A comparative study of models for predation and parasitism

52
in Fig. 9, in which the total number of hosts parasitized, z, is plotted on the vertical
axis against the total number of eggs laid, n, on the horizontal axis. It should be
noticed, however, that this method of presentation is comparable to that in Fig. 8
for the predation model, showing the effect of diminishing return3. Here again, a
wavy tendency is discernible, which shows that the occurrence of superparasitism is
periodically increased in accordance with changes in the number of eggs laid.
One important difference between the predation model and the present parasitism
one is that the number of hosts freshly found hardly increased after 130 eggs had
been laid in this example. This is because the parasite could not get out of the area
already searched because no host individuals were removed from the area and the
parasite repeatedly re-parasitized them. Also, it is noticeable that the observed number
of hosts attacked was always lower than that expected in a random encounter, i.e.
a binomial series (SToY's formula; see w 4g), because of excessive superparasitism.
Although the indiscriminate parasite in this model is unable to avoid superparasit-
ism, to continue searching in the area already searched is obviously inefficient. If,
however, the parasite periodically moved to start a new search elsewhere, it would
raise its hunting efficiency, and so would be favoured by natural selection. If the
parasite is unable to know whether the area where it starts a new search has already
been searched by itself or by other parasites, the shift of hunting area can be only
at random. Of course, a random shift of hunting area certainly involves the risk of
hitting an area which has already been searched by itself or by other parasites; but
the parasite at least avoids the disadvantage of staying in an area which has just
been searched by itself.
In the second series of 'experiments', the parasite left the area after each five
eggs had been laid and travelled for a certain distance in a randomly determined
direction (Fig. 10 a). Again, a wavy trend is seen, but the observed number of hosts
v
: ~- OBS,
80
....... THEOR.
/o".-'" /
__N
nr
ff~ 4O
l.--
ffl
0
212
~ 20
o
6
Z
o 40 ~ 1oo 12o ~o
No. OF EGGS LAiD (n)
Fig. 10a. A result from the second series of Monte Carlo simulations
of parasitism similar to the first series, except that a parasite, after
every five eggs have been laid, moves 7. 5 unit lengths in a randomly
determined direction.