532 MHR • Unit 5 Population Dynamics
Fertile Soil
Humans alter the landscape for a variety of purposes.
We build dams in river valleys to produce electrical
power, strip mountain tops or dig enormous holes
to extract minerals or coal, cover the land with
buildings and pavement, and cut down forests for
lumber or to provide growing space for crops. All
of these activities have an impact on the soil
required to produce our food supply, as does
agriculture itself. In 1998, United Nations experts
estimated that one sixth of the world’s land area
(nearly two billion hectares) is now degraded as a
result of overgrazing and poor farming practices.
Desertification, the transformation of marginal dry
lands into near-deserts, which are unsuitable for
agriculture, is a serious problem in many parts of
the world, as shown in Figure 15.22. This is
particularly true in Africa where the Sahara desert
is expanding, but also in parts of the southwestern
United States (for example, Mojave desert areas)
and India (along the Great Indian desert).
Desertification can be reversed if it has not gone
too far, but only slowly, at great expense, and
with great effort.
Deforestation also has disastrous effects on soil
quality, particularly in tropical regions, but also in
North America, including all of the Canadian
provinces. Some experts estimate that at least
40 percent of tropical rain forests have already been
lost and that they are currently disappearing at the
rate of roughly 400 km^2 per day. These forests are
sometimes logged to supply lumber, but are often
cleared simply to get at the mineral deposits
beneath them. Commonly, they are cut down to
obtain land for agriculture. In many cases, the
fields that result are only fertile for 10 to 20 years
or less. This is partly because, as you learned in
Chapter 13, the soil in tropical rain forests is poorin nutrients. In addition, abundant rain in these
areas causes erosion in cleared areas (particularly
on steep slopes), and mudslides sweep crops and
soil into the ocean. In Madagascar and Haiti, for
example, soil erosion following deforestation is so
severe that thousands of tonnes of soil wash into
the sea each year.Figure 15.22What do you think would be required to
restore this land to a useable state?Biodiversity
All of the human impacts on renewable resources
already described have serious consequences for
other species as well. In other words, air and water
pollution and soil erosion threaten all types of
organisms. In some cases, the effect of humans has
been very direct: over-exploitation of many fish,
mammal, and bird populations for food, clothing,
and other uses has put many species at risk. This
has caused the extinction(complete disappearance
from Earth of all members of a species) of some, and
the extirpation(disappearance of a species from
areas that were once part of its range) of others. In
most cases, however, human impact on other
species has occurred mainly as a result of habitat
alteration, which reduces the ability of organisms
adapted to that habitat to survive and/or reproduce.
As you saw in Chapter 12, extinction is a natural
process and is constantly occurring. At various
times in the history of Earth, there have been mass
extinctions, in which large numbers of species
died out. The period when most of the dinosaurs
and many other species, especially marine forms,
disappeared is one of these. Many researchers,In 2001, a group of pre-eminent scientists, writing in the
journal Science, predicted that a rapid expansion of
agriculture would occur over the next 50 years, driven by
the demands of a wealthier and larger human population.
This expansion would involve the conversion of 10 billion
hectares of natural ecosystems to agriculture, and would
increase the amount of nitrogen and phosphorus added to
water by 2.4 and 2.7 times respectively. They predicted that
this expansion, and the additional pesticide use that would
occur, could cause massive damage to aquatic ecosystems
and result in the extinction of many species.