Introduction
Herman A. Verhoef and Peter J. Morin
First of all, we must clarify what we consider to be
the subject matter of community ecology. In his
classic textbook, Krebs (1972) described a commu-
nity as ‘an assemblage of populations of living
organisms in a prescribed area or habitat’. He de-
scribed this as ‘the most general definition one can
give’. The ‘prescribed area’ suggests that we are
dealing with some sort of spatially bounded sys-
tems, and it is the somewhat arbitrary nature of
possible boundaries that has caused continuing
debate about the nature of communities (e.g. Rick-
lefs 2008). It is debatable whether community ecol-
ogy is a proper discipline at all if communities do
not exist as natural definable units (Begonet al.
1996). At the beginning of the 20th century there
was much debate about the nature of communities.
The driving question was whether the community
was a self-organized system of co-occurring species
or simply a haphazard collection of populations
with minimal functional integration. At that time,
two extreme views prevailed: one view considered
a community as a superorganism whose member
species were tightly bound together by interactions
that contributed to repeatable patterns of species
abundance in space and time. This concept led to
the assumption that natural communities exist as
fundamental units, and this made it possible to
classify communities in a manner comparable to
the Linnaean taxonomy of species. One of the lead-
ing proponents of this approach was the Nebraskan
botanist Frederick E. Clements. His view became
known as theorganismic concept of communities.It
assumes a common evolutionary history for the
integrated species (Clements 1916).
The opposite view has been termed theindividu-
alistic continuum conceptand was advocated by the
American botanist H.A. Gleason. His focus was on
the traits of individual species that allow each to
live within specific habitats or geographical ranges.
A community can then be seen as an assemblage of
populations of different species whose traits allow
them to persist in a prescribed area (Gleason 1926).
This is a much more arbitrary unit than that envi-
sioned by Clements. In Gleason’s view, spatial
boundaries of communities are not sharp and the
species assemblages may change considerably over
time and space.
We feel that the current broadly accepted view
about the nature of communities is much closer to
Gleason’s opinion. A given species can occur in
rather different collections of species, or commu-
nities, under different circumstances. Community
ecology can, therefore, be described as the study of
the community level of organization, rather than of
a spatially and/or temporally definable unit (Begon
et al. 1996).
As suggested by Morin (1999), we include within
community ecology the study of two or more spe-
cies at a location, including predator–prey inter-
actions, interspecific competition, interactions of
mutualism and parasitism. We recognize that
some ecologists will argue that such pair-wise in-
teractions are simply aspects of population biology,
but we also point out that they provide the basic
interactions that contribute to patterns involving
larger numbers of species. We also recognize that,
even in well-studied communities, much of the
complexity and daunting diversity involving bacte-
ria, fungi, protists and small invertebrates remains
poorly known. Nonetheless, fascinating patterns
among well-studied groups of organisms demand
our attention and beg for explanation.1