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

162 Richard J. Hobbs, Anke Jentsch, and Vicky M. Temperton
and reach the threshold for bifurcation and regime shift. As for disturbance
intensity, some disturbances result in straightforward secondary succession
that reestablishes the pre-disturbance composition, structure, and resources,
whereas others affect site quality through long-term decreases or
increases in resource levels, leading to trajectories that are out of bounds of
the pre-disturbance situation (Walker and del Moral 2003). In the state of
equilibrium, there is bounded variation: in a large enough restoration area,
no species or successional states become extinct across the area as a whole
(although they may do so in individual patches), but they can fluctuate in
abundance due to the impact of disturbances. For example, species with
concordant life cycles dominate periods right after a disturbance event,
while species with discordant life cycles dominate later phases of succession
(Pavlovic 1994). Nevertheless, both kinds of species contribute
to community assembly in a disturbance-prone ecosystem. Understanding
disturbance effects and subsequent system responses is crucial for understanding
regime shifts and their thresholds and scales. Here, disturbance
ecology offers valuable pointers to be applied in restoration action, and
more cooperation between the ecologists and restorationists on this issue
will no doubt provide more concrete guidelines for action in the future.
7. Spatial mosaics. Restoration goals are not static. A crucial challenge for
restoration ecology is therefore to understand ecosystem dynamics as a
function of spatial and temporal interrelations of different elements of
the disturbance regime. Disturbances and management action play a crucial
role for initiating and stabilizing successional rhythms (Jentsch et al.
2002a). If a restored site is all in one age state, regardless of whether this
is recently disturbed or long undisturbed, it will lose species that characterize
the other age states (Pickett and Thompson 1978; Beierkuhnlein and
Jentsch 2005) and thus will also lose its ability to respond to disturbance
(Jentsch 2004). Restoration managers need to plan for a sustainable mosaic
of all stages and species (Pickett and Thompson 1978) and for dynamic
processes sustaining this mosaic.
8. Non-linear dynamics. Although we can direct restoration to a certain extent
via knowledge of community assembly and succession in combination
with the appropriate disturbance regime, there will inevitably also be an
added factor of unpredictability within an ecosystem. Internal feedbacks
within ecosystems can interact with broad-scale external forces, such as
global weather patterns or restoration efforts, and trigger shifts to either
alternative regimes or to novel trajectories. Nonlinear system dynamics
imply that a system’s retransformation leads to novel conditions instead of
prior structures and functions (Beisner et al. 2003, Holmgren et al. 2006).
Often, nonlinearity of ecosystem dynamics or regimes shifts are neither
very obvious nor dramatic. For example, factors that undermine resilience
slowly, such as eutrophication in resource-limited systems (Verhagen et al.
2001, Jentsch et al. 2002b, Hölzel and Otte 2003), disturbance-mediated
introduction of invasive species (Sharp and Whittaker 2003) or climate
change (Jentsch and Beierkuhnlein 2003), can be responsible for altered
successional trajectories.
9. Inertia. Although in some cases changes in state can occur suddenly, attention
of restoration managers needs to be drawn to gradual accumulation up
to thresholds of state change. Climate change is one of the major driving