Not just the food chain but the natural environment as well. Animals can migrate (theoretically, if they have somewhere to migrate to), but trees that sprouted in the right climate a hundred years ago might be in the wrong climate now. And animals that depend on those forests are going to be in trouble if they aren't healthy.

When changes happen on timescales that are slower than forests and ecosystems can naturally grow and spread that's not such a problem because you at least have a mostly-stable ecosystem at any given point in time.