Linking Restoration and Ecological Succession (Springer ... - Inecol

Chapter 7 Restoration as a Process of Assembly and Succession Mediated by Disturbance 161
as nutrient accumulation in soils, a common problem in the highly industrialized
countries of northern Europe, can seriously alter the species
composition of ecosystems, despite the overall species richness remaining
very similar over time. To reach a restoration goal of reinstating certain
plant communities adapted to low nutrient conditions (such as calcareous
grasslands or heathlands), one therefore has to actively disturb the process
of continuous accumulation of nutrients, for instance by removing topsoil
(see Chapters 5 and 6). To allow the desired community to assemble, these
actions have to be carried out in close vicinity to viable populations of the
target species, or even in conjunction with introducing the desired species.
4. Thresholds. The restoration goal of self-sustaining dynamics suggests that
we can find thresholds for nonsustainable dynamics which lead to changes
in state (recently also referred to as regime shifts; Scheffer and Carpenter
2003; Mayer and Rietkerk 2004). Within the area of management actions
using disturbance as a tool for modifying ecosystem dynamics, thresholds
leading to nonsustainable dynamics should not be crossed. Therefore,
restoration ecologists need to understand the interplay of discrete versus
continuous processes and to carefully manage for a dynamic balance between
the two. This is, of course, no simple task, and developing an ability
to identify, and hence avoid, thresholds of nonsustainable dynamics for different
systems forms a current challenge for ecologists and restorationists
alike.
5. Restoration goals and alternative stable states. Restoration projects can
and should include multiple goals and the maintenance of a variety of successional
states, and thus several reference dynamics (Grimm and Wissel
1997) also referred to as “multiple equilibria” (Scheffer and Carpenter 2003;
Suding et al. 2004). Considering that restoration projects often aim at restoring
specific types of habitat (such as calcareous grassland or heathland),
this can pose a practical problem. One way of dealing with the possibility of
“multiple equilibria” is to include as much of a mosaic of different habitats/
vegetation patches within a restoration site as possible, to allow for shifts in
states from one successional stage or alternative stable state to the next (see
Chapter 2). Alternatively, active management is usually needed to keep a
habitat in a certain desired state. In addition to the influence of management,
changing environmental conditions, including disturbances as well as more
continuous environmental changes, may contribute to shifting ecosystems
between alternative states (Suding et al. 2004). Within a particular state,
resilience is mainly determined by the interplay of disturbances with internal
system feedbacks, e.g., in terms of disturbance magnitude, which can
be absorbed by the system without changing (Mayer and Rietkerk 2004).
6. Scaling issues. In practice, disturbance regimes are important determinants
of both restoration site trajectories including self-sustaining dynamics and
state shifts. For example, Turner et al. (1993) introduced the concept of
landscape equilibrium caused by various kinds of disturbance regimes.
They predict that the presence or absence of equilibrium and variance in an
ecosystem is defined by the dimensions of disturbance relative to landscape
extent and speed of successional dynamics. If the ratio of disturbance area
to landscape area, or the ratio of disturbance frequency to the time needed
for successional recovery, is very large, single, not necessarily dramatic
disturbance events may destabilize the dynamic equilibrium of one regime