Tuesday, April 7, 2009

Science of the Seasons: Stone cold crazy in love

It snowed again this week and spring seems to have been put off again. After a long winter, many of us are ready to see a color change from pure white snow (perhaps with a little gray volcanic ash as an accent) to the shades of green that spring offers.

Various trees are showing bulging catkins that are preparing to pop open, and swelling leaf buds will soon give rise to the next generation of leaves. A postponement of what we think of as spring may occur, but changes are ongoing in the streams, whether we know it or not.

If we could see underneath the ice cover of streams and rivers today, there are subtle changes happening. A fair number of aquatic insects have been feeding and growing during the winter months, and some will emerge as adults within a couple weeks. One of the early emerging insects from the Kenai River is a group of small stoneflies (Plecopterans), commonly called winter flies or winter stoneflies. They get these names because they can often be seen crawling on the snow and ice in the late winter or early spring.

Two weeks from now there will be thousands of tiny black, Capnidae stoneflies emerging from gravel substrate areas. If you happen to walk along the river, perhaps near Slikok Creek, every large rock along the stream edge will be crawling with slender, half-inch long stonefly adults. A close examination will reveal that these little insects have transparent wings folded flat on their backs, indicating they are in fact adults and no longer nymphs.

They aren’t very strong fliers so they prefer to crawl. You’ll find them crawling all over you if you sit down for any length of time or even stand still for a few seconds. They do not feed as adults and will see you as a tall, drumming platform and a possible place to find a mate. One might argue that they are “looking for love in all the wrong places,” but there will be lots of them using you as their personal dating service.

In some years the Kenai River ice cover has been melted away by now. This year it seems that the ice cover could remain intact during their chosen emergence period; but that will not deter their massive emergence. Most of these insects use light duration as their “zeitgeber” or timing cue, and they don’t really care if there is still ice cover or not.

Hidden from our view are air-filled spaces beneath the ice because of lowering river levels. There are lots of exposed rocks or even ice surfaces for the insects to use as an emergence platform. Winter stoneflies can emerge underneath the ice cover, find a mate, lay eggs and die, all hidden from the gaze of curious entomologists or insect predators.

For shortlived aquatic insects, and these stoneflies might only survive for two weeks, synchronous emergence of the entire population is important. By having most members of the population becoming an aerial insect at the same time, there is a high likelihood of everyone finding a mate. Since they are emerging early in the spring, very few avian predators are around to disrupt their party.

Along the Kenai River, various shore birds, like lesser yellowlegs, will work the shoreline, feeding on as many stoneflies as they can, but they cannot eat them all. These winter stoneflies will emerge by the tens of thousands so the predators that do appear will soon become satiated. The surviving flies will be able to complete their reproductive duties.

Because of a diverse number of microhabitats in the Kenai River, there are a fair number of different species of stoneflies. This species diversity probably comes from the variety of possible food sources available for stoneflies. Many, like the winter stoneflies we’ll be seeing in a few weeks, are leaf shredders. They feed on leaves, mostly those from riparian trees that are infested with nutrient-rich bacteria and fungi. The nymphs (immature stoneflies) chop the leaves into fine fragments and pass them through their gut. Because their guts are relatively inefficient — they can only extract about 5 percent of the nutrients they take in — they process a lot of leaves. Now you know why we don’t find many leaves in the river after the ice melts.

Another guild of stoneflies are active predators on smaller aquatic insects. These predators target the most abundant stream insects, the midge larvae and young mayflies. Some stoneflies combine the two lifestyles by starting their nymphal careers as a detritivore but slowly change into omnivores and eventually become predators.

Perhaps one of the most interesting aspects about many stoneflies is their novel approach to finding a mate. The males will crawl into a nearby shrub or tree and begin to “drum” on the limb with their abdomen. A receptive female, upon sensing the drumming, will answer with her own abdomen tapping and crawl toward the male virtuoso. Eventually the male will find the responding female and they mate. Each stonefly species uses a unique drumming cadence so inappropriate species are not attracted. Eggs are then laid on the stream surface or along the shoreline on submerged structures like a rock or tree limb. Soon stonefly nymphs hatch out and start the yearly cycle again.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River watershed.

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