Biology Reference
In-Depth Information
Effects of competition on microhabitat width
Microhabitat width is just one of the many niche dimensions that may be
affected by competition. Effects may occur in extant communities, e.g.,
when the microhabitat of one species contracts in the presence of another,
or it may have occurred in the past as the result of coevolution with
competitors (the ''ghost of competition past'').
Rohde (
1979a
) gave a detailed discussion of evidence on niche
restriction in parasites and concluded, with respect to microhabitat
restriction, that
(1) interspecific competition in parasites occurs and may lead to compe-
titive exclusion or changes in microhabitat width in some or all
co-occurring species (interactive site segregation), but there is no
evidence that such effects lead to evolutionary changes and avoidance
of competition, i.e., to selective site segregation;
(2) parasites with coinciding or overlapping microhabitats often show no
interactions;
(3) related species commonly have widely overlapping microhabitats;
(4) the effect of species intrinsic factors (facilitation of mating) on niche
restriction is indicated by the finding that competing species often do
not exist and cannot have existed in the past;
(5) circumstantial evidence suggests that niche restriction may be due to
selection for increasing intraspecific contact and thus mating (''mating
hypothesis of niche restriction'');
(6) the probability that two species have completely coinciding niches is
infinitesimally small, and niche differences should not be used as
evidence for competition.
A more detailed discussion of the evidence is given on pp. 81-85.
As pointed out above, one supposed effect of interspecific competition
is change in microhabitat width in the presence of competing species.
Kennedy (
1985
,
1992
), in careful studies, demonstrated that microhabi-
tats of acanthocephalans in the digestive tract of the eel Anguilla anguilla in
Britain contracted in the presence of other species. In contrast, Rohde
(
1981a
) compared microhabitat width (as measured by the number of
sections of the gills and mouth cavities of approximately equal size) of
ectoparasites of poorly and heavily infected marine fish. He found no
correlation. Moreover, there is no significant difference in the number of
host species used by monogenean ectoparasites of marine fishes at differ-
ent latitudes, although tropical fish have considerably greater numbers of
