Biology Reference
In-Depth Information
by Lawton and MacGarvin (
1986
) who discussed the observation that
numbers of species of insects on introduced plants at first increase rapidly
and then more and more slowly, until a maximum is reached. They
consider the possibility that interspecific competition is responsible as
unlikely, because these plants have many empty niches. A more likely
explanation is that species for successful colonization are not available
(pool exhaustion hypothesis of Lawton and Strong
1981
). This may be an
important mechanism explaining poor communities.
The packing rules based on fractal geometry and competition
Ritchie and Olff (
1999
) have used spatial scaling laws (fractal geometry) to
derive a rule for the minimum similarity in the size of species that share
resources. Serengeti (East African) mammalian herbivores and Minnesota
savanna plants were shown to conform to the predictions of the packing rules
(pp. 41-42). However, Rohde (
2001a
) has shown that the packing rules do
not apply to metazoan ecto- and endoparasites of marine fishes (pp. 43-44).
These negative results support the view that parasites of marine fish do not
live in saturated structured communities but rather in assemblages not sig-
nificantly structured by interspecific competition. The positive results pre-
sented by Ritchie and Olff (
1999
) for Serengeti grazing mammals and North
American savanna can be explained by the fact that they are either vagile
(mammals) or disperse well (savanna plants), and both utilize significant
proportions of the resources for which different species compete, plants in
thecaseoftheformerandlightandspaceinthecaseofthelatter.Interspecific
competition is therefore expected (for further details see pp. 178-180).
Even if competition occurs and even if it is important in structuring
communities, effects on infra- and component communities may be quite
different, as illustrated by the example of trematode communities in snails.
Infracommunities of parasites are defined as all populations of all species
infecting a single host individual, component communities are all the
populations of all parasite species infecting a host population.
Different effects of competition in infra- and component
communities
Of great importance, the studies of Sousa (
1990
,
1992
,
1993
) and Kuris
(
1990
) on the community structure of larval trematodes (18 species) of
the coastal snail species Cerithidea californica, at a number of sites on the
Californian coast, led to the conclusion that trematode species interact at
