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

7.3 Assembly
Chapter 7 Restoration as a Process of Assembly and Succession Mediated by Disturbance 153
Concepts of succession and community assembly both address temporal dynamics
within ecosystems. However, while succession focuses on the dynamics
of a system following initial colonization of a denuded site (primary succession)
or the dynamics of system regeneration after a disturbance (secondary
succession), community assembly asks the question “How does the suite of
species present at any particular location arrive and persist there, and how
does that relate to the pool of species available within the region as a whole?”
Hence, questions concerning community assembly, although inherently containing
a dynamic component, are often spatially framed. Indeed, one of the
main methodological approaches to assembly, the categorical approach, aims
to study extant communities and provide a snapshot in time of the species,
functional groups, or guilds present there. Different patterns of abundance of
species in different functional groups that are found in a community are then explained
via so-called assembly rules, which are often tested against null models
of no interaction between organisms. An extension of this is the idea of guild
proportionality, which suggests that, within a particular community type, the
proportions of species of different guilds are almost constant across sites in
different developmental stages (Wilson and Roxburgh 1994, Wilson 1999). As
a consequence of the theory of guild proportionality, one would expect the
nearest plant of a different species to belong to another guild or functional
group.
Applying such concepts of community assembly to restoration situations
could prove difficult, unless one could unequivocally show, for a given system,
that nonconstant proportions of species in different guilds was a sign of a
highly degraded site compared to a reference ecosystem exhibiting clear guild
proportionality. In this case, a lack of guild proportionality in a degraded site
could be used as an indication of the system being stuck in a certain state, and
appropriate management measures (usually involving some kind of disturbance
favoring a particular species) could be taken to move the system from the
undesired stable state to a desired stable state. Although we do not have enough
data on guild proportionality in different ecosystems (mainly in grasslands and
deserts so far) to be able to apply such methods at this stage, it could be a
promising venue for future research linking ecological theory with restoration.
A promising community assembly approach for more direct application to
ecological restoration is the concept of “filters,” whereby a species can only
establish in an area if it can deal with the environmental conditions (i.e., the
abiotic filters) as well as the other organisms it finds there (i.e., the biotic filter)
(Kelt et al. 1995, Weiher and Keddy 1995, Zobel 1997, Díaz et al. 1999,
but see Belyea 2004 for caveats). Various conceptualizations of environmental
“filters,” for example very low or high nutrient levels in an ecosystem, also
tend to indicate that the main effect of filters is to vary the species composition
in relation to environmental (and hence spatial) variation (Díaz et al.
1999; Hobbs 2004). On the other hand, the “response” or dynamic approach
to assembly considers changes in the biotic community and the expression of
community assembly “rules” over time, and recent treatments emphasize the
dynamic nature of filters, which are likely to change over time as well as spatially
(Fattorini and Halle 2004; Hobbs 2004). As discussed in Temperton and
Hobbs (2004), the dynamic filter approach could prove useful, at least before