Biology Reference
In-Depth Information
only a single species could be maintained in a steady state. Or, according
to Levin (
1970
), ''No stable equilibrium can be attained in an ecological
community in which some r components are limited by less than r limit-
ing factors. In particular, no stable equilibrium is possible if some r species
are limited by less than r factors.'' Biodiversity is only possible because the
world is not homogeneous, in other words, because species can exploit
heterogeneous resources and patterns (Levin
2000
). When new, rare
species arise, they can survive by exploiting new niches. Heterogeneity
may be spatial and temporal. (However, see pp. 52-53: Gause's principle
valid only because of environmental stochasticity).
Leslie et al.
1968
) demonstrated competitive exclusion in two
Tribolium species. However, they also noted unexplained coexistence
between the species. As an explanation of this observation, Edmunds
et al.(
2003
) have recently proposed a model in which, as interspecific
competition increases, there is a sequence of bifurcations; that is, a
scenario with two stable competitive exclusion equilibria is replaced by
a scenario with two competitive exclusion equilibria and a stable coex-
istence cycle. In other words, two species may well coexist on one
limiting resource, even if (or because) there is increased competition.
An example of competitive exclusion is the trematode Gorgodera euzeti
and the monogenean Polystoma integerrimum, both infecting the urinary
bladder of the frog Rana temporaria in the Pyrenees (Combes
2001
,
reference therein). The number of frogs examined was 1941, the number
infected with the first species alone was 576, with the second species 280,
with both species 39. If double infections had occurred by chance alone,
the number should be at least 576
280/1941
ΒΌ
83. This example seems
convincing, but can the possibility be excluded entirely that slight differ-
ences in habitat preferences of the two species are responsible for the
smaller than expected number of double infections? Combes (p. 434)
gives some other examples for parasites (see also Kuris
1990
, for larval
trematodes). For a detailed discussion of competition in larval trematodes
in snails, demonstrated by experiments or inference from field studies, see
Combes (
2001
), page 428 onwards (for details see Chapter
8
). Rohde
(
1979a
) pointed out that intraspecific crowding may have effects similar
to competitive exclusion. Moulton and Pimm (
1987
) studied the effects
of the 49 bird species introduced to the Hawaiian Islands from 1869 to
1983 and found convincing evidence for competitive exclusion.
Competitive exclusion in a particular habitat can be overcome by
immigrations from adjacent habitats. Already Gause (
1935
), in his classical
study on competition between Paramecium caudatum and P. aurelia and
