| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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A closer look The Aeolian Zone
It's midsummer in the Sierra Nevada, and the glaciers are melting. A steady diet of sun has dotted the high country with snow island archipelagoes. Meltwater pools reflect a moonscape of naked granite ridges and cloudless sky. There's no welcoming greenery here, no shelter and little shade. It all has the hard, flat, elemental look of the inorganic: blue sky, gray rock, and white ice. Lacking even soil for plants, this harsh world should be a biological wasteland. Yet the high snowfields of the Sierra and other high mountain ranges are a vibrant ecosystem unto themselves. Known as the aeolian zone, high altitude snowfields are fed by drafts of wind from more fertile lands below. A hundred miles to the west lie the farm fields and grasslands of the Great Central Valley. Insulated from cooling ocean breezes, summer temperatures in the valley regularly soar well over 100 ºF. That hot air rises like an escaping balloon, sweeping a multitude of tiny creatures' gnats, midges, and winged ants' along with it. Airborne plant seeds, pollen, and other organic matter are caught by the rushing air as well. The currents carry the hostages up into the Sierra foothills, above ponderosa forests and alpine lakes, then over the highest snowfields. There, the sudden drop in temperature drains the winds of their strength. Their cargo comes tumbling out of the sky like manna from heaven. Once on the snow, insects accustomed to hot valley temperatures soon grow too stiff to fly or even walk. Paralyzed but still alive, they helplessly await their fate. While the sun shines, the birds have this smorgasbord to themselves. Rosy finches and American pipits snap up the leggy popsicles with abandon. The night shift, however, is a strictly six-legged affair. Flightless beetles of the genus Nebria and primitive, cricketlike grylloblattids emerge from beneath boulders and ice crevices at dusk. Ranging from inky black to toffee brown or bottle green, they stand out clearly against daytime snow. To avoid becoming bird fodder themselves, they wait until dark to fill their own stomachs. The windfall can be so rich that mealtimes don't take long. One experimenter clocked the amount of time feeding glacier beetles spent on glacier ice atop Washington's Mount Rainier. Virtually all scuttled home within two hours, so sated they did not appear again for several days. Pickings on the snowfields vary throughout the year. In spring, when snows are still deep and the updrafts still weak, both the food and glacier residents are spread thin. Summer melting shrinks the snow islands and concentrates the crowd. Each lingering patch of frosty white turns into a singles bar, as the beetles bump into more of their kind and wander off to mate. As the months go by without new flurries, the snow may take on a yellow tint from pollen, or a delicate pink from algae, all sprinkled with gritty dirt and dust. By late October, the residents have either gone underground or decamped to the perennial ice of the glacier.
California Academy of Sciences entomologist Dave Kavanaugh has been studying Nebria beetles for the last 35 years. He first encountered one while climbing a 14,000-foot mountain in Colorado. I was sitting on top just eating an orange when this beetle crawled across my boot. It turned out to be the second known specimen of a very rare species belonging to the genus Nebria. The thrill of that find helped steer him out of medical school and towards entomology. Since then, he's braved chill winds and dangerous night climbing conditions to capture more of the quirky beetles. Kavanaugh and others have found that the Nebria aren't high-altitude snobs. More important for them is temperature. They cling to the coolest environments' cold, fast-running streams, alpine peaks, and northerly latitudes. In Alaska and northern British Columbia, they are found all the way down to sea level. Kavanaugh found that daytime temperatures atop high Sierra ice can rival a foggy day in San Francisco. To beat the heat, the beetles retreat beneath large rocks and even within the snowbanks themselves. The crevices beneath snow layers, Kavanaugh says, would be like the Carlsbad Caverns for us. These spaces are huge, relative to their size. Glacier life, however, poses its own problems. Winter temperatures regularly dip below freezing. To endure the chill, Nebria and many of their snowbound neighbors run antifreeze through their bodies. Some use the equivalent of glycol, the same chemical we add to car radiators, while others employ ice-melting salts. If large ice crystals form within their bodies, their organs may rupture and they will die. They'll bleed to death, Kavanaugh says. By controlling the rate of freezing, and the size of the ice crystals that form, a lot of these insects can stand being frozen solid for the winter. Glacier beetle metabolisms are also fine-tuned to handle the chill. Their enzymes, like ours, work best within a very narrow temperature range. But instead of operating around 98.6ºF, Nebria enzymes prefer something closer to 32 ºF. Just how sensitive the beetles are to warmth became clear when Kavanaugh returned from a recent trip to Conness Glacier in the eastern Sierra. He had captured a few live Nebria and was showing them to colleagues in the lab. I had a few out of the refrigerator at room temperature. Within about 15 minutes, one went belly up it just rolled over on its back and started kicking spastically. The ability to withstand the occasional deep-freeze means glacier insects have the bounty of the aeolian zone virtually to themselves. But the twenty-first century isn't the best time to be a snow and ice specialist. The climate seems to be changing at a much faster rate than they can evolve, so they're going to be all out of luck if they can't move up to stay in a cool zone. In California, they're already at the tops of the mountains' there's nowhere else to go, Kavanaugh says. Things may look grim for the aeolians now, but don't underestimate these hardy little insects. They have survived the comings and goings of a half-dozen previous ice ages. With a little luck, and a lot of snow, they could hang on until the time of the glaciers comes again. Kathleen M. Wong is Managing Editor of California Wild. |
