Large heards grazing is necessary for grasslands to thrive.
They till the ground, knock down tall dead plants, graze (but not "browse" the grass), fertilize, and water the grass.
Deer and other fauna do not knock down the grass the way bovine do. We used to have millions of Buffalo. Now we use cattle as a substitute.
If we don't do that, we have to burn the grassland. Or it dies.
That's what we used to do in Kansas. It was quite fun. And the government paid us to do it.
Anyways. Here's some evidence to back up what I'm saying: TED TALK
Ya, it's a generic error from the looks of it. Didn't know what error to throw when it went down, so it appears to have defaulted to this.