Biology Reference
In-Depth Information
predator-prey dynamics of Paramecium and Didinium, pointed out that
coexistence of both species of Paramecium was made possible by periodic
immigrations from adjacent habitats (not possible in simple laboratory
experiments which did not replicate the spatial component of natural
habitats). Huffaker (
1958
) showed this experimentally in a phytophagous
mite-predator system. Tilman et al.(
1997
) and Hanski (
1997
) discuss
several models that simulate population dynamics in spatial habitats (see
also other chapters in Tilman and Kareiva
1997
).
Ecosystems are complex adaptive systems (CAPs), and resilience in
such systems results from maintenance of heterogeneity (Levin
1998
,
references therein). It is important to note that not only spatial, but also
temporal heterogeneity, contribute to diversity and resilience of systems.
It is also important to note that resource heterogeneity may exist in
''essential homogeneous'' environments, i.e., in environments that, in
the long term, have similar statistical characteristics, but differ due to
historical events such as local disturbances caused, for example, by the
extinction of a dominant species (Levin
1998
).
In simple laboratory experiments, the number of potentially compet-
ing species has to be small. However, in nature, the vast majority of
ecological systems comprise a multitude of species, and effects of compe-
tition are not necessarily the same in species-poor and species-rich
systems. The next section deals with the question of how diversity affects
species interactions. It also discusses the effects of supposedly ''harsh'' and
''benign'' environments, and various other factors that may have an effect
on the degree of competition.
Factors that determine the degree of competition in populations
(1) Species diversity and competition, and the effects of environmental factors
The probability of coevolution of a species with competitors may be
inversely related to species diversity. For example, corals and trees in
tropical rainforests have the potential to interact strongly not with one
but with many neighbors (Connell
1980
). In Queensland rainforests, the
mean number of tree species with a height greater than 0.5mper 10
10m
plot had 57 other trees among the nearest neighbors, and for some of the
less common species, all nearest neighbors were different
species.
Coevolution between them is therefore unlikely.
Connell (
1975
) points out that interspecific competition is most likely
in moderately harsh physical environments. Very harsh conditions reduce
population densities to such a degree that competition becomes unlikely,
