Geoscience Reference
In-Depth Information
theorem' (MVT) (
Charnov, 1976
). As a result of this pattern, foragers are
expected to allocate proportionally more time to patches of higher than
patches of lower host density: density-dependent foraging (
Cook and
Hubbard, 1977; Hubbard and Cook, 1978
). With the introduction of multi-
ple foragers, a similar pattern is expected, where individuals should distribute
themselves in space according to host availability, density-dependent aggre-
gation. The 'Ideal Free Distribution' (IFD) predicts that more foragers
should be located in patches of higher host density, in such a manner that
all foragers within the habitat encounter hosts at the same rate (
Wajnberg,
2006
). In terms of ecological network structure, this results in the interaction
strengths between parasitoids and their hosts being determined by the rela-
tive abundance of each viable host. In reality, the relative adherence of
foragers to the above predictions of parasitoid distribution is evident in
different degrees in different studies; it has been suggested that differences
can be explained by parasitoid ecology, host distribution and the abiotic
environment (
Corley et al., 2010; Lessells, 1985; van Veen et al., 2002;
Wajnberg, 2006
). In addition, and of particular importance, when foraging
on a multiple host species, the quality of individual hosts is predicted to play
an important role in foraging decisions.
An important mechanism that determines the distribution of foragers
across patches is the patch allocation time (PAT), which describes the
amount of time that a forager spends within a host patch (
Wajnberg,
2006
); according to the MVT, PAT should be higher in patches of higher
host density. PAT is currently thought to be determined by host encounter
rates, where each encounter with a host increases (incremental PAT) or
decreases (decremental PAT) the time spent within a patch before the forager
moves to the next patch; for an 'in depth' review, see
Wajnberg (2006)
.
Incremental PAT rules, in response to viable hosts for oviposition, result in
positive density-dependent foraging and parasitoid distribution (
van Alphen
et al., 2003
); conversely, decremental PAT rules result in density independent
foraging, where parasitoids do not conform to the IFD. However, the idea
that we see decremental PAT rules seems to contradict the above idea that
species are trying to maximise their oviposition rate. In situ studies have
shown that parasitism rates can be positively or negatively related to, as well
as be independent of, host density within a patch; a single study has reported
that all three types of variation can be found under the same conditions
within the same host-parasitoid networks (
van Veen et al., 2002
). A series of
theories have been suggested that attempt to explain how parasitoids can
exhibit this range of relationships between host abundance and parasitism
rate and still be foraging optimally.
In a study of the parasitoid wasp Ibalia leucospoides, the study population
exhibited a strong adherence to the IFD, but the strength of the adherence
decreased with increased distance from the experimental release point (
Corley
