Biology Reference
In-Depth Information
May and Anderson (
1978
) have shown that parasites can regulate
growth of host populations as well, even in the absence of predation or
intraspecific competition (for further references and discussion see
Combes
2001
). In the context of parasite biology, Combes (
2001
) defines
regulation as maintaining a population by density-dependent mechanisms
below the theoretical carrying capacity. Combes further points out that
parasite populations are characterized by regulation - where it exists - that
occurs independently in each infrapopulation. Exceptions to this are
parasites that have long-lived, free-living stages. Regulation in such
cases may not occur in the parasitic but in the free-living stages.
Furthermore, in strongly aggregated parasites, earlier regulation occurs
in the more heavily infected host individuals. Referring to Price (
1980
)
who stressed that the frequencies and intensities of infections with many
parasites are so small that they must be considered nonsaturated and
nonregulated, Combes counters by claiming that regulation may often
not occur at the time of observation, but only when population densities,
which fluctuate cyclically, reach a peak. Also, the hosts observed may not
be the main hosts. Combes finds support in Keymer (
1982
), according to
whom temporal stability is common in parasite populations, indicative of
regulatory processes. Density-dependent regulation is deemed necessary
in many cases because otherwise densities would become too great, either
by renewed infections or multiplication in the host. Exceptions could be
populations in which there is no multiplication in the host and acquisition
of new parasites is low. Combes makes the following point, ''it is difficult
to reconcile the following two statements with the third: (a) parasite
populations are almost always aggregated; (b) aggregation favours the
appearance of regulation; and (c) parasite populations are usually not
regulated.'' Thus, (c) cannot be true when (a) and (b) are true. Combes
therefore concludes that numerous parasite populations are regulated in
their hosts. Poulin (
1998
) also noted that because aggregation is the rule,
density-dependent regulation can be expected. However, these arguments
are not convincing, because even if populations are aggregated and even
if aggregation favours regulation, effective regulation may require much
greater population densities in the most heavily infected host individuals
than is often observed. Moreover, regulation may be sporadic and thus
influence a relatively small part of the population.Allthis,however,does
not mean that regulation in parasite populations does not occur. There are
indeed well documented cases of such regulation.
Esch and Fernandez (
1993
) give examples of density-dependent and
density-independent processes in parasites, and Combes (
2001
) distinguishes
