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Tuesday, May 2, 2000
Natural genki drink fuels aerial pollinators
By MARK BRAZIL
For most of our planet's mind-numbingly long history of around 4.6 billion years, the most complex life form on Earth was the prokaryotic cell. The ghostly signatures of these simple cells without nuclei first appear in rocks dated to about 3.75 billion years ago. The length of their nearly 2-billion-year reign on Earth, though not their complexity, makes our own brief history seem as fleeting as the barely noticeable flicker of a neon light tube.
Even once their successors, those cells with nuclei known as eukaryotes, had appeared, taking the planet another step closer to more organized life, there was still no rush toward the "bigger and better" organisms we now consider typical of biodiversity. More than a billion years was still to pass before the Precambrian and Cambrian explosions of about 560 and 530 million years ago.
Only then, after about 85 percent of Earth's history had already passed, did life really begin to blossom. Over a period of several million years, cellular organization was raised to new heights and just about all of the major phyla, the essential body plans of life recognized on Earth today, began to appear. Whole suites of other phyla failed to make it through the sieve of evolutionary history, but our own precursors -- the predecessors of creatures with backbones -- first appeared at this time.
Although we perceive our modern world as dominated by humans (after all, we now number over 6 billion), the creatures most clearly ascendant are the insects. They outnumber us, are more resistant and hardier than we, can survive just about anywhere and have been around so long as to trivialize our brief span on Earth. We take them for granted, even to the point of despising them, yet we depend on them indirectly in many ways.
By the Carboniferous Period, 360-286 million years ago, insects were already well established. This was a time of giant insects, when dragonflies up to a meter long existed. These massive predators, like radio-controlled helicopters, zoomed through swamp forests snatching smaller aerial insects on the wing.
Still, though there was abundant insect life then, it lacked an element we now take for granted. The next major development in the insect world was based on a huge change in the plant kingdom.
For millions of years, green and brown dominated the plant world. Giant horsetails -- plants that were to become the fossilized coal deposits that we now burn -- were followed by a new line of experimentation during the Triassic, the coniferous plants, but this did not change the overall appearance of the natural world to any great extent. This was a period of vast homogeneous stands of just a few species of plants, dependent on the wind for pollination and exchange of genes between members of their population.
During the early part of the Cretaceous, 146-65 million years ago, however, plant life made an astonishing leap: Flowering plants evolved, and with them came a tremendous change. The flowers provided a new source of food for pollinators, most of which were insects, and attracting those insects led flowering plants into a competitive arena where color, shape and pattern mattered.
This period saw the spread of color, and the diversification of the plant world. Gone were the great swathes of unbroken green, homogeneous forest; newly evolving flowering plants splashed the landscape with color.
Pollen-rich flowers evolved in parallel with the insects that pollinated them. The pollen itself, which the plants needed to be carried away to other flowers, served as a food for the insects, but that involved the potential loss of the very product the plants needed transferred.
The next stage of development ushered in a new relationship between plants and insects: Flowering plants provided an energy drink of nectar to insects, which in turn carried away the plants' pollen.
As specialist flowering plants evolved, insects evolved to specialize in feeding from them. The colorful day-flying butterflies and the more cryptic nocturnal moths are their familiar descendants. Moths, like the hawk moths shown here, have cranked their flying abilities to the maximum, and can feed without landing by hovering in front of a flower. Their wings blur with speed as they uncoil a proboscis supremely adapted for in-flight refueling. They pump fuel on board in flight much as fighter aircraft do from airborne fuel carriers.
Some of these moths are, like the butterflies, day-flying. Their large size and wing-blurring speed have earned them the name hummingbird hawk moths because they resemble those other nectar feeders of the Americas. The hummingbirds were only able to evolve their high-energy lifestyles because of the pre-existing fuel supply.
A few mammals have taken a leaf out of the nectar feeding book too; flower feeding bats and sugar gliders are to be found in the Southeast Asian and Australian regions, but they are mostly nocturnal, small and hard to see. If you wish to see nectar feeding at its most colorful, take the time to sit and watch a flowering shrub on a warm sunny morning. You will find that a steady procession of insects is attracted, from bees to butterflies, each with its own strategy for feeding.
Flight is an extremely energy-consuming activity, and so to achieve fast-powered flight (rather than gliding or weak flapping), a light fuel, delivering a high-energy return for its weight, is an essential prerequisite.
Nectar is the natural answer, but nectar as a fuel source carries one drawback. From the plant's perspective, dilute nectar is cheaper to supply. It must contain enough energy to attract insects in the first place, but any more would be wasteful. From the insect's perspective a plant that provides more energy in its nectar is going to be a more popular food source.
Natural selection forces plants into a tightrope balancing act of providing sufficient energy to entice the aerial pollinators, but not so much that vital energy is wasted. The water content to dilute the nectar is easily supplied by the plant, but that in turn becomes a problem for the nectar feeders -- they must, somehow, be able to carry and process the fuel efficiently and void all the additional water released as they burn the fuel. Disposing of liquid waste is an important hurdle for all flyers -- as anyone will realize who watches the contortions of roosting bats when they need to urinate!
A long-held human ambition to fly has taken us from the birth of aviation to space travel in less than a century, and liquid fuel has been the key for our high-powered flight too. Our airborne behemoths, the jumbo jets, leave white contrails behind them, writing on the sky in water vapor released from burning fossil fuels.
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