Environmental Engineering Reference
In-Depth Information
upon the notions of
disturbance
and
stability
intro-
duced already in Chapter 1. Note that these topics are
applicable not only to ecosystems (e.g. in terms of
nutrient cycling, hydrology etc.) but also to biotic com-
munities (e.g. trophic interactions), populations (e.g.
genetic equilibria) and individuals (e.g. health). Also,
we need to introduce the reader to notions such as
ecosystem functioning, ecosystem services and the
functional role of biodiversity.
Please note that in this topic we avoid the term
per-
turbation
, which originates from physics where it has a
very precise meaning (indicating a small vibration),
and is not taken to imply an interruption or disruption
in a normal process. By contrast, in ecology, the term
is often used either to denote a trigger or cause of severe
disturbance or else as a close synonym of disturbance.
There is no consistency in the way the two terms are
used in the scientifi c ecology literature, and we do not
concur with the two sole efforts at elucidation. To wit,
we do not follow the terminology of Rykiel (1985) who
proposed to use 'disturbance' to refer to causes, and
'perturbations' to refer to effects. Nor we do not agree
with White and Jentsch (2001), who proposed to
measure disturbance in absolute terms, for instance by
the reduction of biomass of a mown grassland. To wit,
the notion of 'disturbance' takes on meaning only once
a reference system or state has been defi ned. In the case
of the Arctic tundra, mentioned in this chapter, the
reference was the ecosystem state before the distur-
bance was induced, when grazing by geese was still part
of the 'normal functioning' of the vegetation.
To avoid confusion, researchers and authors often
distinguish between kind, intensity, frequency and
scale of disturbance. The kind of disturbance depends
on the environmental factor concerned, whether living
or nonliving, and whether human or nonhuman. The
degree or intensity of a disturbance is determined by
the difference between the new conditions and the pre-
vious steady state (or reference) conditions. Frequency
is also important because of different effects from iso-
lated, recurrent and continuous disturbing events;
they can be irregular or regular and of differing dura-
tions. Finally, the scale or extent of disturbance refers
to different spatial and temporal patterns, and to differ-
ent levels of ecological organization: ecosystem, com-
munity, population or individual.
2.4.1
Disturbance and disturbance factors
The midcontinent population of the lesser snow goose
(
Chen caerulescens
), which breeds in the eastern and
central Canadian Arctic and sub-Arctic, and winters in
the southern United States and northern Mexico, was
relatively stable from 1950 to 1970, but it increased
fourfold until a peak in 1998 (Abraham
et al
. 2005 ).
This increase was largely because of increased survival
in the winter areas in response to an agricultural food
subsidy. Due to an expanding and increasingly inten-
sive agriculture, they have adapted their migration
pathways and largely graze on food crop residues, rice
and wheat, and waste corn in particular (Jefferies &
Rockwell 2002 ). Also, well - meaning nature - conserva-
tion managers established an increasing number of
wildlife refuges in the winter areas and along the
fl yway, sometimes alongside agricultural fi elds. The
inadvertent result of these coinciding changes in agri-
culture and conservation was that with ever greater
densities, the geese over-exploited the tundra vegeta-
tion of their breeding ground, for example the coastal
Hudson Bay salt marshes, which led in turn to irrevers-
ible degeneration of this formerly highly stable eco-
system to an
alternative stable state
of exposed
sediment. The present pattern of vegetation loss is
likely to continue in the foreseeable future (Abraham
et al
. 2005). This is an example of an ecological distur-
bance, defi ned as a long-term disordering of a constant
or steady state, due to an external event or phenome-
non, to which a given system is not capable of respond-
ing through its inherent resistance or resilience; the
terms are explained below. We call the artifi cially
infl ated geese population a
disturbance factor
, and
the resulting effect on the salt marsh a
disturbance
(see Figure 1.2). In the case of a serious disturbance of
this sort, an ecosystem is often no longer stable in its
previous state or condition, but moves instead to an
alternative steady state (see also Chapters 6 and 21).
2.4.2
Stability and alternative stable states
What does the notion of stability - in other words, a
long-standing steady state - imply? Since there are
dozens of defi nitions of stability in the ecological litera-
ture, and defi nitions of resilience and resistance some-
times overlap, we adopt the most useful ones for our
purposes when we discuss stability, and resistance and
resilience (see Figure 1.2).
•
Stability
is the capacity of a system to return to
a starting state following a signifi cant change in its
