Mar. 11, 1999 FAYETTEVILLE, Ark. -- Forget the damage done by mountain bikes and all-terrain vehicles -- a University of Arkansas researcher has found that sometimes leaving only footprints can wreak long-term ecological havoc on Western U.S. deserts.
Dr. Dave Evans, biology professor at the University of Arkansas, and Jayne Belnap of the U.S. Geological Survey, study the black biological soil crust that blankets Western deserts. The barely visible dark material teems with life, harboring lichens, mosses, cyanobacteria and algae.
People have traditionally viewed the desert as a wasteland. Historically ranchers used the dry, apparently barren landscape to raise cows. Today city dwellers use the vast land tracts as playgrounds, riding mountain bikes or all-terrain vehicles across the arid landscape.
Such behavior may alter these areas and the species that live there permanently, even if the behavior stops, according to Evans and Belnap.
The researchers report their findings in the January issue of Ecology.
The scientists studied two spots in the Needles District in Utah's Canyonlands National Park -- one where cattle grazed more than 30 years ago and one where their hooves touched the ground eight years ago. Both areas have remained undisturbed since.
"The sites used in our study don't look that different until you get up close and look at the crust," Evans said. "We have other sites that are heavily disturbed and they look very different. The crust is missing and you get mostly cheatgrass."
Evans and Belnap used a round, fluted instrument called a soil corer to take small samples of the desert earth. They measured the nitrogen content of the soil and the ratio of two nitrogen isotopes in the soil, 14N and 15N.
The nitrogen content in soil is a balance between nitrogen added through fixation and nitrogen lost through gas emissions. Evans likens the process to a bathtub with both the faucet and the drain open. Water comes in from the faucet (nitrogen fixation) and leaves through the drain (gaseous emissions). In an undisturbed area, the addition and loss of nitrogen are balanced, and the nitrogen level stays the same. But in areas where plant life has disappeared, less nitrogen gets fixed, causing a net nitrogen loss in the ecosystem.
At eight years and even after 30 years, the Canyonlands areas still show profound nitrogen loss, Evans said. Furthermore, the proportion of 15N in the soil has increased, because 14N leaves the soil more readily. This will ultimately lead to less fertile soil and fewer plants, Evans said. Evans likens the urgency in protecting deserts to that of saving the rain forests.
"Arid ecosystems are the most widespread on earth," Evans said. "The problem is that much of the land has been disturbed so the crusts just aren't that obvious to people."
With about45 percent of the earth's land surface area designated arid or semi-arid and 35 percent deemed at risk of becoming desert, preserving plant life in these areas becomes a crucial component in global climate and environmental issues.
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