Surely they breed at exactly the same speed as whatever they are using for hosts ?
So in the case of beast X which breeds at Y speed the alien Z would breed at an initial rate X * Z then at a reduced rate of X * Y until eventually all you had left were Z.
At that point. Z will go something like 'oh'
and X won't be doing much of anything and it certainly won't be getting up to any of the fun that precludes Y.
If you get my meaning.
Spiro
