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late reproduction; specifically, the proportion of an individual's lifetime egg
load that is mature upon eclosure (
Jervis et al., 2001
). The allocation of
resources towards early reproduction, pro-ovigeny, results in species with
greater reproductive potential but a shorter lifespan; conversely, synovigenic
species, which mature eggs during their lifetime, have fewer offspring but live
longer (
Blackburn, 1991; Jervis et al., 2001, 2008
). Previous studies have
considered how egg-limitation or time limitation drives the proportional
allocation of resources, during development within the host, to allow species
to optimally utilise host resources (
Jervis et al., 2008
). Recently, the division
between the two trains of thought has been bridged. In actuality, individual
parasitoids exhibit different levels of egg or time limitation over their
lifetime, differing according to species ecology, where they both, roughly,
equally contribute to selection of optimal allocation strategies (
Rosenheim
et al., 2008
). However, the relationships between life history and resource
limitation, which have already been considered in previous studies regarding
evolution, have strong applications to optimal foraging and host-parasitoid
network structure (
Casas et al., 2000; Ellers et al., 2000; Heimpel and
Rosenheim, 1998; Jervis et al., 2008; Rosenheim et al., 2008
).
In order to reconcile the two schools of thought based on the importance
of time versus egg-limitation in parasitoid evolution,
Rosenheim (1999)
considered the two conditions in terms of their effect on future fitness
returns. This outlook is an important way of considering the effect of
resource limitation on foraging strategy, as parasitoids are thought to forage
in a manner where they are maximising individual fitness (
Cook and
Hubbard, 1977; Hubbard and Cook, 1978
). The cost of laying an egg is
twofold: firstly, that egg cannot be used again, which only incurs a cost to
future fitness if the forager does not have an egg available when a better host
becomes available (egg limitation); secondly, the time taken to lay the egg
cannot be used to find and attack another host, which only incurs a cost if the
forager does not have enough time to allocate all of its eggs (time limitation).
Therefore, an increased likelihood of egg limitation means there are greater
costs associated with the use of each egg, and time limitation means there are
greater costs to utilising each unit of time (
Figure 7
), which will affect the
profitability of hosts (
Rosenheim et al., 2008
). The rest of this review con-
siders how these two costs are related to the life history of parasitoids and
how egg and time limitation can be related to foraging strategy.
C. Egg-Limitation, Life History and Electivity
In egg-limited foragers, eggs are the limited resource that must be allocated
optimally. Individuals that are more likely to be egg-limited during their
lifetime incur higher fitness costs to utilising suboptimal hosts because the
