Environmental Engineering Reference
In-Depth Information
concept of disturbance contributes to refining the understanding of temporal heterogeneity
(
Peters et al. 2011
).
Refinement of translating time into actual events is facilitated by differentiating pulses
from presses in ecological systems (
Glasby and Underwood 1996
). A pulse event is one
that has a sudden onset, relative to the time window (or grain) and time span (or extent)
under consideration. Pulse events also end soon after their onset. They may either repeat
or not, and if they repeat, they may be irregular or cyclical. The specific events may
include disturbance, in the strict sense of an alteration of the three-dimensional structure
of an ecological system or part of the system, or the events may be stresses. Stresses, in
contrast to disturbances that affect structure, directly affect the metabolism or functioning
of systems. Stresses may be caused by introduction of toxins or by shift of environmental
regulating factors to extremely high or low levels. Temperature and moisture are two of
the most common environmental regulators that can cause stress when they reach extreme
levels, and hence, generate heterogeneity in time.
Press events contrast with pulse events (
Figure 10.11
). Press events, once instigated,
remain in place for a long time. A dry year is a pulse event, while climate shift to a differ-
ent regime of temperature, moisture, or seasonality is a press event. Pulse events may be
avoided by certain organisms, while press events can be avoided only by those with per-
sistent dormant stages or ability to migrate. The complexity of ecosystem response to a
mixture of press and pulse events can be considerable. It is important to recognize that
ecological events are characterized by several things, and that pulse and press identify
only a small subset of the logically complete set of kinds of ecological events (
Reiners
2005
). Ecological events are characterized by the relative suddenness of their onset, the
length of their persistence, the suddenness or gradualness of their disappearance, and
their effects (
Figure 10.12
). The effects of ecological events can also be temporary, persis-
tent, or may reappear after a long dormant interval. So a more complete terminology
might include pulses versus presses, long- versus short-lived, slow versus fast releasing,
and effects that are immediate, temporary, slow, persistent, or echoes. A taxonomy of
events through time has not been explored in ecosystem ecology, and suggests something
Pulse
Press
Time
FIGURE 10.11
Diagrammatic representation of the contrast between pulse and press events. Pulse events,
whatever the form of their onset, are relatively quick to appear as well as quick to dissipate, though they may
have persistent effects. Press events, in contrast, represent persistent changes in the driving factors or structure of
an ecological system. Pulse events are exemplified by fires, hurricanes, and floods. Press events include such
things as climate shifts, persistent environmental stressors, the exposure of new surfaces by Earth movements, or
the invasion of new dominant organisms into a region.
