Environmental Management: Principles and practice

How sensitive and vulnerable are ecosystems?
The concept of ecosystem stability was discussed in chapter 9 (Hill, 1987; Stone et
al., 1996). Mitchell (1997:51) felt that basic concepts of ecosystem diversity and
stability were too simplistic adequately to describe reality; so, as ecosystems were
inherently complex, environmental managers would need to accept that often they
could not manage ecosystems, although they might manage human interactions
with them.
On the whole, biogeochemical and biogeophysical processes tend to resist
change and are self-regulating within limits, so one can expect a sort of dynamic
equilibrium. However, some global cycles, environments and organisms (and groups
of people) are more sensitive to change than others. Stability is in large part a
function of resistance to change and resilience following it. Resistance (or
sensitivity) may be defined as the degree to which a given ecosystem undergoes
change as a consequence of natural or human actions or a combination of both.
Resilience may be used to refer to the way in which an ecosystem can withstand
change. It is widely held that ecosystem stability is to a significant degree related
to biological diversity: the greater the variety of organisms there are in an ecosystem,
the less likely is there to be instability (Pimm, 1984). However, it is quite possible
that a change in some parameter could have an effect on all organisms regardless
of diversity: thus diversity may help ensure stability but does not guarantee it.
Resilience is often used as a measure of 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).
What the environmental manager seeks to avoid is environmental degradation
—a rough definition of which is: the loss of utility or potential utility or the reduction,
loss or change of features or organisms which may be difficult: costly or impossible
to replace. Recognizing degradation can be difficult: it may be slow and gradual, or
it may take place long after disturbance. People may fail to notice change which may
sometimes be too imperceptible for a single generation to see. Nowadays it is rare
for an environment to be ‘natural’. The chances are that there has been degradation
by humans, e.g. development in southern France may degrade the maquis scrubland,
but that is already much degraded compared with the prehistoric forest cover. The
present condition of the an ecosystem may not indicate what has been lost, or show
if there has been improvement. An ecosystem may not be stabilized when disturbed:

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it could already be much degraded or improved compared with its natural state, or it
could be undergoing cyclic, more or less constant or erratic change (Kershaw,
1973:65–84). Return to a pre-disturbance state when disturbance ceases is by no
means certain (Burton et al., 1977; Blaikie and Brookfield, 1987; Goldsmith, 1990).
For example, grazing can lead to increased scrub cover; a reduction in grazing might
be expected to lead to a reduction of the scrub—but that sometimes causes a thickening
of woody vegetation.