The living things that drift along in the currents of air.
The air above your head? It's not devoid of life. But the life that's there is nearly invisible. At one end of the scale, bacteria and viruses drift in the breeze. Slightly larger are spores and pollen. Clouds play host to algae, fungi, and bacteria (there are a few species endemic to the stratosphere)-- and why not have an ecosystem in a cloud?--there's water up there. Various seeds have adapted to be spread by wind. And there are numerous inverterbrates, from spiders to caterpillars to wingless mites, who put themselves at the mercy of wind and air to move (in search of food or sex). Spiders can use their silk to balloon (they've been found at altitudes of seven miles, and scientists have evidence that remote islands in the Pacific Ocean, such as Hawaii, were colonized by spiders who ballooned their way there). And though some insects have wings, some are so lightweight that updrafts (even that from a warm stone) can be powerful enough to lift them beyond the biological boundary layer, the zone where their wing speed can exceed the wind speed. So it is that airplanes can find termites and aphids at high altitudes, and that alpine ecosystems can have predators (birds and insects) who prey on the aerial plankton brought back down to the surface by the downdrafts provided by ice and snow.
Robert R. Dunn. "A head in the clouds: do the microorganisms that circulate in the atmosphere get there by chance or by contrivance?" Natural History. July-August 2009. FindArticles.com. http://findarticles.com/p/articles/mi_m1134/is_6_118/ai_n32339705/ (November 15, 2010)
Robert Krulwich, "Look Up! The Billion-Bug Highway You Can't See," Morning Edition. July 15, 2010. http://www.npr.org/templates/story/story.php?storyId=128389587 (November 15, 2010)
David Lukas. "Of Aerial Plankton and Aeolian Zones." Writing on Air. David Rothenberg, Wandee J. Pryor, eds. MIT Press, 2003. http://books.google.com/books?id=4DeX7Gb-LJ4C&lpg=PA39&ots=InqKd4d1F_&dq=aerial%20plankton&pg=PA39#v=onepage&q=aerial%20plankton&f=false (November 15, 2010)