Concepts in restoration ecology - Restoration Ecology - page 19

sion (resulting from removal of fish that feed at the
bottom of shallow lakes).
A reference ecosystem, therefore, serves as a model
for planning a restoration project, and later for its
evaluation (Egan & Howell 2001, SER 2002). The
identification of a reference system is related to the
level of ambition of a reconstruction project (see
Chapter 1). For individual species, knowledge of a
reference system requires the determination of its
habitat (see for example Rickers et al . 1995); for
communities it is the determination of an association
or assemblage of species and its synecology (see for
example Ellenberg 1986). However, historical data
sets of plant species and the associated plant com-
munity members can become outdated within half a
century (Bakker et al . 2000, Strykstra 2000). This
potential discrepancy is a general problem, as it also
limits the success of identifying functional groups
from species traits. Evolutionary mechanisms do only
partly result in future success. This holds for species,
but more so for communities, even for so-called pris-
tine tropical rainforests (van Gemerden et al . 2003).
Considerations of natural dynamics imply that refer-
ence systems, which we know from the past, can there-
fore only serve as a point of orientation, not so much
as a goal in terms of returning to the past. Restoration
ecology demands insight into potential perspectives of
nature development from the past towards the future,
which can be modelled in terms of scenarios or tra-
jectories. These ideas will be dealt with in concreto
in Chapters 8-15 in this volume, where case studies
will also be described to illustrate the practice of eco-
logical restoration.

desired
state

alternative
states

subsequent
decline

degraded
state

STAYS THE SAME

continued
decline

Time

Fig. 2.2 Potential paths of ecosystem development
after disturbance to a degraded state. Restoration
objectives may or may not be met. After Hobbs
and Norton (1996). Reproduced by permission of
Blackwell Publishing.

factor will not result in recovery of components of
the original ecosystem that have been lost, because it
has caused irreversible damage to the ecosystem
involved, which may cause switches that are some-
times described in terms of 'thresholds of irreversib-
ility' (Aronson et al . 1993b). Drainage of wetlands,
for instance, leads to irreversible changes in the soil
structure (decomposition of peat, iron depletion,
acidification, etc.). Just rewetting the drained wetland
will not lead to the return of the original species of
that ecosystem.
Restoration efforts may not result in a goal that is
wanted, because the developing system may be
locked in an alternative stable state. For example, reduc-
tion of nutrient loading in eutrophied turbid shallow
lakes rarely leads to a satisfactory recovery of the
clear state, even if the nutrient level is considerably
reduced (Scheffer et al . 1993), which can be explained
by their 'bi-stability theory'. The clear state is domin-
ated by aquatic vegetation, and a turbid state by a
high algal biomass. Nutrient reduction has to be
accompanied by reduction of the fish stock in the lake
to enforce the switch back to clear water. This may
be due to a trophic cascade effect (reduction of the
predation pressure by phytoplanktivorous fish allows
populations of large-bodied zooplankton to peak and
graze down the algal biomass, causing clear water in
spring), and to the effect of reduced sediment resuspen-

2.3 A systems approach: the concepts
of disturbance and stability

In view of the central task of ecological restoration
to restore or rehabilitate disturbed ecosystems, we will
reflect upon the notion of disturbance. In general terms,
a disturbance is a long-term disordering of a constant
or steady state, due to an external event to which the
system is not capable of responding through resist-
ance or resilience (Fig. 2.3; see also Holling 1973,
DeAngelis 1992, Mitchell et al . 2000, SER 2002). The
latter terms refer to the notion of stability, a concept
also to be dealt with below.