Air is invisible, but far from empty—it is its own dense brew of life. Consider that the atmosphere over a single square mile of the earth’s surface contains 25 million airborne insects. Consider that the fraction of organic detritus that falls out of orbit can become thick enough to plow. Consider that within the space of a very tall imaginary top hat, 50 to 100 million microorganisms swirl above your head.
Airborne life-forms—tens of thousands of species collectively called aerial plankton—form a web of ecological relationships that researchers barely understand. Not only are the sheer numbers astonishing, so is the variety: caterpillars, spiders, aphids, butterflies, moths, beetles, mites, and many other invertebrates, plus countless seeds, spores, and pollen grains of fungi, algae, mosses, liverworts, and flowering plants.
A series of collections, made first with a net on a kite in the 1930s and later on the wings of biplanes, determined that the band of fairly stable air located between 100 and 15,000 feet above the ground literally swims with life. Early observers and a contingent of skeptics wondered if aerial plankton simply represented the passive, accidental transport of lightweight objects. At best, they said, these objects were early life stages of plants and animals merely dispersing to new sites and only temporarily aloft. But evidence now suggests that the aeolian zone—named in the 1960s by San Francisco State College’s Lawrence Swan, after the Greek wind god Aeolus—is more than a temporary shelter for homeless organisms. It is an aerial community of both intentional and unintentional participants.
The life-forms found here rise and fall on atmospheric tides, circling the globe, sometimes to great heights. No one knows for certain just how high life travels in the atmosphere, but in 1974, Russian rockets collected air samples that contained six species of common opportunistic microbes at heights of 35 to 50 miles. Likewise, no one knows how long individual organisms choose to stay aloft, though it is known that tiny aphids control their journeys to the extent that they can descend, against adverse winds, to reach favored food patches.
While the majority of aeolian zone travelers are either too small or too distant to be observed by humans, we occasionally glimpse the dazzling fecundity overhead. Macroinsects, especially migratory forms of locusts, butterflies, and dragonflies, provide good examples. One cloud of locusts observed in the Middle East in 1926 contained more than 10 billion individuals. In California, a continuous stream of 3 billion painted lady butterflies once passed an observer during a three-day period. A recent tally near Chicago counted 1.2 million dragonflies during fall migration.
Ballooning spiders are another conspicuous component of the aeolian zone. I have stood on high, barren peaks with spiders appearing out of thin air to crawl on my head and shoulders. These spiders, typically young ones, are dispersing in search of new homes. Each spider rides on its own silken balloon (technically, a parachute) constructed by reeling out filaments until the wind fills it. This is effective travel—airborne spiders have been reported at heights of seven miles and frequently land on ships hundreds of miles at sea. At times, the air becomes so dense with ballooning spiders that their strands completely encrust exposed surfaces and form sheets of silk known as gossamer.
I recently scanned the sky with binoculars while the afternoon sun hovered behind a tree. It was ablaze with darting luminescent jewels of backlit insects, layer upon layer of them, as far as my optics could focus. They seemed to stretch to the stratosphere, but of course I was only seeing the closest hundred feet or so of the aeolian zone.
In biological terms, the atmosphere has three significant layers. Closest to the ground is the biological boundary layer. Here insects can most easily control their flight because their flying speed is greater than wind speed. By day, winds and air movement reduce this layer to only a few feet in height, but in the calm of night, it rises much higher.
Above this lies the planetary boundary layer. This is where global air currents eddy and tumble across the planet’s rough surface, producing strong gusts and vertical mixing of air. During the heat of the day, this turbulent zone is at its thickest, up to 6,000 feet high. After the sun sets and the earth’s surface cools, this band shrinks to under 1,000 feet.
Even higher, the geostrophic layer, free of friction with the earth’s surface, is a zone of strong, constant wind shear. Organisms that are able to ascend at night through the narrowed planetary boundary layer can lock into this geostrophic beltway and disperse great distances with ease.
Microscopic spores, bacteria, seeds, and insects all grapple with these ceaselessly shifting parameters. Though humans experience wind in its horizontal dimensions, lightweight organisms encounter wind as a vertical force. Even small, sun-warmed twigs and pebbles generate tiny updrafts, while large violent updrafts can sweep to the top of the troposphere (five to ten miles high) in an hour. Carried aloft in 60 minutes, a small insect may require 20 days to work its way back to the ground, although by then winds may have transported it around the world.
A less favorable fate befalls plants and animals that run into fingers of earth stretching high into the atmosphere. In 1802, while climbing Chimorazo, then the world’s highest known peak, Alexander von Humboldt made the first written record of invertebrate fallout from the atmosphere. He noted that at extremely high altitudes insects usually found at low elevations were cast incongruously about the surface of snowfields, too numbed by cold ever to rise again.
Researchers have documented this daily organic rain on mountain peaks and other regions where large snowfields cool the air and cause local downdrafts. In the Himalayas, they counted 400 specimens dropping onto a 10-square-meter plot in 20 minutes. Then they discovered something else: Predators were feasting on this steady supply of food airlifted from the plains below. A species of jumping spider at 22,000 feet on Mount Everest and gray-crowned rosy finches on alpine peaks in the Sierra Nevada live off aerial plankton. Specialized communities of beetles and scavenging flies live at the melting edge of snowfields. And at a melting glacier in the Rockies, bears return each year to gorge on an entombed 600-year-old locust swarm.
After Mount Saint Helens erupted in 1980, researchers made another discovery: Invertebrate fallout is first, laying down the organic basis for primary plant succession in a barren wasteland—contrary to the popular understanding that simple plants are the first building blocks of succession. Within eight weeks of the devastating eruption, a host of invertebrate scavengers and predators were found subsisting entirely on this fallout and, while none survived long, their bodies became nutrients for new plants. By the second summer, 43 species of spiders alone had been found in the blast zone, all arriving by air from at least 20 miles away.
Aerial plankton and aeolian zones are now subjects of serious scientific inquiry. Web sites and international conferences focus on subdisciplines such as aerobiology and radar entomology, and for good reason—allergens, insect pests, and epidemic diseases are also residents of the aeolian zone. Radar technology has developed to the point that an aphid in flight can be identified to species by the frequency of its wingbeat. Less encouraging is the implied intent of some current research—to develop models of how converging wind patterns concentrate aerial insects so that spraying of insecticides can kill the maximum numbers.
As I sit on my porch and write, I sense that the air is vital with life around me. It’s a sunny afternoon and a light breeze drifts across California’s Central Valley and gently ascends the west slope of the Sierra Nevada. Somewhere, I imagine, a large puffball begins to expel the first waves of the 7 trillion spores it will produce. Somewhere else, a cloud of rose-grain aphids begins lifting from a wheat field in densities of 400 million per acre.
In the Andaman Islands there is a wind call billiku, "the spider," for it wraps itself about everything—just like the wind now winding its way up the foothill slope. I catch myself watching small particles float through a shaft of sunlight. An animated fly buzzes my head, and I consider the cyclonic highway above me. Air—I take a deep breath and fill my lungs with spores, pollen, and microorganisms.
From Orion (Spring 1999). Subscriptions: $25/yr. (4 issues) from the Orion Society, 195 Main St., Great Barrington, MA 01230.