A trifecta of forces now threatens all of these goods and services. The first is global climate change: As temperatures rise, scientists expect that shifting rainfall patterns will help push many current grasslands—including vast swaths of the great American prairies—into desert. The second threat involves human land-use decisions: Rising global populations will force us to convert more land into cities and suburbs for living and into croplands for food. Their wide, flat expanses and nutrient-rich soil make grasslands ideal for both. The third, and many say largest, threat that grasslands face is the one posed by overgrazing.

Ironically, grazing is normally very good for grasslands. Because wild herbivores evolved to subsist on grasslands alone, both grazers and grasses are well adapted to grazing. Grasses can grow from the base upward, so by clipping off the top part of the blade, herbivores expose new growth shoots to the sunlight, thus stimulating the plants’ growth. By breaking up the soil with their hooves, they allow water to penetrate the ground and enable seeds to germinate and take root. And as they defecate and urinate, large grazing mammals return nutrients such as nitrogen and phosphorus to the soil in a form that plants can easily absorb.

But when grass is overgrazed, or chewed down to its roots, the growth area on the blade is destroyed, the blade can no longer regenerate, and the plant eventually dies. When too many plants die at once, the soil has nothing to hold it in place. And when too many large grazing animals stomp their hooves directly onto the soil, the soil becomes compacted, which makes it harder for water to penetrate, seeds to germinate, or seedlings to grow. INFOGRAPHIC 27.3

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