Page 251 - Environment: The Science Behind the Stories

Worldwide, overgrazing causes as much soil degra-
dation as cropland agriculture does, and it causes more
desertification. Humanity tends more than 3.4 billion cattle,
sheep, and goats. Degraded rangeland costs an estimated
$23 billion per year in lost productivity. Grazing exceeds
the sustainable supply of grass in India by 30% and in parts
of China by up to 50%. To relieve pressure on rangelands,
both nations are now beginning to feed crop residues to
livestock.
Range managers in the United States assess the carrying
capacity of rangelands and inform livestock owners of these
limits, so that herds are rotated from site to site as needed to
conserve grass cover and soil integrity. Managers also can
establish and enforce limits on grazing on publicly owned
land. Yet U.S. ranchers have traditionally had little incentive
to conserve rangelands because most grazing has taken place
on public lands leased from the government and because U.S.
taxpayers have heavily subsidized grazing. As a result of this
classic “tragedy of the commons” situation (p. 183), overgraz-
Figure 9.19 Overgrazing occurs when livestock eliminate ing has resulted in extensive environmental impacts across the
the vegetation that covers the soil. American West.
Today increasing numbers of ranchers are working coop-
eratively with government agencies, environmental scientists,
and even environmental advocates to find ways to raise live-
impeding plant regrowth and preventing the replacement of stock more sustainably and safeguard the health of the land
biomass, the result is overgrazing (Figure 9.19). (see THe SCieNCe beHiND THe STOrY, pp. 252–253).
When livestock remove too much plant cover, soil is
exposed and made vulnerable to erosion. In a positive feed-
back cycle (pp. 124–125), soil erosion makes it difficult for
vegetation to regrow, a problem that perpetuates the lack of Watering and Fertilizing Crops
cover and gives rise to more erosion (Figure 9.20). Moreo-
ver, non-native weedy plants that are unpalatable to live- For as long as humanity has been farming, we have provided
stock may invade and outcompete native vegetation in the our crops with supplemental water and nutrients in order to
new, modified environment. Too many livestock trampling boost production. In the modern age of industrial agriculture,
the ground can also compact soils and alter their structure. the large-scale and mechanized provision of extra water and
Soil compaction makes it more difficult for water to infil- nutrients has raised our productivity to unforeseen heights,
trate, for soils to be aerated, for plants’ roots to expand, and allowing us to feed a world of 7 billion people. However, these
for roots to conduct cellular respiration (p. 50). All of these practices have also contributed to soil degradation and land
effects further decrease the growth and survival of native degradation, and can affect ecosystems and people far away
plants. from farm fields.

Figure 9.20 Overgrazing has
Overgrazing
ecological consequences. When
grazing by livestock exceeds the
carrying capacity of rangelands and
Compacts soil and Removes native their soil, this can set in motion a
damages structure grass
series of consequences and positive
feedback loops that degrade soils
and grassland ecosystems.
Decreases water Decreases Invasive species Exposes
infiltration aeration gain foothold and bare topsoil What is the immediate
outcompete natives in cause of exposure of bare
altered environment topsoil? What is the immediate
consequence of exposing bare
Decreases grass Wind and topsoil? How many immediate
growth and survival water erosion consequences of wind and water
250 erosion are shown in this diagram?

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