Geoscience Reference
In-Depth Information
increases the 'killing power' of the parasitoid population, increasing its
capacity to control the pest population (
Mills and Wajnberg, 2008
).
Sex ratio related host-choice decisions also present an important opportu-
nity to study apparent competition mediated by a shared natural predator.
If parasitoids preferentially utilise host species B for males, which represent
an egg sink as they do not interact with hosts, in a patch containing species
A and B, then there will be fewer eggs available for allocation towards species
A. Conversely, the presence of species A will increase the female ratio of the
parasitoid population, which will result in the asymmetric suppression of
species B (
Figure 10
;
Heimpel et al., 2003
). Quantified web data can be used
to test for any strength of indirect effects, such as sex related host-choice
60%
High killing
power
Host A
40%
60%
Host A
High killing
power
Host B
40%
40%
Low killing
power
Host B
60%
Figure 10
Hypothetical offspring sex ratio allocation decisions made by female
parasitoids in the presence of three separate host patches. Foraging parasitoids are
thought to allocate a larger number of fertilised (female) eggs to populations of larger
hosts (Host A in this case) because the allometric scaling of fitness is stronger/steeper
for female offspring than for males, that is, larger female offspring provide a greater
increase to inclusive fitness than large male offspring. This results in a female-biased
sex ratio for parasitoid populations reared on Host A, which, as female parasitoids
(and not male ones) are responsible for the death of hosts, results in an high killing
power (more eggs to allocate to available hosts) of the parasitoid population. Con-
versely, Host B is a small, poor host, and parasitoid populations reared on it have a
male biased sex ratio and a low killing power. Parasitoid populations reared upon
both hosts have a high killing power, resulting in an increased rate of attack for Host
B compared to when it exists on its own. However, Host A experiences a reduced rate
of attack compared to when it exists on its own as fewer eggs as it shares the burden of
parasitism with Host B. This disproportionate effect of parasitism by a shared
parasitoid is known as 'apparent competition' and may represent a significant struc-
turing force within host-parasitoid networks.
