Part of the discussion at a recent Sierra Club Grazing Committee meeting that I attended centered on a pivotal question: Do plants "need" grazing?

Some people, notably holistic range r management promoter Allan Savory, argue that plants require grazing to survive, lest they become "overmature" or "decadent." This kind of assertion provides a justification for livestock production, but it is not uniformly accepted by ecologists.

The effect of grazing on plants is somewhat like the effect of sunlight on people. Even small amounts of sunlight hurt the skin of all people. As a consequence, we all develop mechanisms that help us "tolerate" the sun. The usual defense against exposure is skin pigmentation. However, one must be careful not to conclude that such an adaptation means we "need" to expose ourselves routinely to the sun. Except for a very small need of vitamin D, which limited exposure to sunlight will supply, most of us are better off avoiding the sun, thereby decreasing our risk of skin cancer.

In a similar way, different grassland ecosystems have different degrees of susceptibility to herbivory. Grasses on the Great Plains, where herbivory pressure, historically, was greater, have developed more tolerance to grazing, just as people in the tropics have developed mechanisms to tolerate the strong tropical sunlight. Plants in the Great Basin or other areas where there were fewer herbivores are less tolerant.

Most plants have developed various mechanisms that allow them to tolerate or avoid herbivory pressure to some degree. Cactus have thorns to discourage browsing. Plants like sagebrush have chemical terpenes that make them unpalatable to many, but not all, species. Some grasses have awns to discourage grazing. Others have underground runners so they can spread in spite of grazing pressure.

What all of these adaptations suggest is that herbivory comes at a cost to the plant. Plants have developed all kinds of ways of dealing with that cost, but the adaptation is still a cost rather than a benefit.

When a grass plant is grazed, it sends a signal to the rest of the plant: Mayday! Mayday! I lost my photosynthentic material. A plant that loses its leaves cannot survive long since leaves produce the food that the plant requires.

What does the plant do? It translocates nutrients from other parts to sustain new growth of leaves. But the process does not happen without a cost. When nutrients are taken from roots, the root system suffers and may even die in a prolonged drought. When nutrients are taken from seed development, fewer seeds are produced. These are real costs to the plant. The fact that some plants can tolerate herbivory pressure does not mean they should be grazed.

Many of the studies purporting to support the notion that grasslands need or benefit from livestock or other large herbivory pressure are narrowly defined and reflect economic not ecological interests. The measurement commonly employed in range studies is "above-ground biomass." When a plant is cropped, it produces new shoots so that it can continue to photosynthesize. This leads to greater amounts of aboveground biomass since new young shoots tend to grow at a faster rate than old shoots.

This is what happens in a forest when it is logged. A young tree grows faster than an old, or "decadent," tree. "Overmature" forests do not produce wood as fast as young, "healthy" growing forests. If you are a logger, this is an Important economic consideration. If you are an ecologist, it is a consideration best avoided for the health of species that depend on old-growth forests.

New shoots that develop after a plant is cropped are higher in nitrogen and other nutrients. If you are a rancher, new growth is better than "decadent, overmature" grasses because it supplies more of what cattle need to grow. But again, the new growth comes at a cost, especially if the plant is grazed more than once in a season. The overall biomass of a plant - root system included -- is often reduced when there is significant herbivory pressure. But because below-ground biomass is difficult to measure, most studies are content to document above-ground biomass, thereby presenting a biased perspective on the "benefits" of herbivory.

I remain unconvinced that grasslands - even on the Great Plains - require large herbivory pressure to remain viable. Many relict, ungrazed grassland sites I've visited - from the tops of buttes, sides of cliffs, highway and railroad right-of-ways, where native grasslands exist without significant herbivory pressure from cattle or bison - indicate quite clearly to me that Great Plains grasslands do not require herbivory pressure to thrive. I invite livestock producers and agency range managers to visit these sites and see for themselves.

George Wuerthner is a Western Watersheds Project advisory board member who lives in Richmond, Vermont.