Environmental Management: Principles and practice

Source: Adapted from Van Dyne (1969:337, Fig. 7)

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CHAPTER NINE
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Also, humans often upset regulatory mechanisms of natural ecosystems, so response
is distorted. When ecosystems are exposed to stress, some responses may be
immediate, others delayed, perhaps for decades. To manage ecosystems effectively
it is necessary to know longer term behaviour as well as short-term response. This
means palaeoecology and historical records have a part to play, as has long-term
monitoring.
Ecosystems adjust to perturbation through regulatory mechanisms. When the
relationship between input and output to the system is inverse (e.g. increased sunlight
causes more cloud which reduces the impact of that sunlight on the surface), it is
termed a negative feedback. The opposite is a positive feedback, whereby an effect
is magnified. There is a risk that a positive feedback might result in a runaway reaction
affecting crucial biogeochemical or biogeophysical cycles—environmental
management must watch for such threats.
Prompted by Darwin’s work on evolution, it was hypothesized (notably by
W.M.Davis) that, under conditions of constant climate and sea-level,
geomorphological processes would slowly evolve a steady-state landscape
(geomorphologists now argue that similar earthforms in comparable environments
might not follow so predictable an evolution, given the complexity of other factors
which operate). A similar reasoning was adopted by Clements (1916), that
vegetation evolves to a steady-state—climax via ecological succession (or a
regression) of communities (Johnson and Steere, 1974:8). The ecosystem should
remain in such a steady state unless a critical parameter alters sufficiently; if change
occurs, the process of ecological succession or biotic development will try to return
to the steady-state. This concept of ecological succession is debated: change may
be directional or random, and climax communities are not as common as textbooks
suggest.