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three types of density-dependent mechanisms for parasites: (a) decision-
dependent regulation (infective stages avoid hosts that are already infected);
(b) competition-dependent regulation (population size is limited by limit-
ing resources or some active elimination processes); and (c) host death-
dependent regulation (most heavily infected host individuals die). Combes
(references therein) gives many examples for all three mechanisms. Thus,
certain parasitoids do not oviposit in hosts already infected (a). Several
intestinal trematodes and cestodes were shown to have reduced fecundity
at high intensities of infection (b), and density-dependent regulation may be
host-mediated (certain birds heavily infected with trematodes are more
easily preyed upon by predators) (c). The last two mechanisms are probably
the most significant.
Concerning the first point, Combes gives the example of the wasp
Venturia canescens, which is a parasitoid of the moth Ephestia kuehniella.
The wasp can (but does not always) decide not to oviposit in moths
already infected.
Concerning the second mechanism, Combes ( 2001 ) and Esch and
Fernandez ( 1993 ) describe in some detail the example of the cestode
Hymenolepis diminuta, which has been well investigated by several authors.
Final hosts are rats that become infected by eating intermediate beetle
hosts containing larval cysticercoids. In various experiments, different
dosages (1-20) of cysticercoids were applied. Two months after infection,
the length, weight, and egg production of each tapeworm were measured,
and found to be inversely proportional to the number of cysticercoids
given. In fact, the greatest number of eggs produced per rat was found in
those rats that had received a single cysticercoid.
Concerning the third point (host-mediated regulation) mentioned by
Combes, this may be brought about in several ways. For example, heavily
infected host individuals may be weakened and either die as a result of
the infection, or are more likely to be eaten by predators, The latter has
been shown for red grouse in Scotland infected with the nematode
Trichostrongylus tenuis. Those grouse killed by predators were shown to
be heavily infected. Because of the highly aggregated distribution of most
parasites in their host populations, such selective disappearance of the
most heavily infected host individuals may have very significant effects on
the parasite population as a whole.
Bradley ( 1974 ), also contradicting Price (see above), suggested that
parasite populations are controlled by low colonization probabilities only
in unfavourable habitats; in more favourable ones they are controlled
by factors such as reduction in the number of parasites that can develop,
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