Principles of terrestrial ecosystem ecology.pdf

ecosystems and current climate. Similarly, the
legacies of the current environment will exert
large effects on the future structure and functioning
of ecosystems.
Disturbance
Conceptual Framework
Disturbance is a major cause of long-term fluctuations
in the structure and functioning of
ecosystems. We define disturbance as a relatively
discrete event in time and space that
alters the structure of populations, communities,
and ecosystems and causes changes in
resource availability or the physical environment
(Pickett and White 1985, Pickett et al.
1999). Many natural disturbances, such as
herbivore outbreaks, treefalls, fires, hurricanes,
floods, glacial advances, and volcanic eruptions,
exert these effects through reductions in live
plant biomass or sudden changes in the pool of
actively cycling soil organic matter. Disturbance
is difficult to define unambiguously.
Events such as intensive grazing and subzero
temperatures that are normal features of some
ecosystems seriously disrupt the functioning of
others. Disturbance must therefore be defined
in the context of the normal range of environmental
variation that an ecosystem experiences.
The dividing line between disturbance
and normal function is somewhat arbitrary.
Herbivory, for example, is often treated as part
of the steady-state functioning of ecosystems,
whereas stand-killing insect outbreaks are
treated as disturbances. The processes are
similar, however, and there is a continuum in
size, severity, and frequency between these two
extremes. Disturbance is clearly not an external
event that “happens” to an ecosystem. Like
other interactive controls, disturbance is an
integral part of the functioning of all ecosystems
that responds to and affects most
ecosystem processes. Naturally occurring disturbances
such as fires and hurricanes are
therefore not “bad”; they are normal properties
of ecosystems.
Human activities have altered the frequency
and size of many natural disturbances, such as
Disturbance 285
fires and floods, and have produced new types
of disturbance, such as large-scale logging,
mining, and wars. Many human disturbances
have ecological effects that are similar to those
of natural disturbances, so the study of either
natural or human disturbances provides
insights into the regulation of ecosystem
processes and human impacts on these processes.
Natural and anthropogenic disturbances
interact with environmental gradients to create
much of the spatial patterning in landscapes
(see Chapter 14).
After disturbance, ecosystems undergo
succession, a directional change in ecosystem
structure and functioning resulting from biotically
driven changes in resource supply. Disturbances
that remove live or dead organic matter,
for example, are colonized by plants that gradually
reduce the availability of light at the soil
surface and alter the availability of water and
nutrients (Tilman 1985). If there were no
further disturbance, succession would proceed
toward a climax, the end point of succession
(Clements 1916). At this point, the structure
and rates of ecosystem processes approach
a steady state in which resource demand by
vegetation would be balanced by the rate of
resource supply. In practice, however, new disturbances
usually occur before succession
reaches a climax, so individual stands of an
ecosystem are seldom in steady state. Nonetheless,
the concept of directional changes in
vegetation after disturbance provides a useful
framework for analyzing the role of disturbance
in ecosystem processes.
Succession occurs in response to biotically
driven changes in resource supply, which
typically occur over time scales of years to
centuries. Succession does not therefore
include the seasonal fluctuations in ecosystem
processes from summer to winter, which are
driven more directly by climate than by the
internal dynamics of ecosystems.
Disturbance Properties
The impact of disturbance on ecosystem processes
depends on its severity, frequency, type,
size, timing, and intensity. Together these attributes
of disturbance constitute the disturbance