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no relationship between the size of the resource and the size of the consumer
(
Oleson et al., 2010; Riede et al., 2011; Vazquez et al., 2009; Yvon-Durocher
et al., 2011
). This review has provided strong evidence that, similar to benthic
suspension feeders, forbidden interactions in host-parasitoid networks are
not predominantly determined by body-size scaling relationships: this sup-
ports the growing body of evidence that size-structuring models are best
applied to predator-prey interactions (
Oleson et al., 2010; Riede et al.,
2011; Yvon-Durocher et al., 2011
). This lack of size structuring in host-
parasitoid networks is especially evident when considering the allometric
scaling of handling constraints, where, while there is evidence for size related
handling capabilities constraining or facilitating host-use events (
Morris and
Fellowes, 2002
), there is no compelling evidence of complete exclusion of an
interaction because of forager size. It is important to note, however, that
there is a distinct correlation between body size and trophic level in host-
parasitoid networks, where the inhabitants of higher trophic levels are smal-
ler than those of lower levels (
Cohen et al., 2005
). This is not a result of
mechanistic relationships between parasitoid size and host suitability and
availability, but arises because parasitoid size is constrained by the
energy available during the larval stage, from a single host, and because
energy transfer between trophic levels is never 100% efficient (
Cohen et al.,
2005
).
Alternatively, fundamental niche structure in host-parasitoid networks is
very strongly determined by the interaction between parasitoid life history
and host phylogeny constraints upon host suitability (summarised in
Figure 12
;
Cagnolo et al., 2011; Sullivan and Volkl, 1999
). In parasitoids,
diet breadth is related to the intimacy of the interaction between the parasit-
oid and its hosts, which concerns both the length of time during which the
parasitoid or its offspring are in contact with the host and the species
specificity of the countermeasures required to overcome host defences.
Life-history traits, such as ecto/endoparasitism and idio/koinobiosis, deter-
mine the intimacy of interaction; traits that increase the degree of intimacy,
such as endoparasitism and koinobiosis, increase the degree to which the diet
breadth of a parasitoid is constrained by host phylogeny, which results in a
narrower diet breadth than for ectoparasitoids or idiobionts (
Godfray, 1994;
Hawkins, 1994
). Trophic levels in host-parasitoid networks are, in most
cases, fixed, and there is no direct relationship between diet breadth and
trophic level as is found in food webs (
Sullivan and Volkl, 1999
). The cases
where trophic levels are not fixed can again be related to parasitoid life
history and the intimacy of interaction. For example, ectohyperparasitoids,
such as Asaphes vulgaris, by nature of their non-specific host exploitation
tactics, have been shown to facultatively switch between both primary and
secondary parasitoid life histories (
Sullivan, 1987; Sullivan and Volkl, 1999
),
and secondary and tertiary life histories (
Sanders and Van Veen, 2010
).
