Environmental Engineering Reference
In-Depth Information
5
Communities: interspecific
interactions
Jelte van Andel
5.1 Introduction: the community
concept
Interactions between species are generally considered
as mechanisms that affect community structure and
provide the community with emergent properties as
compared to the sum of the component individuals
(see Looijen & van Andel 1999). Due to the complexity
of these interactions, not much progress seems to have
been made in understanding the assembly rules since
Diamond (1975) coined the term to deterministically
explain stable communities, despite a more recent
attempt by Weiher and Keddy (1999). The analytical
review by Belyea and Lancaster (1999) is of great help
in understanding the literature dealing with assembly
rules. It clearly distinguishes within trophic levels
from across trophic levels, and between environmen-
tal and dispersal constraints. In the present chapter, I
will pay attention to the dispersal-assembly perspec-
tive and the niche-assembly perspective of commun-
ities (see Hubbell 2001), leaving the discussion on
assembly rules out of scope. (See also Chapter 16 in
this volume.)
The term
ecological communities
is being used for
different kinds of species assemblage. In the previous
chapter I dealt with the
biotic community
, which
includes all the biota in an ecosystem (Odum 1971,
SER 2002; www.ser.org). We distinguish biotic com-
munities from species assemblages classified on the
basis of the species' taxonomic status (e.g. an insect
community) or life form (e.g. a tree community),
which we define as 'the set of individuals of two or
more species that occur in the intersection of the local
distribution areas of these species' (Looijen & van Andel
1999, 2002); in the present chapter I will call these
assemblages
communities
. Population dynamics and
genetics cannot be included in community ecology in
a straightforward manner, because community bound-
aries do not coincide with population boundaries, and
as a result different individuals of a population may
be part of different communities. This is probably one
of the reasons why vegetation science has tradition-
ally been linked much more to ecophysiology than to
population ecology.
Communities develop through a process called
community assembly, in which individuals of species
invade, persist or become extinct. The search for
assembly rules makes ecological knowledge explicit,
rather than implying a philosophy on the identity of
a plant community. Though a community is not just
the summing up of its individual components, it
should not be considered as an organismal entity either.
5.2 The dispersal-assembly perspective
5.2.1 The species pool
According to Hubbell (2001), the dispersal-assembly
perspective asserts that 'communities are open, non-
equilibrium assemblages of species largely thrown
together by chance, history, and random dispersal'. In
Chapter 2 in this volume I have explored the concept
of the species pool and how to identify the adequate
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