Geography Reference
In-Depth Information
ivory tusks. From historical records we know that over
650 species of plants and over 480 animal species have
become extinct in just the last 400 years. These represent
only the documented extinctions. The actual number of
extinctions that occurred during this period is almost cer-
tainly much higher.
Humans have also indirectly contributed to extinc-
tions. Human travel, for instance, introduced new spe-
cies to areas around the globe—rats are among the more
destructive of these; they have had devastating effects on
oceanic islands. Introduced species may cause extinctions
by preying upon native species or competing with them
for resources. A famous example is the dodo bird (
Raphus
cuculatus
), which was hunted to extinction by humans,
dogs, and rats on the island of Mauritius. Introduced spe-
cies may also carry new diseases, leading to the decimation
and extinction of local populations. Species on islands are
particularly susceptible to extinction because of the more
insular ecosystems found on islands. An estimated 2000
species of birds on tropical Pacifi c Islands became extinct
following human settlement.
Identifying the nature and extent of environmen-
tal changes is only a fi rst step toward understanding the
extent of human alteration of the planet. A second, and
more complicated step is to consider the forces driving
these changes.
in the human population, a fourfold increase (from about
1.5 billion to 6 billion) in the twentieth century alone. Per
capita consumption of virtually everything, from water
and farm products to metals and energy resources, has
increased rapidly as well. Fast-developing technologies
allow us to alter the natural environment ever more. Each
of these factors can be studied at all scales from global
to local.
Global environmental changes express themselves at
all scales, from local to global. For example, deforestation
can have a local effect by reducing the diversity of spe-
cies in a particular area. It can have regional impacts by
increasing sediment runoff into rivers. Finally, at a global
scale, deforestation is associated with the release of carbon
dioxide into the atmosphere and may affect global climate
by altering processes that occur at the land surface. To
fully understand deforestation, we must keep all of these
scales in mind.
Several interrelated factors are responsible for the
expanding impact of humans on the environment over
the past two centuries. One of these factors is the dra-
matic growth of the human population. Even consider-
ing the minimal needs for human survival, there can be
little doubt that the fourfold increase in the human popu-
lation in the twentieth century had signifi cant environ-
mental impacts. Another factor is consumption, which has
increased dramatically in the modern world. Yet another
is technology, which has both expanded the human capac-
ity to alter the environment and brought with it increas-
ing energy demands. Each of these interrelated factors
that contribute to environmental change can be studied
broadly, focusing on the general impacts of each factor on
the global environment. Yet, when we shift scales to the
local and regional, and we consider the context of human
actions at these scales, we often fi nd that the causes of
environmental change vary depending on the local and
regional context.
What is the greatest environmental concern facing the
region where you live, and in what other regions of the world
is that concern also present? How do differences between
your region and the other regions sharing the concern infl u-
ence how it is understood and approached?
Political Ecology
Leslie Gray and William Moseley describe the fi eld of
political ecology, beginning in the 1960s and 1970s, as a
way of considering the roles of “political economy, power
and history in shaping human-environmental interac-
tions.” Political ecologists use scale to consider how
attempts to affect environmental change, such as defores-
tation, differ depending on the level of spatial detail used
to examine the issue.
At the local scale, Moseley has studied the conserva-
tion behaviors of farmers in southern Mali (Fig 13.15). He
found that one view that is widely held at the global scale—
that poorer people degrade the land more than wealthier
people—was not true at the local scale. Through exten-
sive fi eldwork, interviews, and soil surveying, Moseley
WHAT ARE THE MAJOR FACTORS
CONTRIBUTING TO ENVIRONMENTAL
CHANGE TODAY?
Environmental change occurs at all levels of scale,
from local to global. For example, deforestation has local
effects by reducing the diversity of species even in a small
area. It has regional consequences by increasing sediment
runoff into streams and rivers. And, globally, it results in
the increased release of carbon dioxide in the atmosphere,
which affects planetary climates.
Several interrelated factors infl uence the escalating
impact of human activities on the natural environment.
The past two centuries have witnessed dramatic growth
