Board Thread:Off Topic/@comment-26018514-20150401033018/@comment-24891101-20150405043007

The WoR about Grimm said that Grimm occasionally get into territorial disputes with animals. Given that the only animals able to contest them are predators, this implies a full food chain. The existence  of derived foodstuffs is also indicative of their existence, as Phantom said.

I would go the opposite direction with assumptions, actually. Given that we've seen the traces of many terrestrial organisms, a full biosphere is necessary for the ecosystem to exist, since Grimm don't particularly interact with animals.