Environmental Engineering Reference
In-Depth Information
disturbances of geological substrates are rare or absent, substrates and soils may become
quite old. Such old substrates, those on the order of several hundreds of thousands to mil-
lions of years since major geologic disruption, also differ based on age, just as they do in
shorter successions. In both short and ancient successions, the interaction of the substrate
and the organisms, modified by any input of nutrients or stress factors from elsewhere,
determines the current biogeochemistry of the ecosystem (
Vitousek 2004
).
Past human actions can generate heterogeneity that determines an ecosystem's current
metabolism. Human management adds or subtracts species from ecosystems, shifts the rel-
ative proportions and distributions of species, or adds or removes substrate and the nutri-
ents it contains. Alternatively, management can add or remove disturbance or stress
agents, or change the timing of disturbances. All of these kinds of human actions can have
persistent effects, and their effects can change in intensity through time. Indeed, the action
or management may be long past but still influence the current behavior of ecosystems
(
Foster et al. 2003
). The past can echo into the present.
Other patterns of temporal heterogeneity and variation are relevant to ecosystems. Some
temporal changes are not directional. Stochastic variation in weather and climate are examples
of this sort of time dynamic. Wet versus dry years can appear unpredictably. Stream flow,
migration or extirpation of populations, and the shift from denitrifying to nitrifying conditions
can follow as the result of interposition of a very wet or dry year in riparian systems (
Stanley
and Fisher 1992
). Wet versus dry years will experience different kinds of disturbances—for
example, drought mortality, flooding, landslides, or fire.
Still longer-term shifts also occur in climate. Of course, the current anthropogenic shift
in climate regimes is an example, with projected alterations in the distribution of tempera-
ture extremes and rainfall patterns. Both of these are major drivers of ecosystem function.
Climate shifts likely will alter disturbance patterns as well because they change the energy
and intensity of weather events, as well as the susceptibility of ecosystems through their
effects on composition, productivity, and architecture.
Long climate excursions illustrate coarse-scale temporal heterogeneity (
McKinzey et al.
2005
). The “Little Ice Age” affected Europe and North America from at least 1650 to the
middle of the nineteenth century. An average decrease in annual temperature of 1
C led
to extreme winters, advance of mountain glaciers, and changes in success and then loca-
tion of farming, and finally, shifts in human population distribution in some areas. Given
the potential for persistent effects seen in long-lived trees and soils, such climate excur-
sions are expected to have perceptible ecosystem signals. Climate scientists have been able
to isolate the effects of this climate shift from the currently ongoing anthropogenic phase
of global climate change (
Jones and Mann 2004
).
Conceptual Refinements for Temporal Heterogeneity: Pulses and Presses
When considering temporal heterogeneity there are important refinements to consider.
Temporal heterogeneity is recognized through the occurrence of ecologically relevant
events. The mere passage of time is not really an environmental cause. It can be a useful
proxy variable representing environmental mechanisms, but it is important to discover the
actual causes or events that have ecological outcomes. Unpacking and operationalizing the
