Geoscience Reference
In-Depth Information
Blackburn (1991)
reported a significant negative correlation between
egg size and fecundity in parasitoids, suggesting that larger eggs incur a
greater cost to potential fecundity than smaller eggs. This correlation
presents a similar scenario to that suggested above, representing a trade-
off between investment in offspring success and investment in the total
number of offspring produced. In cases where a greater energetic cost is
associated with the production of an egg, there is a greater cost to fitness
associated with wasting the egg on an unsuitable host (
Rosenheim et al.,
2008
). Species with large eggs are likely to be egg-limited due to the high
cost of producing each egg, combined with the increased cost to potential
reproduction associated with each egg, suggesting that optimal foraging
strategies in these species are based on host quality and not encounter
rate. Egg size in parasitoids is strongly determined by life history: for
example, embryos of ectoparasitoids do not have access to host resources
during development to the larval stage and often require large, yolk-rich
(anhydropic) eggs to complete this initial stage of development; converse-
ly, endoparasitoid embryos may utilise the host haemolymph and species
will most often lay small, yolk-deficient (hydropic) eggs. We suggest for
endoparasitoids, which also have a more constrained fundamental niche,
interactions with available hosts within the network are structured by host
abundances and not relative host qualities; with the opposite being true
for ectoparasitoids. The relationship between egg size and the structure of
fundamental and realised niches in parasitoids has not, to the best of our
knowledge, previously been considered; a study comparing the egg sizes of
different parasitoid species and how their networks are structured could
help validate the above hypothesis.
D. Time Limitation, Life History and Electivity
If we consider time limitation to be the opposite of egg-limitation, we would
expect parasitoids that have a greater likelihood of dying before allocating
their eggs to experience a higher cost to future fitness associated with not
attacking an available host; that is, spending time not ovipositing incurs a
greater cost when time is a more limited resource than eggs. This idea only
holds when the lifetime foraging success is limited by searching time alone,
and there is a negligible time cost associated with oviposition (
Figure 9
;
Outreman and Pierre, 2005
). However, where time is limited because a high
cost to future reproduction is associated with using an available host and not
finding it, that is, high handling time as opposed to searching time, we expect
species to exhibit high levels of electivity (
Figure 9
). This is because, in this
instance, utilising high quality hosts is the best way to maximise fitness per
unit of time spent. There is some empirical evidence corroborating this idea: a
