Biology Reference
In-Depth Information
Species nonsaturation and nonequilibria
A central concept in the discussion of nonsaturation is that of a vacant or
empty niche. Use of these terms has led to some controversy. It has been
argued that a niche does not really exist unless it is used by an organism,
and the term vacant niche is therefore meaningless. However, many
studies have shown that some niches are much less utilized than other,
similar ones. A vacant or empty niche, thus, is simply a concise way of
saying that more species could exist in a habitat, as suggested by compara-
tive studies. This is easiest to visualize by considering the spatial niche
component, i.e., the habitat or microhabitat of species. If one freshwater
pond is found to contain 10 species, and another of the same size and with
the same characteristics 50, nothing prevents us from concluding that the
former could accommodate more species, in other words that it contains
vacant niches. However, how can we be sure that the characteristics
determining species richness really are the same? In the case of ponds, it
may be difficult, but such difficulties are much reduced or disappear
altogether if we compare very simple habitats, for example fronds of fern
or microhabitats on the gills or in the intestine of fishes. Selecting fish of
equal size from identical habitats removes any error that may arise from
overlooking minor differences that might be responsible for setting differ-
ent ''ceilings'' to species diversity of parasites. The gills of some fish were
found to harbour 1000s of parasites of close to 30 species, whereas most
species of similar size and from similar habitats were found to have a few
specimens of one or two species, or were not infected at all (for details see
below). So, the evidence for the existence of vacant niches is convincing.
There are of course reasons for the differences, but they are likely to be
evolutionary, the result of historical contingencies. Given enough evolu-
tionary time, more species may and will accumulate in the vacant sites.
Many studies have demonstrated nonsaturation of habitats, but non-
saturation does not necessarily imply nonequilibrium, the central theme
of this topic. If population size as a proportion of capacity is plotted against
reproductive rates, the resulting bifurcation diagram never reaches the
carrying capacity. In other words, equilibrium is established well below
growth with increasing population size. The same can be expected for
communities and ecosystems in evolutionary time: the number of species
will always be below the maximum number possible (the ''carrying
capacity for species''), even if equilibrium conditions prevail at a particular
point in evolutionary time. Therefore, in order to use nonsaturation as
