Environmental Engineering Reference
In-Depth Information
environment that serve to maintain the integrity of
the ecosystem;
9
the restored ecosystem is self-sustaining to the
same degree as its reference ecosystem, and has the
potential to persist indefinitely under existing envir-
onmental conditions.
ecosystem tend to produce certain obvious changes in
its species composition. Processes were thus thought
to govern structure. Other authors, for example Paine
(1980), laid more emphasis on the biotic interac-
tions, from which the processes would result. Removal
of a weakly interacting - that is, a functionally
insignificant - species would yield no or slight change,
and removal of keystone species may have a cascade
of effects on the community composition, transmitted
by a chain of strongly interacting links. According
to Pimm
et al.
(1991) food-web theory predicts that
highly connected, complex communities should be most
sensitive to the loss of species from the top of the web
because secondary extinctions propagate more widely
than in loosely connected, simple communities,
whereas simple communities should be more sensitive
to the loss of plant species than complex communit-
ies because in simple communities the consumers are
dependent on only a few species and cannot survive
their loss. Keystone species can occur at all trophic
levels. According to classical results from mathem-
atical food-web theory, omnivory (feeding on more
than one trophic level) would destabilize ecological
communities (Pimm & Lawton 1978), whereas more
recent conceptual syntheses suggest that omnivory
should be a strongly stabilizing factor in food webs
(Polis & Strong 1996, Fagan 1997). For aquatic
systems in particular, it should be recognized that
various components of a food web may demand dif-
ferent timescale-dependent dynamics to be included
in food-web modelling (Kerfoot & DeAngelis 1989).
McCann (2000) reviewed the diversity-stability de-
bate. Recognizing that much of ecological theory is
based on the underlying assumption of equilibrium popu-
lation dynamics, he proposed to distinguish between
two categories of stability in ecology: stability defini-
tions that are based on (i) a system's dynamic stability
and (ii) a system's ability to defy change (resilience,
resistance). Differential responses of populations may
sum, through time, to result in stable community
dynamics. Field tests at the scale of the food web are
few in number. McNaughton (1985), studying herbi-
vores in the grazing ecosystem of the Serengeti under
naturally variable conditions, has shown that greater
diversity reduced the relative magnitudes of fluctu-
ations in productivity induced by seasonal change.
Similarly, Prins and Douglas-Hamilton (1989) illustrated
that the consumption pressure (grazing and browsing)
This list demonstrates clearly that the focus of
ecological restoration is on ecosystems, while recog-
nizing that the biotic community - all the biota in an
ecosystem - is the most important component.
Assemblages of organisms can be recognized by their
species composition as pattern or structure, but also
by their functional roles in the ecosystem as for
example primary producers, herbivores, carnivores,
decomposers, nitrogen fixers or pollinators, in which
case they are known as functional groups.
Research on the biotic community in an ecosystem
is often limited to food-web studies. The trophic struc-
ture has been considered in terms similar to those
used in ecosystem studies: in terms of equilibria and
stability. For example, Noy-Meir (1975) explained
the stability of grazing systems in the Serengeti as a
special case of predator-prey systems, with two steady
states: a high-biomass, high-productivity steady state
which is stable to fluctuations within a certain range,
and a low-biomass, low-productivity steady state.
Either of the two states may be the more stable. Holling
(1973) pointed out that an equilibrium-centred view
is essentially static and provides little insight into
the transient behaviour of systems that are not in equi-
librium. He proposed that the behaviour of eco-
logical systems could well be defined by two distinct
properties: resilience and stability (see Chapter 2 in
this volume). From Holling's point of view, the
notion of an interplay between resilience and stabil-
ity might also resolve the conflicting view of the role
of diversity and stability in communities, because
instability in numbers might result in more diversity
of species and in spatial patchiness, and hence in
increased resilience.
4.2 The diversity-stability debate
A large pile of literature has become available on the
topic of the diversity-stability relationship. Lindeman
(1942) suggested that dynamic processes within an
