Biology Reference
In-Depth Information
species, thus resulting in nonequilibrium conditions. Interspecific compe-
tition is most likely when at least two potential competitors are likely to
co-occur in the same patch, and when population responses are rapid
leading to exhaustion of many of the resources. Price (
1984
) predicts that
intestinal helminths of vertebrates should show little competition, as
resources are constant (space) or steadily renewed (food), hosts are patchy
and population responses of helminths slow (no reproduction in the host).
A review of the findings on fish parasites as a general model for
evaluating intra- and interspecific interactions in low density populations
in resource-rich habitats was given by Rohde (
1991
). He used ectopara-
sites of marine fish as a model but extended the conclusions to other taxa,
concluding that most animal species are likely to live in low-density
populations in resource-rich habitats. In such habitats, many potential
niches are vacant and interspecific competition is of little importance
although it may occur. Microhabitats may or may not expand when
population density is high, and niche restriction may have the function
to enhance chances of mating. Segregation often may not be due to
competition but reinforcement of reproductive barriers (see pp. 85-89
for further details).
Nevertheless, there can be no doubt that resources are sometimes
limited and that species compete for them. Brown et al.(
1979
, further
references therein) have shown experimentally that competition for seeds
is important in determining community structure of desert granivorous
rodents, and Pimm (
1978
), also experimentally, has shown that, in hum-
mingbirds, competition occurs when resources are predictable, but
decreasingly so when resources became less predictable.
The finding that resources are sometimes and perhaps often limiting,
leads to the question of how similar species competing for the same
resource can be, a problem discussed in the following section.
Effects of competition on species
Limiting similarity and ecological character displacement
If resources are indeed limiting, competition for similar resources should
lead to some displacement between species in order to reduce competi-
tion. But how close can two competing species be? Hutchinson (
1959
)
believed that there was a body size difference (in length units) of 1:1.3 in
coexisting species pairs, indicative of the difference necessary for species
to coexist at the same level of the food web. However, is divergence
