Environmental Engineering Reference
In-Depth Information
bacterial populations and led to genetic resis-
tance to these chemicals.
Fourth, we have
eliminated some preda-
tors.
Some ranchers want to eradicate bison
or prairie dogs that compete with their
sheep or cattle for grass. They also want to
eliminate wolves, coyotes, eagles, and other
predators that occasionally kill sheep. A few
big-game hunters have pushed for elimina-
tion of predators that prey on game species.
This simplifies and disrupts natural ecosys-
tems and food webs.
Fifth, we have
deliberately or accidentally
introduced new or nonnative species into com-
munities.
Most of these species, such as food
crops and domesticated livestock, are bene-
ficial to us but a few are harmful to us and
other species.
Sixth, we have
used some renewable
resources faster than they can be regenerated.
Ranchers and nomadic herders sometimes
allow livestock to overgraze grasslands until
erosion converts these communities to less
productive semideserts or deserts. Farmers
sometimes deplete soil of its nutrients by ex-
cessive crop growing. Some fish species are
overharvested. Illegal hunting or poaching
endangers wildlife species with economi-
cally valuable parts such as elephant tusks,
rhinoceros horns, and tiger skins. In some areas, fresh
water is being pumped out of underground aquifers
faster than it is being replenished.
Seventh, some human activities
interfere with the
normal chemical cycling and energy flows in ecosystems.
Soil nutrients can erode from monoculture crop fields,
tree plantations, construction sites, and other simpli-
fied communities, and then overload and disrupt
other communities such as lakes and coastal waters.
Our inputs of carbon dioxide into the carbon cycle
have been increasing sharply (Figure 3-26, p. 56)—
mostly from burning fossil fuels and from clearing and
burning forests and grasslands. This and other inputs
of greenhouse gases from human activities can trigger
global climate change by altering energy flow through
the troposphere. The human input of nitrogen into the
nitrogen cycle exceeds the earth's natural input (Fig-
ure 3-28, p. 58). We are also altering energy flows
through the biosphere by releasing chemicals into the
atmosphere that can increase the amount of harmful
ultraviolet energy reaching the troposphere by reduc-
ing ozone levels in the troposphere.
Eighth, while most natural systems run on sun-
light,
human-dominated ecosystems have become increas-
ingly dependent on nonrenewable energy from fossil fuels.
Fossil fuel systems typically produce pollution, add
Property
Natural
Systems
Human-
Dominated
Systems
Complexity
Biologically diverse
Biologically
simplified
Energy source
Renewable solar
energy
Mostly
nonrenewable fossil
fuel energy
Waste production
Little, if any
High
Nutrients
Recycled
Often lost or wasted
Net primary
productivity
Shared among many
species
Used, destroyed, or
degraded to support
human activities
Figure 6-16
Some typical characteristics of natural and human-dominated
systems.
To survive and provide resources for growing num-
bers of people, we have modified, cultivated, built
on, or degraded a greatly increasing number and area
of the earth's natural systems. Excluding Antarctica,
our activities have, to some degree, directly affected
about 83% of the earth's land surface. See Figures 2
and 3 in Science Supplement 2 at the end of this topic.
Figure 6-16 compares some of the characteristics of
natural and human-dominated systems.
We have used technology to alter much of the rest
of nature to meet our growing needs and wants in eight
major ways (Figure 6-17). First, we have
reduced bio-
diversity by destroying, fragmenting, degrading, and sim-
plifying wildlife habitats.
We have cleared forests, dug up
grasslands, and filled in wetlands to grow food or to
construct buildings, highways, and parking lots. This
represents a loss of overall biodiversity and a degrada-
tion of the earth's natural capital.
Second, we have
used, wasted, or destroyed an in-
creasing percentage of the earth's net primary productivity
that supports all consumer species (including humans).
This type of alteration is the main reason we are
crowding out or eliminating the habitats and food
supplies of a growing number of species.
Third, we have unintentionally
strengthened some
populations of pest species and disease-causing bacteria.
Our
overuse of pesticides and antibiotics has speeded up
natural selection among rapidly reproducing pest and
