Trees, rocks, and a couple other things have a chance to spawn biters when a they are destroyed. It is fully customisable with all fetures being enabled or disabled independently. With this mod you can spawn a number of biters after a tree dies, a rock dies, a spawner dies, or after part of your base dies (the last is disabled by default). It will chose a biter or spitter apropreate for the current evolution to a max evolution (0.9 by default).
Mods introducing new content into the game.