Museum of unintended consequences/Reproduction as a weapon

From CSWiki

NetLogo has a Wolf Sheep Predation model. One mode of operation lets the sheep reproduce without bound of there are not enough wolves. (Another mode include a requirement that there be enough grass to support the sheep. But it is possible to "turn off" the grass.)

To prevent the sheep (or the wolves) from reproducing without limit, I added code to the model that prevents either sheep or wolves from reproducing if the total number of sheep and wolves exceeds a certain threshold, i.e., if there is too much crowding. (One can do this in terms of global crowding or local crowding. The effect is essentially the same.)

As is usual in predation models, the populations grow and shrink. If there are more sheep, the wolves reproduce faster. This cuts down on the number of sheep, and the wolves die off.

What always seems to happen, though is that when the number of wolves comes close to zero, the sheep reproduce until they essentially fill up the available space. Then even though the few wolves who remain are fat and happy, there are so many sheep that they can reproduce faster than the wolves can eat them. Furthermore, since the system arbitrarily limits any births if the total population is too large, the wolves are often prevented from reproducing since the sheep take up all the room. So even though this would be sheep paradise, they can't find room to reproduce. Hence the sheep are using their ability to reproduce quickly and take up all the space as a weapon that prevents the wolves from reproducing.

This works because of the way I wrote the population limit rule. It limited the absolute total number of elements. If I had written the rule differently so that an excessive number of sheep would still not prevent the wolves from reproducing, the results would have been different. But as it is, this is a nice illustration of a rule with unintended consequences.