Environmental Engineering Reference
In-Depth Information
In view of possibilities for ecological restoration,
it is interesting to know whether a disturbance is
definitely irreversible or can be counteracted by
restoration measures. Systems can be disturbed in such
a way that there is no way back to the initial steady
state, unless particular measures are taken. For
example, if artificial fertilizer is applied to transform
a semi-natural hayfield into an intensively used
agricultural grassland, the latter can be considered
a new steady state (degenerated from a nature-
conservationist point of view, intact from an agricul-
tural point of view). The way back to the earlier steady
state can be difficult to pave. Similarly, a semi-
natural hayfield that is left unmanaged may turn into
a woodland or forest. In nature, a system may be
triggered - sometimes quite suddenly - to turn into
one or another alternative stable state. The patchy
spatial distribution of tree savanna, grass savanna and
bare ground may result from different frequencies and
intensities of herbivore grazing and browsing that may
induce irreversible changes in vegetation structure
(Noy-Meir 1975; Rietkerk & van de Koppel 1997; van
de Koppel
et al
. 2002). In such semi-arid grazing
systems the interactions between water infiltration or
nutrient retention and plant density potentially give
rise to the existence of alternating stable vegetation
states and threshold effects, even without the effect
of non-linear herbivore functional response or plant
competition (Rietkerk & van de Koppel 1997). These
interactions may trigger a positive feedback between
reduced plant density and reduced resource availabil-
ity, and lead to a collapse of the system. Similarly,
Grootjans
et al
. (1998) have shown that the pioneer
plants in wet coastal dune slacks prevent the accu-
mulation of organic matter in the soil as long as
they are capable of keeping up a positive-feedback
mechanism, in this case by creating an oxic-anoxic
gradient in the rhizosphere resulting in a subsequent
combination of nitrification and denitrification
(Adema
et al
. 2001). The pioneer stage of succession
(with
Littorella uniflora
and
Schoenus nigricans
) can
be sustained for an extended period of time (decades),
and then suddenly move towards the next, more pro-
ductive, stage of succession (with
Carex nigra
and
Calamagrostis epigejos
). This process is irreversible,
unless measures such as sod-cutting are applied to reset
the successional clock. Problems in the context of
restoration ecology have been reviewed recently by
Suding
et al
. (2004). Indeed, degraded systems can also
be resilient through internal feedback that constrains
restoration.
Although it seems only a small step to elaborate
the notion of stability towards defining ecosystem
health and ecosystem integrity, these terms may sug-
gest that an ecosystem can be considered as some kind
of a superorganism, which it is not. We have to cope
with the term, recognizing that it is being used in a
more societal context, where ecological and socio-
economic valuation systems meet.
2.3.3 Ecosystem health and ecosystem
integrity: useful metaphors
Ecosystem health can be described as the state or con-
dition of an ecosystem in which its dynamic attributes
are expressed within normal ranges of activity relat-
ive to its ecological state of development (SER 2002).
As with the notions of disturbance and stability,
we make a choice for using a relative definition of
ecosystem health. From an ecological point of view,
ecosystem health can be evaluated in terms of the eco-
system's overall dynamic state at a given time based
on ecosystem functioning (Winterhalder
et al
. 2004).
However, the notion of ecosystem health implies
more than an ecological valuation; socio-economic
criteria are generally also taken into account. Rapport
(1995) stated that 'evaluating ecosystem health in
relation to the ecological, economic and human
health spheres requires integrating human values
with biophysical processes, an integration that has been
explicitly avoided by conventional science'. Boulton
(1999), dealing with river health assessment, similarly
emphasized that, in contrast to definitions of healthy
ecosystems based on solely ecological criteria, judge-
ments of river health must include human values, uses
and amenities derived from the river system. Indeed,
rivers are not just ecosystems, but can also be con-
sidered in terms of functions for mankind, for
example, as sources of clean water for drinking and
washing, for industrial and agricultural purposes, as
conduits for pollutants, and as places for recreation
and aesthetic pleasure. The notion of ecosystem
health, therefore, adds a socio-economic valuation to
the strictly ecological qualifications in terms of stab-
ility and disturbance (see Chapter 1).
