Energetic appetites take ecologic toll.

Energetic appetites take ecologic toll

An extraterrestrial observing Earth "might conclude that cattle is the dominant animal species in our biosphere," muses David H. Wright, an ecologist at the University of Georgia in Athens. In a new analysis of "species energy curves," he notes that bovines consume more than 2 percent of all the photosynthetically derived energy in Earth's land-based environment and some 3.3 times more calories than the planet's roughly 5.32 billion humans consume. And yet, Wright asserts, cows appropriate far less photosynthetically derived energy than humans do.

The fallacy inherent in comparing bovine and human calorie consumption, he argues in the July AMBIO, is that it ignores the human tendency to "waste" valuable calories. For example, Wright notes that people don't eat whole cereal grasses or whole animals. Moreover, he says, what the human species discards is as likely to be landfilled as returned to the food web and shared with other species. The human/bovine comparison also ignores humanity's tremendous appetite for land and biological materials--from real estate for paddocks and paved roads to flax and trees for paper and lumber. In siphoning off these resources -- especially the land, which collects solar energy -- humans severely limit the energy available to meet the most basic needs of other species, Wright observes.

Wright estimates that human activities tie up between 20 and 30 percent of the potentially available photosynthetically fixed energy in terrestrial ecosystems -- some 650 to 800 exajoules (1 exajoule = [10.sup.18] joules) annually. And as the global human population increases to an expected 6.1 billion by the year 2000, its share of photosynthetically derived energy will likely reach 24 to 37 percent, he says. Through a complex series of calculations, Wright shows how this increased use threatens to extinguish 3 to 9 percent of the planet's land species. And pollution and sport hunting "will only make it more difficult for the average species to survive in the face of human restrictions on natural energy flow," he says.

Wright acknowledges that energy-use estimates provide at best a "coarse" statistical prediction of species risks, but says they do offer ecologists a needed gauge of the magnitude of effort required to protect species diversity.