THERE'S a lot of talk about energy supply these days. But few discuss, or even perceive, what may be the the biggest issue of all - how we share the planet's photosynthetically derived energy with the rest of Earth's land animals. It appears humans are hogging the supply. In fact, we waste far more than our fair share of the energy when that share is measured by what we need to run our bodies.
University of Georgia ecologist David H. Wright, who noted this ecological danger in the journal Ambio, acknowledges that his estimates are coarse. Yet they do indicate a concern that we should take seriously.
Plants provide a lot of energy. For land-based living communities, it amounts to the energy content of 100 to 130 billion barrels of crude oil a year, to use the currently fashionable energy measure. Humans need relatively little of this to live. In fact, according to Dr. Wright, the world's cattle consume 3.3 times as many photosynthetically supplied calories as do the 5.32 billion people.
It's not what people eat that raises the danger Wright perceives. It's what we ``waste.'' We eat only parts of plants and animals. What we throw away often goes into landfills where its energy content isn't shared by other species, except microbes. Our diversion of the natural energy flow may be even more ``wasteful.'' We use vast quantities of plants for lumber, paper, and other products. We even cut back the basic energy production by paving over land.
All these practices limit the energy supply of other land-based animal species. We corner something like 20 to 30 percent of the potential photosynthetic energy in Wright's estimation. (Cattle take only about 2 percent.)
Furthermore, if population growth and economic development continue as now projected, Wright thinks this human energy sequester could reach 24 to 37 percent by the end of the decade.
It's hard to know how this growing energy deficit will affect other species. Wright's coarse estimate suggests that it might threaten 3 to 9 percent of land species, thus adding a new factor to other causes of extinction. He notes that, at the very least, it will make it harder for ``the average species to survive in the face of human restrictions on natural energy flow.''
We know relatively little about the energy needs of Earth's diverse species. What we do know suggests that energy requirements can be subtle. Did you know, for example, that it takes nearly the same amount of energy to run a mile at a leisurely pace as to run it at top speed?
Rodger Kram and C. Richard Taylor of Harvard University's Museum of Comparative Zoology reported data in Nature in July that suggest it is primarily the cost of supporting an animal's weight and the time it takes to generate the supporting force that determines the energy cost of running, not the animal's speed. Or consider animals that don't need to support weight with legs. You might think they use less energy to move than limbed creatures.
Yet, in a paper in Nature in August, Michael Walton, Bruce C. Jayne, and Albert F. Bennett of the University of California, Irvine, show that the snake Columber constrictor slithering with sidewise undulations over the ground spends as much energy as a running animal of the same weight. If the snake moves down a tunnel concertina fashion, the energy cost is even higher.
The world's biological energy flows have not been much of an environmental concern. It's time ecologists gave them a high research priority so we can understand this subtle environmental challenge.