Geoscience Reference
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profitability, in terms of individual consumer fitness, associated with
the consumption of resources within the consumer's fundamental niche
(
Petchey et al., 2008
). Trait-pairing characteristics facilitate or hinder inter-
actions between species within ecological networks, changing the profitabili-
ty of resource items (
Vazquez et al., 2009
). Consumer and resource body sizes
have been identified as important trait-pairing characteristics in food webs
and mutualistic networks (
Brose et al., 2006; Petchey et al., 2008; Rooney
et al., 2008; Stang et al., 2009
).
In ADBM studies, which focus on modelling predator-prey interactions,
prey profitability is determined by searching time costs, handling costs
(which are both functions of the sizes of both predators and their prey) and
nutritional benefit (which is a function of prey size), and these costs and
benefits interact in such a way that larger, more nutritious prey items are
more profitable for larger predators (
Petchey et al., 2008
). As optimal forag-
ing theories predict that consumers should preferentially consume the most
profitable resources, the ADBM predicts that larger predators consume
larger prey (
Petchey et al., 2008
); these patterns have been confirmed in a
wide range of marine, fresh-water and terrestrial predator-prey networks
(
Petchey et al., 2008; Riede et al., 2011; Woodward et al., 2010a
). Although,
under conditions where prey items are scare, predators should, and have been
shown to, exhibit a reduced level of size-selectivity in their prey consumption
decisions (
Galarowicz et al., 2006
).
We have provided evidence that, similarly to food webs, foraging decisions
made by parasitoids are determined by costs associated with searching for and
handling different host resources (
Heimpel and Casas, 2008
). However, para-
sitoids may be limited in their foraging success by either the time available for
finding and utilising viable hosts (time limitation), or by the number of eggs
available to allocate to the hosts that they find (egg-limitation) (
Figure 13
).
This additional limitation on foraging success can change the way that host-
parasitoid networks are structured when compared to food webs as it changes
the currency of optimisation: that is, for predators, optimal foraging always
considers energy consumption per unit time, but for parasitoids, foraging
decisions can be related to the optimal allocation of time and/or eggs
(
Hubbard and Cook, 1978; Petchey et al., 2008; Rosenheim et al.,2008
).
Parasitoids that are more likely to experience egg-limitation can only use a
certain number of hosts irrespective of their host encounter rate. Consequent-
ly, these parasitoids maximise their individual fitness by preferentially allocat-
ing their limited eggs to the best quality hosts (
Minkenberg et al.,1992
).
Conversely, parasitoids that are not egg-limited should make host-choice
decisions according to the relative costs associated with finding a resource
and utilising it, similar to predators in food webs (
Wajnberg, 2006
). If foraging
success is more strongly limited by the time taken to encounter a host, then
parasitoids should utilise every host they encounter, provided that it is viable.
