60
M. E. SOLOMON
Among the main lines of thought surrounding this model, one has
been to challenge or investigate the assumptions, including the assumption of insatiable parasites. While it is obvious that parasites do not
have unlimited fecundity, it is not necessary, for the purposes of the
model, to assume that they do. One is required to assume only that the
limit of their fecundity is not reached, and that fecundity does not
decline at the levels of host and parasite abundance involved (cf,
Varley, 1947).
This assumption receives no support from the above-mentioned
experimental data on the functional response of insect parasites reviewed
by Holling (1959b). The diagrams, similar in form to Fig. 17, show that
the number of hosts parasitized rises proportionately less than host
density does, from the lowest levels of density studied. While these
results are not simply an index of parasite fecundity, they do remind
us that parasites, besides seeking and finding hosts, spend some time
in dealing with the discovered hosts and depositing their eggs; the
greater the density of hosts, the greater is the proportion of the parasites’ time that is spent in this way instead of seeking more hosts (cf.
Tinbergen and Klomp, 1960). They also show that, however high is the
hosts’ density, the parasites are unable to attack more than a definite
number of them. (To explain how such parasites could ever, without
the help of additional factors, put a stop to the increase of their hosts,
we must invoke also the numerical response, but this is a separate
matter from the assumption under discussion.)
One recent example shows an opposite effect to that of parasite
satiation. In experiments. with the whitefly Trialeurodes vaporariorum
(Westw.) and its Hymenopterous parasite Encarsia formosa Gahan,
Burnett (1958b)c) demonstrated that with low but increasing densities
of the host, the percentage of hosts found also increased (with further
host increase this percentage became approximately constant). The
reduced percentage parasitism at low host density evidently had something to do with the parasites’ searching eaciency (it did not occur
when the experimental area was reduced). The maintenance of the same
percentage parasitization at higher host densities showed that fecundity
of the parasites was not a limiting factor within the range of densities
considered. Burnett (1959), in the course of a very useful review of
parasite-host experiments, pointed out that we do not know how much
departure from the initial assumptions is required to upset the conclusions from the Nicholson-Bailey model. This is a question that
could well be investigated theoretically. Nicholson (1933, 1954b) and
Nicholson and Bailey (1935) have elaborated the original model rather
than tested the effects of relaxing the basic assumptions. However, in
the present instance it seems that any falling off of effective parasite
M. E. SOLOMON
Among the main lines of thought surrounding this model, one has
been to challenge or investigate the assumptions, including the assumption of insatiable parasites. While it is obvious that parasites do not
have unlimited fecundity, it is not necessary, for the purposes of the
model, to assume that they do. One is required to assume only that the
limit of their fecundity is not reached, and that fecundity does not
decline at the levels of host and parasite abundance involved (cf,
Varley, 1947).
This assumption receives no support from the above-mentioned
experimental data on the functional response of insect parasites reviewed
by Holling (1959b). The diagrams, similar in form to Fig. 17, show that
the number of hosts parasitized rises proportionately less than host
density does, from the lowest levels of density studied. While these
results are not simply an index of parasite fecundity, they do remind
us that parasites, besides seeking and finding hosts, spend some time
in dealing with the discovered hosts and depositing their eggs; the
greater the density of hosts, the greater is the proportion of the parasites’ time that is spent in this way instead of seeking more hosts (cf.
Tinbergen and Klomp, 1960). They also show that, however high is the
hosts’ density, the parasites are unable to attack more than a definite
number of them. (To explain how such parasites could ever, without
the help of additional factors, put a stop to the increase of their hosts,
we must invoke also the numerical response, but this is a separate
matter from the assumption under discussion.)
One recent example shows an opposite effect to that of parasite
satiation. In experiments. with the whitefly Trialeurodes vaporariorum
(Westw.) and its Hymenopterous parasite Encarsia formosa Gahan,
Burnett (1958b)c) demonstrated that with low but increasing densities
of the host, the percentage of hosts found also increased (with further
host increase this percentage became approximately constant). The
reduced percentage parasitism at low host density evidently had something to do with the parasites’ searching eaciency (it did not occur
when the experimental area was reduced). The maintenance of the same
percentage parasitization at higher host densities showed that fecundity
of the parasites was not a limiting factor within the range of densities
considered. Burnett (1959), in the course of a very useful review of
parasite-host experiments, pointed out that we do not know how much
departure from the initial assumptions is required to upset the conclusions from the Nicholson-Bailey model. This is a question that
could well be investigated theoretically. Nicholson (1933, 1954b) and
Nicholson and Bailey (1935) have elaborated the original model rather
than tested the effects of relaxing the basic assumptions. However, in
the present instance it seems that any falling off of effective parasite