Biology Reference
In-Depth Information
Furthermore, parasite distributions within host populations are usually
overdispersed, leading to undercolonization of most host individuals.
Price notes that disruptions in parasite-host systems commonly occur,
resulting in nonequilibrium states. The complexity of biotic interactions
characteristic of a parasitic way of life, according to Price, is the main
factor generating nonequilibrium; (also May
1973
: complexity decreases
stability). Price discusses several examples in detail. In each of these
examples, patchy resources (host individuals) lead to a low probability
of colonization, and ephemeral patches result in a high probability of
extinction. Schistosoma populations provide one of his examples. Here,
patchiness of resources is indicated by the patchy distribution of water, the
independent differentiation of snail host populations, highly specific
parasite strains, and patchiness of snail and vertebrate host distribution.
Ephemeral patches are due to temporary availability of water, high
mortality, sterility or reduced fecundity of snail hosts, the dynamics of
snail and vertebrate hosts, and developing host resistance.
However not only in populations, but also in communities of parasites,
equilibrium conditions are unlikely, since the parasite populations of
which they are comprised are all in nonequilibrium (Price
1980
).
Mechanisms leading to nonequilibrial communities are the same as for
populations: if colonization probabilites are low and extinction probabil-
ities high - which is usually the case - nonsaturation and nonequilibrium
will result. Price discusses a number of examples and concludes that, if
interspecific competition occurs in such systems, it is intermittent.
Strong (
1984
) introduced the term density-vagueness to describe par-
ameters of birth and death rates that are only weakly explained by density.
Sometimes, the density effect can be inferred only with imagination,
or the correlation may be low (although statistically significant).
Explanations other than density may be age, weather, migration, and
others. He provides evidence that density dependence is not evident in
a large number of studies and life-cycle stages, but he also points out why
density dependence may sometimes be underestimated (but also over-
estimated). Nevertheless, for insects at least, density effects, where they
occur, are often weak, intermittent, or discontinuous.
The belief that equilibrium conditions and competition are the major
factors determining ecological processes, underlies and informs most
ecological modelling. The concepts of evolutionary stable strategy
(ESS), continuously stable strategy (CSS), and neighborhood invader
strategy (NIS), for example, are based to a large degree on such an
assumption (e.g., Apaloo
2003
, further references therein).
