Biology Reference
In-Depth Information
Figure 5.1. Distribution of one species of Monogenea (lines), one adult copepod
(large filled circle), and one larval copepod (small filled circle) on the gills and in the
mouth cavity of six Oligoplites palometa off the coast of Brazil. Note: the habitat is
largely empty (related fish species were found to have much larger numbers of species
occurring at greater abundances). In spite of this, species have narrowly defined
microhabitats. From Rohde (
1991
). Reprinted by permission of the editor of Oikos.
the mouth cavity, suggesting that niche restriction is not the result of
interspecific effects, but genetically programmed. The proximate causes for
niche finding in this case have not been studied, but may be water currents,
size of the gill filaments, or some other factors. There is now ample
evidence that preference for microhabitats is, at least to a large degree,
not affected by the presence of other species: even species that always
occur singly may have very narrowmicrohabitats, although environmental
factors such as oxygen concentration, or crowding (i.e., presence of large
numbers of individuals of the same species) may have effects.
Two non-competitive mechanisms for niche restriction have been
proposed, the necessity to specialize in order to guarantee survival in a
particular microhabitat, and mate finding. Niche space is largely empty, as
shown in Chapter
2
(''Species nonsaturation and nonequilibria'').
All species must be specialized to varying degrees, and this alone will
lead to niche restriction even in largely empty niche space. Price (
1980
),
in particular, emphasized this aspect of niche restriction. Rohde (
1976a
,
b
;
1977b
) suggested that one ultimate reason (i.e., one biological function) for
narrow niches may be the enhancement of mating encounters.
Evidence for the ''mating hypothesis of niche restriction'' was again
provided by using fish and other parasites. Many of these parasites are
hermaphroditic (all Monogenea, most Digenea, all Aspidogastrea, all
Gyrocotylidea, all Amphilinidea, the Eucestoda). Therefore, it is important
to point out that these species often show obligatory cross-fertilization.
