Environmental Management: Principles and practice

How stable are environments?
Environmental management is likely to want to know whether an ecosystem is stable,
and what would happen if it were disturbed. As discussed earlier, the concept of
ecosystem stability has provoked much debate and is not fully resolved (Hill, 1987).
Natural ecosystems are rarely static: the best environmental management can expect
is a sort of dynamic equilibrium, not a fixed stability. Equilibrium is in part a function
of sensitivity and resilience to change. Sensitivity may be defined as the degree to
which a given ecosystem undergoes change as a consequence of natural or human
actions. Resilience refers to the way in which an ecosystem can withstand change.
Originally it was proposed as a measure of the ability of an ecosystem to adapt to a
continuously changing environment without breakdown. It would be misleading to
give the impression that these concepts, stability and resilience, are straightforward.
There is disagreement as to whether an ecosystem evolves in the long term
towards a steady-state with equilibrium of its biota through slow and steady evolution
of species (phyletic gradualism) or generally steady slight and slow evolution
punctuated by occasional sudden catastrophes and extinctions, after which there may
comparatively rapid and considerable biotic change (punctuated equilibrium) (Gould,
1984; Goldsmith, 1990). Whatever the process, the end result is held to be a ‘climax
stage’, reached via more or less transient successional stages (at any of which
succession might be halted by some parameter) (Clements, 1916).
Stability (some prefer to use ‘constancy’) is often invoked by those interested
in establishing whether conditions will remain steady or will return via a predictable
path to something similar to the initial steady-state after disturbance. It is widely
held that ecosystem stability is related to biological diversity: the greater the variety
of organisms there is in an ecosystem, the less likely is there to be change in biomass
production, although population fluctuations of various species may still occur
(Tilman, 1996). However, it is quite possible that a change in some parameter could

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have an effect on all organisms. Thus diversity may help ensure stability, but does
not guarantee it. An ecosystem may not be stabilized when disturbed: it may be close
to a starting-point, or it could be undergoing cyclic, more or less constant or erratic
change. Return to a pre-disturbance state is therefore uncertain.
Resilience is often measured by the speed of recovery of a disturbed ecosystem,
but can refer to how many times a recovery can occur if disturbance is repeated (Holling,
1973). Topsoil, for example, has little resilience; easily lost, it may well take centuries
to develop a new cover. An ecosystem may return to stability after several disturbances
but fail to after a subsequent upset for various reasons. The concept of resilience has
been applied to human ecology: some societies absorb or resist social change and
continue with traditional skills and land uses or develop satisfactory new ones; other
societies fail and their resource use and livelihood strategies degenerate.
Referring to sensitivity and resilience, Blaikie and Brookfield (1987:11)
suggested a simple classification of land, which may be modified to apply to
ecosystems in general:
1
Ecosystems of low sensitivity and high resilience These only suffer degradation
under conditions of poor management or natural catastrophe. Generally these
are the best ecosystems to stretch to improve production of food or other
commodities.
2
Ecosystems of high sensitivity and high resilience These suffer degradation
easily but respond well to management and rehabilitation efforts.
3
Ecosystems of low sensitivity and low resilience These initially resist
degradation but, once a threshold is passed, it is difficult for any management
and restoration efforts to save things.
4
Ecosystems of high sensitivity and low resilience These degrade easily and do
not readily respond to management and rehabilitation efforts. It is probably
best either to leave such ecosystems alone or to alter them radically—for
example, forest might be converted to rice paddyfield and suffer less ongoing
degradation than if it were converted to tree crops.
Managers or researchers often wish to establish in advance, or sometimes after
a disturbance, what the consequences will be:
(a)
Will the ecosystem re-establish its initial state?
(b)
Will there be a shift to a new state?
(c)
If (a) takes place, how rapid will the recovery be and how complete?
(d)
What path does the recovery take?
(e)
How often can recovery occur?
(f)
Will the same recovery path always be followed?
(g)
Will successive, similar disturbance have the same effect?
(h)
Would change still occur if there were no disturbance?

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Some ecosystems are in constant non-equilibrium or frequent flux, rather than
in a stable state at or near carrying capacity. The behaviour of an ecosystem (physical
or human) can be modelled. However, there is often such complexity that the outcome
is difficult to predict reliably with simple systems analysis. Succession may not be
as reliable and useful a concept as some would like. In particular, ecological change
may not be as predictable as might be wished. For example, in some environments
heavy grazing leads to increased scrub cover; a reduction of grazing might be expected
to lead to a reduction of the scrub because it sometimes causes a thickening of the
woody vegetation. Some plant communities do not exhibit succession as a directional
change but follow a cyclic fluctuation about a mean (the classic case being bog or
tundra hummock formations) (Kershaw, 1973:65–84).