Principles of terrestrial ecosystem ecology.pdf

13
Temporal Dynamics
Ecosystem processes constantly change in response to variation in environment over
all time scales. This chapter describes the major patterns and controls over the temporal
dynamics of ecosystems.
Introduction
Ecosystems are always recovering from past
changes. In earlier chapters we emphasized
ecosystem responses to the current environment.
Ecosystems are, however, always responding
to past changes that have occurred
over all time scales (Holling 1973, Wu and
Loucks 1995). These changes include relatively
predictable daily and seasonal variations, less
predictable changes in weather (e.g., passage
of weather fronts, El Niño events, and glacial
cycles), and occurrence of disturbances (e.g.,
tree falls, herbivore outbreaks, fires, and volcanic
eruptions). Consequently, the behavior of
an ecosystem is always influenced by both the
current environment and many previous environmental
fluctuations and disturbances.
The global environment is changing more
rapidly than it has for millions of years. These
changes result from an exponentially rising
human population that shows an everyincreasing
technological capacity to alter
Earth’s environment and ecosystems. Perhaps
the most urgent need in ecosystem ecology is to
improve our understanding of factors governing
the stability and change in ecological
systems (Box 13.1). This understanding is critical
to managing ecosystems so they sustain
their diversity and other important ecological
attributes and so ecosystems continue to
produce the goods and services required by
society. This chapter addresses the basis of the
temporal dynamics of ecosystems.
Fluctuations in
Ecosystem Processes
Interannual Variability
Ecosystem processes measured in 1 year are
seldom representative of the long-term mean.
Many ecosystem processes are sensitive to
interannual variations in weather and to fluctuations
in the internal dynamics of ecosystems,
such as outbreaks of herbivores or pathogens.
The same ecosystem, for example, can change
from being a carbon source in one year to a
carbon sink in the next (Goulden et al. 1998).
The production at a particular trophic level can
change from being limited by food to being
limited by predation. Some of the interannual
variability in ecosystem dynamics reflects
processes that are potentially predictable, such
as the cyclic variation in climate. El Niño events
are a consequence of large-scale oscillations in
the global ocean-atmosphere system that recur
every 2 to 10 years (see Chapter 2). These are
associated with relatively repeatable climatic
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