Essentials of Ecology

232 CHAPTER 10 Sustaining Terrestrial Biodiversity: The Ecosystem Approach

to hold water). Overgrazing also enhances invasion by

species such as sagebrush, mesquite, cactus, and cheat-

grass, which cattle will not eat.

Scientists have also learned that, before settlers

made them into rangeland, natural grassland ecosys-

tems were maintained partially by periodic wildfires

sparked by lightning. Fires were important because they

burned away mesquite and other invasive shrubs, keep-

ing the land open for grasses. Ecologists have studied

grasslands of the Malpai Borderlands—an area on the

border between the southwestern U.S. states of Arizona

and New Mexico—where ranchers, with the help of

the federal government, not only allowed overgrazing

for more than a century, but also fought back fires and

kept the grasslands from burning. Consequently, trees

and shrubs replaced grasses, the soil was badly eroded,

and the area lost most of its value for grazing.

Since 1993, ranchers, scientists, environmental-

ists, and government agencies have joined forces to

restore the native grasses and animal species to the

Malpai Borderlands. Land managers conduct periodic

controlled burns on the grasslands, and the ecosystem

has now been largely reestablished. What was once a

classic example of unsustainable resource management

became a valuable scientific learning experience and a

management success story.

About 200 years ago, grass may have covered nearly

half the land in the southwestern United States. Today,

it covers only about 20%, mostly because of a combi-

nation of prolonged droughts and overgrazing, which

created footholds for invader species that now cover

many former grasslands.

Limited data from FAO surveys in various coun-

tries indicate that overgrazing by livestock has caused

as much as a fifth of the world’s rangeland to lose pro-

ductivity. Some grasslands suffer from undergraz-

ing, where absence of grazing for long periods (at least

5 years) can reduce the net primary productivity of

grassland vegetation and grass cover.

We Can Manage Rangelands

More Sustainably

The most widely used method for more sustainable

management of rangeland is to control the number of

grazing animals and the duration of their grazing in a

given area so that the carrying capacity of the area is

not exceeded (Concept 10-3). One way of doing this is

rotational grazing in which cattle are confined by por-

table fencing to one area for a short time (often only

1–2 days) and then moved to a new location.

Livestock tend to aggregate around natural water

sources, especially thin strips of lush vegetation along

streams or rivers known as riparian zones, and around

ponds established to provide water for livestock. Over-

grazing by cattle can destroy the vegetation in such

areas (Figure 10-21, left). Protecting overgrazed land

from further grazing by moving livestock around and by

fencing off these areas can eventually lead to its natural

ecological restoration (Figure 10-21, right). Ranchers

can also move cattle around by providing supplemental

feed at selected sites and by strategically locating water

holes and tanks and salt blocks.

A more expensive and less widely used method of

rangeland management is to suppress the growth of un-

wanted invader plants by use of herbicides, mechanical

Figure 10-20Natural capital
degradation: overgrazed (left) and
lightly grazed (right) rangeland.

USDA, Natural Resources Conservation Service