Geoscience Reference
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are determined by evolutionary history, with little scope for generalisation
based on traits such as body size. Correspondingly, a recent study that
explored the structure of a host-parasitoid network of parasitoid wasps
and leaf miners revealed that the network was highly compartmentalised,
with host phylogeny playing a significant role in the organisation of
these compartments (
Cagnolo et al., 2011
). However, as we show here,
differences in certain life-history traits are linked with diet breadth and
structure host-parasitoid networks in a manner analogous to variation in
body size in food webs.
IV. LIFE HISTORY AND HOST RANGE
Parasitoid species traits related to how a parasitoid forages determine which
hosts can, and which cannot, be used successfully as a nursery for their
offspring: that is, they determine the parasitoid's fundamental niche. This
section introduces some of the axes of insect parasitoid life history that have
been used to distinguish between species and discusses how they determine a
species' fundamental niche.
A. Egg Placement
Parasitoid species are often categorised according to where they place their
eggs in relation to the host during the act of oviposition. The position of the
egg determines the level of the interaction between the parasitoid and the
host; species with more intimate interactions with their hosts, such as endo-
parasitoids, will have narrower diet breadths (
van Veen et al., 2008
). Endo-
parasitoid larvae have to overcome specialised internal host defences, such as
egg encapsulation. The development of countermeasures to these internal
defences requires high levels of phylogenetic specificity and dramatically
reduces the number of viable host species for endoparasitoids compared
with ectoparasitoids, which only have to overcome generalised external
defences, such as kicking or rolling (
Quicke, 1997; Sullivan and Volkl,
1999
). We would expect, therefore, endoparasitoids to have narrower diet
breadths than ectoparasitoids and promote a phylogeny-based compartmen-
talised structure within the host-parasitoid network (
Quicke, 1997; Sullivan
and Volkl, 1999; van Veen et al., 2008
).
Some ectoparasitoids have been suggested to be able to switch facultatively
from a primary to a hyperparasitoid life history (
Sullivan and Volkl, 1999
).
This is the only example of flexible trophic levels in host-parasitoid net-
works. All endohyperparasitoids are obligate hyperparasitoids and show
