Principles of terrestrial ecosystem ecology.pdf

14 1. The Ecosystem Concept
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F
Predator
E
Herbivore
D
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-
Plant A Plant B
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A - - B
Process Nature of
feedback
Resource uptake A -
Competition A+B -
Mutualism C +
Herbivory D -
Predation E -
Population growth F +
Shared resources
Mycorrhizal
fungus
Figure 1.4. Examples of linked positive and negative
feedbacks in ecosystems. The effect of each
organism (or resource) on other organisms can be
positive (+) or negative (-). Feedbacks are positive
when the reciprocal effects of each organism (or
resource) have the same sign (both positive or both
negative). Feedbacks are negative when reciprocal
effects differ in sign. Negative feedbacks resist the
tendencies for ecosystems to change, whereas positive
feedbacks tend to push ecosystems toward a new
state. (Modified with permission from American Naturalist,
Vol. 148 © 1996 University of Chicago Press,
Chapin et al. 1996.)
climate (Fig. 1.5) (Vitousek et al. 1997c). At
least some of these anthropogenic (i.e., humancaused)
effects influence all ecosystems on
Earth.
The most direct and substantial human alteration
of ecosystems is through the transformation
of land for production of food, fiber, and
other goods used by people. About 50% of
Earth’s ice-free land surface has been directly
altered by human activities (Kates et al. 1990).
Agricultural fields and urban areas cover 10 to
15%, and pastures cover 6 to 8% of the land.
Even more land is used for forestry and grazing
systems. All except the most extreme environments
of Earth experience some form of direct
human impact.
Human activities have also altered freshwater
and marine ecosystems. We use about
C
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half of the world’s accessible runoff (see
Chapter 15), and humans use about 8% of the
primary production of the oceans (Pauly and
Christensen 1995). Commercial fishing reduces
the size and abundance of target species and
alters the population characteristics of species
that are incidentally caught in the fishery. In the
mid-1990s, about 22% of marine fisheries
were overexploited or already depleted, and an
additional 44% were at their limit of exploitation
(Vitousek et al. 1997c). About 60% of the
human population resides within 100km of a
coast, so the coastal margins of oceans are
strongly influenced by many human activities.
Nutrient enrichment of many coastal waters, for
example, has increased algal production and
created anaerobic conditions that kill fish and
other animals, due largely to transport of nutrients
derived from agricultural fertilizers and
from human and livestock sewage.
Land use change, and the resulting loss of
habitat, is the primary driving force causing
species extinctions and loss of biological diversity
(Sala et al. 2000a) (see Chapter 12). The
time lag between ecosystem change and species
loss makes it likely that species will continue to
be driven to extinction even where rates of land
use change have stabilized. Transport of species
around the world is homogenizing Earth’s
biota. The frequency of biological invasions
is increasing, due to the globalization of the
economy and increased international transport
of products. Nonindigenous species now
account for 20% or more of the plant species
in many continental areas and 50% or more of
the plant species on many islands (Vitousek
et al. 1997c). International commerce breaks
down biogeographic barriers, through both
purposeful trade in live organisms and inadvertent
introductions. Purposeful introductions
deliberately select species that are likely
to grow and reproduce effectively in their
new environment. Many biological invasions
are irreversible because it is difficult or
prohibitively expensive to remove invasive
species. Some species invasions degrade
human health or cause large economic losses.
Others alter the structure and functioning of
ecosystems, leading to further loss of species
diversity.