Supper On Ice
Protected sites in the mountains of Alaska and Canada support snow beds that may last well into the summer, or even throughout the year. These snow beds may seem inhospitable sites for life, yet birds are often seen foraging on them. The attraction is a surprising abundance of dead or inactive insects that make good eating.
Many insects and other arthropods have a dispersal phase in the life cycle, in which they may travel great distances. Some, like juvenile spiders and mites, are aided by threads of silk that act like balloons in the wind. Others use their wings. In either case, they may be carried long distances by the wind. Insects have been captured in nets hundreds of miles at sea, and thousands of feet above the mountain tops.
Large snow beds cause local downdrafts, and thus act as traps for this "aerial plankton." The insects that end up on the snow may have originated many miles away. Studying snow beds at Eagle Summit, in interior Alaska, Dr. John S. Edwards and his colleagues from the University of Washington found that large carpenter ants formed a major part of the insects on snow beds, even though carpenter ants were not found living in the surrounding tundra. These ants were winged reproductives, unsuccessful participants in the annual nuptial flight from colonies in the spruce forests found at lower elevations. Small flies and aphids also contributed large numbers of insects to the snow surface.
Eight species of birds, ranging in size from small Lapland longspurs and water pipits to common ravens, were observed feeding on the snow bed feast, neatly removing and eating the fat-filled abdomens of carpenter ants while leaving behind the head and thorax, with their unpleasant dose of formic acid.
The arrival and subsequent death and decay of insects originating at lower elevations contributes valuable nutrients to the alpine ecosystem. In any one year, the quantity of nutrients added is small, but, accumulated over many years, this may represent a significant source of critical nutrient elements.
