Principles of terrestrial ecosystem ecology.pdf

and alters patterns of vegetation development
(Bradshaw 1983). Invasion by exotic grasses
can alter fire frequency, resource supply, trophic
interactions, and rates of most ecosystem
processes (D’Antonio and Vitousek 1992).
Elimination of predators by hunting can cause
an outbreak of deer that overbrowse their food
supply. The types of species present in an
ecosystem depend strongly on other interactive
controls (see Chapter 12), so functional types
respond to and affect most interactive controls
and ecosystem processes.
Human activities have an increasing impact
on virtually all the processes that govern ecosystem
properties (Vitousek 1994a). Our actions
influence interactive controls such as water
availability, disturbance regime, and biotic
diversity. Humans have been a natural component
of many ecosystems for thousands of years.
Since the Industrial Revolution, however, the
magnitude of human impact has been so great
and so distinct from that of other organisms that
the modern effects of human activities warrant
particular attention. The cumulative impact of
human activities extend well beyond an individual
ecosystem and affect state factors such as
climate, through changes in atmospheric composition,
and potential biota, through the introduction
and extinction of species. The large
magnitude of these effects blurs the distinction
between “independent” state factors and interactive
controls at regional and global scales.
Human activities are causing major changes in
the structure and functioning of all ecosystems,
resulting in novel conditions that lead to new
types of ecosystems. The major human effects
are summarized in the next section.
Feedbacks analogous to those in simple physical
systems regulate the internal dynamics of
ecosystems. A thermostat is an example of a
simple physical feedback. It causes a furnace to
switch on when a house gets cold. The house
then warms until the thermostat switches the
furnace off. Natural ecosystems are complex
networks of interacting feedbacks (DeAngelis
and Post 1991). Negative feedbacks occur when
two components of a system have opposite
effects on one another. Consumption of prey by
a predator, for example, has a positive effect on
the consumer but a negative effect on the prey.
The negative effect of predators on prey pre-
Human-Caused Changes in Earth’s Ecosystems 13
vents an uncontrolled growth of a predator’s
population, thereby stabilizing the population
sizes of both predator and prey. There are also
positive feedbacks in ecosystems in which both
components of a system have a positive effect
on the other, or both have a negative effect on
one another. Plants, for example, provide their
mycorrhizal fungi with carbohydrates in return
for nutrients. This exchange of growth-limiting
resources between plants and fungi promotes
the growth of both components of the symbiosis
until they become constrained by other
factors.
Negative feedbacks are the key to sustaining
ecosystems because strong negative feedbacks
provide resistance to changes in interactive
controls and maintain the characteristics of
ecosystems in their current state. The acquisition
of water, nutrients, and light to support
growth of one plant, for example, reduces availability
of these resources to other plants,
thereby constraining community productivity
(Fig. 1.4). Similarly, animal populations cannot
sustain exponential population growth indefinitely,
because declining food supply and
increasing predation reduce the rate of population
increase. If these negative feedbacks
are weak or absent (a low predation rate due
to predator control, for example), population
cycles can amplify and lead to extinction of one
or both of the interacting species. Community
dynamics, which operate within a single ecosystem
patch, primarily involve feedbacks
among soil resources and functional types of
organisms. Landscape dynamics, which govern
changes in ecosystems through cycles of disturbance
and recovery, involve additional
feedbacks with microclimate and disturbance
regime (see Chapter 14).
Human-Caused Changes in
Earth’s Ecosystems
Human activities transform the land surface,
add or remove species, and alter biogeochemical
cycles. Some human activities directly affect
ecosystems through activities such as resource
harvest, land use change, and management;
other effects are indirect, as a result of changes
in atmospheric chemistry, hydrology, and