Biology Reference
In-Depth Information
2
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Nonequilibrium in communities
Definition and evolution of communities
Ecological community is not a term that is used uniformly by all authors,
and there is much disagreement about how communities may have
evolved. In this section, I discuss definitions given for ecological commu-
nities and their evolution.
Giller and Gee (
1987
) review the different ways the term community is
used, and the problems arising from these different usages. Fauth et al.
(
1996
) defined communities as all species co-occurring at the same time,
irrespective of taxon; guilds are species that use the same class of resources.
They defined assemblages as groups of species of one taxon (e.g., birds)
within a community. According to Cornell (
2001
), communities are
collections of species living contemporaneously in the same place, con-
sisting of individuals that are spatially interspersed, with the potential of
direct or indirect interaction. Following Whittaker, Levin (
1992
) points
out that ''communities'' and ''ecosystems'' are arbitrary subdivisions of a
''gradation of local assemblages.'' Communities are not well integrated
units, because species within them respond individualistically to the
environment. Lawton (
2000
) asks: how many species constitute a com-
munity? There is no logical break between populations of single species
and of many, and there is a tendency to take several species of a single
taxon as comprising a community. Lawton points out that entire com-
munities are almost impossible to study, with the exception of some in
very simple habitats such as water-filled tree holes. However, there are
many other habitats that are well defined and have a characteristic set of
species which are not found elsewhere. Such species sets comprise easily
defined communities. Many of them are quite simple and can easily be
studied. Examples are parasite communities on the gills of fish and in the
alimentary tract of various vertebrates.
The central question of community ecology is whether communities
in a habitat comprise all species that have happened to arrive there, or only
