few more years until we know for sure that the population has grown. | ||