Biology Reference
In-Depth Information
5
.
Noncompetitive mechanisms
responsible for niche restriction
and segregation
Niche restriction is usually attributed to interspecific competition: a
species does not spread from its optimal niche into less suitable ones
because competing species, better adapted to these niches, prevent it
from doing so. However, the chapters dealing with interspecific compe-
tition have shown that evidence for this assumption is in many cases far
less convincing than generally assumed. But why, then, are niches
restricted? This chapter deals with this problem by firstly demonstrating
that niches may be restricted even if potentially interacting species are
absent, and secondly by suggesting other, non-competitive mechanisms
responsible for niche restriction. It further proposes alternative mechan-
isms for explaining niche segregation.
Evidence for niche restriction even in the absence
of potentially interacting species, and mechanisms
responsible
Gill parasites of fishes are particularly good models for examining this
question because the distribution of parasites can be easily mapped,
because the number of species is limited (even in the richest communities
they do not exceed about 30 species and are usually much fewer), and
because an almost unlimited number of replicas are available. Rohde, in a
series of papers (e.g., Rohde
1976a
,
b
;
1977a
,
b
,
c
;
1978a
,
b
;
1980b
; and
reviews
1989
;
1991
;
1994a
;
2002
), has shown that monogenean gill
parasites of marine fish from all latitudes use microhabitats that are some-
times very restricted, even when competing species do not exist or, when
they do exist, are not present on individual fish. Figure
5.1
gives an
example. Six fish were examined and found to have one species of
Monogenea on the gill filaments (maximum 18 worms on one fish), all
of which chose the same microhabitat, the base of the gill filaments in the
middle portion of the gills. Only two individuals of other parasites were
present, one copepod larva on the gill filaments and one adult copepod in
