Environmental Engineering Reference
In-Depth Information
of the richness that remains to be accomplished in the study of temporal heterogeneity
and ecosystems.
A final thought is necessary concerning the complexity of ecological events. What
counts as an ecologically relevant event, and how the events are scaled, depends on the
model of the system adopted by investigators. For one system model some environmental
event may appear as a pulse, while for another system model, the same event might
appear as a press. For example, a series of drought years may be represented in a long-
term forest model as a pulse, while for a model of a salamander population in a vernal
pool, the same drought may appear as a press. Therefore, the concepts of pulse and press,
as well as the more complete lexicon of event onsets, persistence, and releases, are not
absolutes. Rather, they are determined by the scale—grain and extent—used in models of
particular systems.
INTERNAL AND EXTERNAL
HETEROGENEITY
The second dimension along which to consider the effects of heterogeneity identifies
whether the heterogeneity of interest is inside or outside the focal ecosystem. Of course, an
ecosystem study may well investigate both internal and external sources and effects of het-
erogeneity. These two, like temporal and spatial heterogeneity, are not mutually exclusive.
Internal heterogeneity is exemplified by such things as the pits and mounds within
individual forest stands mentioned earlier, or the spatial arrangement of cut versus intact
Onset
Duration
Release
Abrupt
Gradual
Short
Long
Slow
Fast
None
FIGURE 10.12
A conceptual alphabet illustrating the complexity of ecological events not represented by the
simple pulse
press contrast. For each graph, the
x
-axis represents time and the
y
-axis represents the intensity of
the event or of the effect. The top row shows different forms of onset, duration, or release of an ecological event,
such as disturbance or imposition of stress. Specific real events can be described by combining the appropriate
forms of onset, duration, and release. The bottom row indicates aspects of complexity in the responses to ecologi-
cal events. Effects can be rapidly emerging, slowly emerging, short persistence, long persistence, gradually declin-
ing, or quickly declining. In the lower right panel, the existence of a peaked form of effect is shown by combining
rapid emergence, no persistence, and rapid decline. This panel further illustrates the difference between immedi-
ately felt and lagged effects. The immediate effect arises directly after the event, and lagged effects are illustrated
by the dashed line connecting the event with the right, delayed peak. Lags may exist for effects possessing any of
the other combinations of emergence, duration, and decline as well.
(Figure courtesy of the Baltimore Ecosystem
Study, Long-Term Ecological Research Project.)
