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important role in determining a parasitoid's diet breadth in the presence of
host mutualists ( Sanders and Van Veen, 2010; Sullivan and Volkl, 1999 ).
V. REALISED NICHE IN PARASITOIDS
A. Optimal Foraging
In regard with how parasitoids maximise their reproductive output, Cook and
Hubbard (1977) stated that: 'In a consideration of the strategies adopted by
insect parasites when searching for their hosts it is realistic to assume that an
underlying objective of their behaviour is to leave as many offspring as possible
which survive to reproduce in the next generation. Natural selection will favour
those strategies which result in a reproductively efficient distribution of the
parasite's egg complement among the available host population' (p. 115).
As the above quotation suggests, in parasitoids, foraging success is directly
related to reproductive success, meaning that parasitoid foraging behaviour
should be more closely related to optimal foraging theory than is evident in
food webs because of a reduced level of conflict between reproductive and
feeding processes ( Wajnberg, 2006 ). Optimal foraging theory is thought to
determine how fundamental niches are structured into realised niches, where
interactions between consumers and their resources are strongest/most likely
with the resources that best increase consumer fitness ( Petchey et al., 2008 ).
It is important, therefore, if this theory is to hold, that we are able to
demonstrate that parasitoids forage in an optimal manner, what the limited
resources are that need to be allocated optimally, how these relate to parasit-
oid ecology and thereby how optimal foraging impacts upon host-parasitoid
network structure.
B. Maximising Host Encounter Rate
In order to produce as many offspring as possible, parasitoid foragers need to
make sure that they experience the maximum number of oviposition oppor-
tunities during their lifetime ( Cook and Hubbard, 1977; Hubbard and Cook,
1978; Wajnberg, 2006 ). For the purpose of parasitoid optimal foraging
theory, hosts are typically considered to exist as a network of aggregated
patches distributed in a habitat, patches of higher host density theoretically
offer an increased host encounter rate, which decreases as hosts within a
patch are utilised ( Hubbard and Cook, 1978 ). In order to maximise the
number of hosts a parasitoid encounters, a forager should spend time within
each patch in such a way that host encounter rate is the same across all
available patches; this time allocation strategy is called the 'Marginal value
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