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Weather Almanac for September 1998
Living On the Air
As gaze out my window this morning, a large flock of starlings wheel across my field of vision in continually changing formation -- first like a giant egg, then like a ribbon waving above the tree tops. When I walked out into the neighbourhood park earlier, many of the trees and shrubs were tinselled with gossamer threads, shining in the early morning sun. My walk this early autumn morning also reminded me of those years when later summer/early autumn meant considerable discomfort as my nasal passages reacted to the concentration of pollens, spores and moulds in the air.
The thought rose in my mind that life inhabits the atmosphere in much the same way it does the oceans. Some organisms, trees and grasses, for example, are rooted in the surface while humans and elephants, cats and dogs, deer and cattle "crawl" along the bottom of the ocean of air. Above the surface, "swim" birds and butterflies, bats and bees. And if we look close enough, we can see a myriad of small life forms floating in the breeze. These are called aeroplankton, the atmospheric equivalent to oceanic plankton. The term literally means "air wanderer" because aeroplankton, tiny plants and animals and bacteria that live -- eating, excreting, even reproducing -- on the air are ever at the mercy of the wandering wind.
In the 1960s, research by Malcolm Brown discovered living algae and other microbes in the orographic clouds (clouds forming when air moves over mountain summits) but none in the air above the clouds. Since then, many other studies have found lifeforms living high in the sky. Russian scientists, for example, have collected microbes (bacteria and fungi) between 58 and 75 km (36 - 47 miles) above the deserts of Kazakhstan.
Observations show that air masses over land contain approximately 500 bacteria per cubic metre of air; the concentration falls over the ocean to about 50 out at 160 km (100 miles) out from shore and to one per cubic metre in the open ocean atmosphere. The census of aeroplankton species includes viruses, approximately 1000 species of bacteria, 40,000 varieties of fungi and hundreds of species of protozoa, algae, mosses and liverworts that live some part of their life cycle as aeroplankton.
How do these species get into the atmosphere, you ask? In a number of different ways. Microbes living in the waters of sea or fresh water can be ejected into the air when bubbles break at the surface: for example, in the froth and foam of waves breaking along the shore or at sea in whitecaps. Once ejected into the air, the wind currents may force the microbes upward to higher and higher altitudes. Once high in the sky, these small lifeforms may take hours or days to fall back to the surface. For example, a typical fungi spore pushed to an altitude of 1000 metres (3280 ft) would take about 24 hours to fall back to ground under only the pull of gravity. But, under the influence of updrafts and downdrafts in the atmosphere, it may take 10 days (on average) for the spore to fall out of the sky.
The development of cumulus clouds and orographic clouds also have mechanisms which may launch lifeforms skyward. Caught in these updrafts, aeroplankton are pushed to great altitudes. If the clouds do not form rain and eventually evaporate back into the air, the aeroplankton may find themselves high in the sky, riding the winds around the globe. In addition, the aeroplankton may be able to carry on life processes while residing in or on large cloud droplets found in a thunderstorms or raindrops.
Of course, we all know that many plants use the winds to scatter their pollens. More than 10,000 species of flowering plants are wind-pollinated. And some of these, such as the grasses, trees, ragweed and goldenrod, bring misery to allergy sufferers and fortunes to those selling medicinal remedies.
In addition, many plants send their seeds off toward new growing sites by air mail. Trees, such as the maple and sycamore, provide their seeds with papery wings so that they can glide some distance from the mother tree. A seed from a Scotch Pine (Pinus sylvertris) was observed to have traveled 7 km (4.4 miles) from its parent during a storm. And we are all familiar with the tufted seeds of dandelion and milkweed which are released into the wind to spread across the landscape.
All 15,000 species of orchids produce seeds which are small and have a thin covering which allows them to remain airborne for long periods. A single plant may send out over 370 million seeds to be dispersed by the winds. The aerial wanderings of these seeds can cover amazing distances. Orchids are found on remote ocean islands 1000 to 3000 km (625 to1900 miles) from large land masses.
The records for seed or spore dispersal belong to the fungi family, particularly the giant puffball (Lycoperdon giganteum) and the Artist's Fungus (Ganoderma applanatum). The giant puffball, when struck by a raindrop, ejects a spore-laden puff of air. As many as 7 trillion spores may be released by this fungus into the wind. The Artist's Fungus (so named because the white under surface of this hard, woody fungus can be used as an artist's canvas) drops spores out of spore tubes into the wind rather steadily during the six-month reproductive season, the total number released by a mature fruit can reach 3.7 trillion spores.
The riders of the wind I find most fascinating, however, are the spiders. Many species of spider deliberately use the winds to move around an area. Here is how they do it. The spider, prompted by some environmental or ecological condition, moves to a vantage point where the winds are swirling and strongest. This may be the end of a branch, on a rock surface, fence post, or bush top. They position themselves with their abdomens pointed to the air and eject fine threads of silk from their spinnerts. At some point, the friction of the air upon the silk thread(s) is great enough to lift the spider from its perch. It lets go and is carried off by the breeze.
One spectacular event of spider air travel occurs in Yosemite National Park in California. There when conditions are ideal, the air is filled with the silks of traveling spiders, enough to form a gossamer canopy arching almost 2000 m (6500 ft) long and 1200 m (4000 ft) high over Sentinal Rock. At other times, millions of small spiders form a smaller structure across Bridal Veil Falls.
Once the spiders become airborne, the winds may carry them great distances. Charles Darwin found small red spiders suddenly appearing in the rigging of HMS Beagle when the ship was 100 km (60 miles) off the coast of Argentina. Researchers collecting spiders in the air over the southern United States and northern Mexico captured 1500 spiders belonging to more than 45 species at a variety of altitudes from 10 to 4500 m (33 to 14,700 ft).
There is even evidence that some of these aerial travelers may be able to control their destination to some degree by altering their silken para-sail. By reeling in some of the threads, the spider can descend or by ejecting more silk, they may further catch the wind. And, if they do land and do not like their location because of unsuitable conditions, they are capable of catching the next breeze "out of town".
When we walk out on an early morning in the autumn and see dew-laden gossamer shimmering in the light, we are often seeing the discarded spider flying-machines, hanging off branches and leaves, criss-crossing the grasses, fluttering in the light air from buildings and trees. Such is the joy of weather combined with the lives of a myriad of species that makes watching nature so enjoyable and fulfilling.
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