Geoscience Reference
In-Depth Information
predicted that there are significantly more undescribed parasitoid Hymenop-
teran species than non-parasitoid species. Estimates suggest that if all insect
species were described, then 10-20% of them would be parasitoid wasps,
according to estimates of total Hymenopteran diversity being between
300,000 and 3 million species (
Pennacchio and Strand, 2006; Sharkey,
2007; Whitfield, 1998
). As a result of the ubiquity of parasitoid wasps within
ecosystems, most host-parasitoid networks—and thus the empirical evidence
referenced in this review—have considered these organisms, rather than
parasitoids from other insect orders.
Previously defined parasitoid networks have generally considered
two fixed trophic levels, in an analogous manner to mutualistic networks,
consisting of primary parasitoids (for definitions and examples of terms
describing the traits of parasitoids, their hosts or host-parasitoid networks,
please see
Table 1
) and their hosts (
Cagnolo et al., 2011; Lewis et al., 2002;
Memmott et al., 1994; Rott and Godfray, 2000; van Nouhuys and Hanski,
2002
). The hosts of the primary parasitoids considered in parasitoid network
studies are generally phytophagous arthropods of a particular guild
(
Figure 2
;
Hawkins, 1992
), for example, leaf miners (
Lewis et al., 2002;
Memmott et al., 1994; Rott and Godfray, 2000
). Other studies have consid-
ered another higher trophic level that comprises the secondary parasitoids,
which feed on the primary parasitoids (
Bukovinszky et al., 2008; Eveleigh
et al., 2007; Muller et al., 1999; van Veen et al., 2002
). This secondary
parasitoid trophic level is not always as fixed as the lower levels, and some
species are facultative hyperparasitoids, switching from a primary to a sec-
ondary parasitoid life history under conditions of high inter/intraspecific
competition (
Eveleigh et al., 2007; Mustata and Mustata, 2009; Sullivan,
1987
). Further trophic levels of hyperparasitoids have been documented that
include tertiary and quaternary parasitoids, but such instances are probably
rare (
Mustata and Mustata, 2009
). Due to the importance of plant assem-
blage in determining host, and therefore parasitoid, communities, host-
parasitoid network studies may also include information regarding the
host-plants, which make up the bottom trophic level in these networks
(
Petermann et al., 2010; Tylianakis et al.,2007
).
The hosts of primary parasitoids often comprise parts of the diets of 'true'
predators within the same ecosystem, suggesting that there is potential for
indirect competitive interactions between components of host-parasitoid
networks and food webs (
Memmott et al., 2000; van Veen et al., 2008
).
Further, direct interactions between predators and parasitoids may have a
significant impact on the structure of both types of network; for example, the
ladybird beetle, Harmonia axyridis, feeds upon larvae of A. evri, a parasitoid
wasp, while they are inside their aphid hosts (
Synder and Ives, 2003
), or the
parasitoid wasp Dinocampus coccinellae which utilises ladybird species as
hosts (
Figure 3
A and B).
