I don't know about an agent, Yme, but in the meantime if it's causing you problems or keeping you from playing you can do that with CAOS by removing and reinjecting the relevant cos files with the various "ject" commands.
I believe it's....
ject "whatever.cos" 1
ject "whatever.cos" 4
ject "whatever.cos" 7
to remove (1), inject (4), or "reset" (7, but I'm not totally sure what reset means in this context) a given cos file, in that order. If you have a population bloom of for instance tubas you could use that to remove all of them and re-add only as many as you want, until you find the agent you're thinking of, but you'd have to do it for each species you wanted to reset.
I had no luck finding the agent, so I either have to go back to the broken version of the cos files, that leave the terrariums lifeless and barren, or I could use these commands.
How can I figure out what cos files the different animals and plants use?