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

Chapter 7 Restoration as a Process of Assembly and Succession Mediated by Disturbance 155
The concept of non unidirectional succession allows a reconsideration of theories
of succession as initially formulated at the beginning of the 20th century,
in particular the dichotomy between deterministic climax models (Clements
1916) and stochastic individualistic models of terrestrial succession (Gleason
1926). In part, the recognition of a decline phase in succession after reaching a
phase of maximum biomass accumulation or species richness could be viewed
as providing support for Clements’ view that ecosystems can develop quite deterministically
over time toward a convergent state with a maximum biomass or
a stable end point. However, retrogression can happen in different ways and at
different times in various seres. Perhaps two distinct categories of retrogressive
successions can be recognized: “natural” retrogressions, such as that observed
on the Coooloola dunes in Australia (see Chapter 4), and human-induced retrogressions
arising from altered grazing and fire regimes, mining, and the like
(see Chapters 2, 4, and 6)
On the other hand, this pattern and the subsequent decline phase can result
from the individualistic growth and maturation of particular dominant species,
indicating that Clements’ emergent properties can be explained using Gleason’s
individualistic approach. Intriguingly, recent work on the assembly of
vegetation on old fields over time (Fukami et al. 2005) found convergence of
functional plant traits over time—indicating more deterministic dynamics—
but divergence of species composition as a result of priority effects—indicating
more stochastic dynamics. This work raises important issues of how the level of
focus of our research affects the outcome, i.e., how dependent is our hypothesistesting
of different ecological theories on community succession and assembly
on the unit of focus, be it species, community structure, functional groups, or
species traits? To ensure more cross-fertilization between community ecology
and restoration, in the future we need to make sure we try to answer the critical
questions at several levels of focus. Otherwise, we may be unwittingly missing
out on crucial information for restoring ecosystems. It could well be that
continued debate on deterministic versus stochastic theories of ecosystem is in
part due to the possibility that both theories (as well as the theory of alternative
stable states) can apply at the same time (depending on the level of focus) or
within the same system at different times. Resolving this major issue in ecology,
by knowing at what level of focus one or another theory applies, would help
greatly improve the applicability of ecological theory to restoration.
In succession, the lack of extensive examination of the decline phase after
the maximum biomass stage is probably due to the shorter timescale of earlier
studies. Most studies have been carried out on relatively young landscapes, lack
data from very long-term chronosequences, and illustrate a bias of successional
focus on build-up and not break-down concepts (see also Chapter 4). Experience
in restoration projects has repeatedly shown, however, that the story of
successional direction is complex, particularly in a world experiencing drastic
changes in disturbance regimes (Hölzel and Otte 2003, Jentsch and Beierkuhnlein
2003, Aronson and Vallejo 2006). Disturbance and its interactive effects
on species colonization and extinction potential are a vital part of the dynamics
of ecosystem development. Disturbance regimes, including intensity and frequency
of disturbance events, are recognized as critical drivers of successional
trajectories (Trudgill 1977, White and Jentsch 2001).
We often see restoration as an acceleration of succession via human intervention
after major or chronic human disturbance. The issue is whether the