Biology Reference
In-Depth Information
(Goater et al.
1987
), because their ecological requirements are closer and
they therefore have a greater potential for resource competition, which
should lead to a reduction of sympatric congeners.
But what is the null model? How many congeners are expected if they
are acquired at random? Congeners are extremely common in some
habitats, but not in others. Particularly striking are the large numbers of
congeneric parasite species infecting the same host, for example flocks of
nematodes in Australian marsupials. Thus, up to 20 species of the genus
Cloacina (Nematoda, Strongyloidea) occur in a single species of kangaroo
(Beveridge et al.
2002
). Parsimony analysis of the nematodes showed that
the species flock in each host species is polyphyletic in origin, although a
few related parasite species occur in some of the hosts. Interestingly, even
closely related host species may differ considerably in numbers of con-
generic nematodes. For example, Macropus agilis is most commonly
infected with a single species of Cloacina, whereas M. dorsalis normally
harbours a large number. But does the small number of congeners in the
former species really indicate competitive exclusion? If this were the case,
why are there so many species in a closely related host? There is certainly
no evidence whatsoever that interspecific competition in one host should
be stronger than in another. The most parsimonious explanation for the
differences appears to be that they are fortuitous, due to some ecological
differences between host species that make infection more difficult in one
than in the other. For negative results to find reduction of sympatric
congeners, see also Pianka (
1973
) for lizards and Terborgh and Weske
(
1969
) for birds.
Concerning fish parasites, Rohde (
1989
) pointed out that a null-
hypothesis, i.e., how many congeners could be expected, cannot be
established, but the great number of congeners found makes it unlikely
that a reduction in their number due to competition has occurred. Data
are incomplete, but it seems likely that fish species without congeneric
parasites are the exception rather than the rule.
Nevertheless, in some cases a reduction in the number of congeners
co-occurring in the same microhabitat has been demonstrated. Rohde
and Hobbs (
1986
) have shown that congeneric monogeneans on the gills
of marine teleost fish are more strictly segregated than non-congeners.
However, such segregation of congeners occurred only in species with
identical copulatory organs. Congeners with distinctly different copula-
tory sclerites often share the same microhabitat. This means that not
interspecific competition, but reinforcement of reproductive barriers is
responsible for segregation. This is discussed in detail in Chapter
5
.
