Geoscience Reference
In-Depth Information
20
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Figure 25.6
Theoretical example of how equilibrium of ecosystem processes is most
commonly observed at medium temporal scales (e.g., decades). In this case, theoretical
trends in average summer temperatures of an ecosystem are shown over years, decades
and millennia. Non-equilibrium is observed over years, due to annual fl uctuations in
weather conditions and climatic variations. When these variations are recorded over
decades, an approximate equilibrium (average temperature) can be observed. Over
millennia however, broad changes in (for example) climate creates notable variation
in summer temperatures, so that observations once again revert to non-equilibrium.
Ecosystem management is often conducted on a short-term basis with equilibrium
conditions assumed, despite these mainly being appropriate only at larger scales.
ecosystem, and community composition is relatively unchanging. Although the term
'steady state' is used here, it is apparent that some level of dynamism would still be
exhibited by the ecosystem, but only at very small spatial and temporal scales.
In practice, the concept of 'steady state' equilibrium is linked directly to the
spatial and temporal scale considered (see Sayre, this volume). At 'medium' spatial
and temporal scales, which may be based around anthropocentric space and time-
scales (e.g., hectares of land, decades to centuries), the equilibrium concept remains
valid; ecosystems do exhibit predictable changes and discrete stages observable over
human lifetimes, leading up to a relatively 'stable state' ecosystem as given in fi gure
25.6. At fi ner spatial and temporal scales, equilibrium in biotic and abiotic patterns
and processes is not always so apparent. Consider, for example, gap analysis of
woodlands. If a tree dies, a new biological community and range of physical pro-
cesses will occur within the relatively small space the tree previously occupied, which
may operate at different rates to those in adjacent habitat. Likewise, at very broad
spatial and temporal scales (centuries to millennia) ecosystems are likely to change
according to broader changes in climate, geomorphological processes, and increased
