Environmental Engineering Reference
In-Depth Information
Speciation
large-scale migration
small-scale migration
Environmental sieve
with two screens
acting in concert:
abiotic factors and
biotic interactions
dispersal
filtering
Community
species pool
Fig. 2.4 The role of large- and small-scale
processes in determining species richness.
After Zobel (1997). Reproduced by permission
of Elsevier.
Local species pool
Regional species pool
2 Local species pool : the set of species occurring in
a type of ecosystem or landscape (e.g. a river valley).
3 Community species pool : the set of species present
in a site within the target community, including the
soil seed bank.
Functional groups and keystone species
The terms functional groups and functional types
(Walker 1992) have been used in studies on global
change and also in restoration ecology to cope with
the complex response of very many species to pro-
nounced changes in the environmental conditions.
Unfortunately, the terminology is inconsistently
employed. While the term group seems to indicate a
grouping of individually known species in one par-
ticular class, the term type suggests a (concrete or
abstract) representative of a class of species. In the
present volume we stick, therefore, to using the term
functional groups . Several ecological classifications are
being used to escape from dealing with individual
species and to focus on ecological species groupings.
Use can be made of earlier ecological classifications that
aimed at identifying species groupings to describe the
structure and functioning of communities or ecosys-
tems. Examples are, in order of appearance, life forms
(Raunkiaer 1934), guilds (Root 1967), r - K strategies
(MacArthur & Wilson 1967), C-S-R strategies (Grime
1979, 2001), adaptive syndromes (Angevine & Chabot
1979, Swaine & Whitmore 1988) and groups that
respond similarly to disturbances (Friedel et al . 1988,
Lavorel et al . 1997) or resource shortfalls (Grubb,
1998).
We have not noticed any reflection in the literature
on the notion of function or functioning of species
groupings in communities. Instead, several approaches
have been developed to identify functional groups, often
Species pools can, according to Zobel et al . (1998), be
determined on the basis of environmental similarity,
functional similarity and/or phytosociological similarity
between the species, or by applying an experimental
approach (evaluating the results from sown seed
mixtures). Weiher and Keddy (1999) provided several
examples of how communities are assembled from
species pools. They mentioned two developing para-
digms for the assembly of communities: (i) the island
paradigm, dealing with mainlands, islands, immigra-
tion and coexistence, and (ii) the trait-environment
paradigm that begins with pools, habitats and filters,
and convergence. The term 'assembly rules', used to
describe the problem of assembly communities from
pools, had been derived from Diamond's (1975) ori-
ginal usage of the word. Wilson and Gitay (1995)
defined it as a set of ecological restrictions on the
observed patterns of species presence or abundance,
based on the presence or abundance of one or more
other species or groups of species (not simply the
response of individual species to the environment).
These approaches are reflected in the scheme of
Fig. 2.4, where the environmental filters (resources,
abiotic conditions and biotic interactions) could be
considered assembly rules.
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