How stable are environments?
‘Stability’ can have a number of meanings, including: lack of change in the structure
of an ecosystem; resistance to perturbations; or a speedy return to steady state after disturbance
(Troumbis, 1992: 252). Environmental managers are likely to want to know
whether an ecosystem is stable, and what would happen if it were disturbed. As discussed
above, the concept of ecosystem stability has provoked much debate which is
not yet fully resolved (Hill, 1987). It is clear that natural ecosystems are rarely static:
the best environmental management can expect is a sort of dynamic equilibrium, not a
fixed stability (Smith, 1996). Furthermore, human activity is increasingly disrupting ecosystems.
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 of stability and
resilience are straightforward and fully established.
Ecosystems are subject to natural and anthropogenic changes, some catastrophic and
sudden, others gradual and less marked (Stone et al., 1996). It is widely held that, given
long enough, a steady state will be reached by an ecosystem because a web of relationships
allows it to adjust to serious localised or moderate widespread disturbances.
Such an ecosystem is supposed to remain in steady state unless a critical parameter
alters sufficiently. If change then occurs, it is termed ‘ecological succession’ or ‘biotic
development’ (Johnson and Steere, 1974: 8). Some economists and political studies
specialists have suggested economics, politics and social development follow predictable
evolutionary paths to steady states.
There is debate as to whether an ecosystem: (1) evolves in the long term towards a
steady state with equilibrium of its biota through slow and steady evolution of species
(phyletic gradualism); or (2) experiences generally steady, slight and slow evolution
punctuated by occasional sudden catastrophes and extinctions, after which there may
be comparatively rapid and considerable biotic change ‘punctuated by equilibrium’
(Gould, 1984; Goldsmith, 1990). Whatever the process, the end result is widely held to
be a ‘climax stage’, reached via more or less transient successional stages, at any of
which succession might be halted by some limiting factor. The concept of ecological
succession, pioneered by Clements (1916), is complex and still debated. According to
the concept, organisms occupying an environment may modify it, sometimes assisting
others – a birch wood may act as a nursery for a pine forest, which ultimately replaces
the birch – thus birch is a successional stage en route to a pine stage. These transitional
stages leading to a mature climax community are known as seres. Two types of succession
are recognised: (1) primary succession and (2) secondary succession. The former
is the sequential development of biotic communities from a bare, lifeless area (e.g. the
site of a fire, volcanic ash, or newly deglaciated land). The latter is the sequential development
of biotic communities from an area where the environment has been altered but
has not had all life destroyed (e.g. cut forest, abandoned farmland, land that has suffered
a flood or been lightly burnt). Many communities do not reach maturity before being
disturbed by natural forces or humans, and so type (2) situations are common. Where
succession is taking place from a bare area, the first stage is known as the pioneer stage;
although, in practice, the expression may be applied to growth taking place in areas that
do have some life – such as regrowth after logging. Natural forests may be assumed to
maintain maturity, rather than becoming senile and degenerating, through ‘patch-and-gap’ dynamics – clearings caused by storms and other disturbances allow regeneration.
Pioneer communities usually have a high proportion of plants and animals that are hardy,
have catholic niche demands, and disperse well (e.g. weeds with wind-carried seeds,
and insects which can fly). Mature, climax communities are supposed to have more
species diversity, recycle dead matter better, and be more stable.