A drastic change in the weather such as the ice age could change the enviornment. Another possibility would be the introduction of a new species into the enviornment that did not have a natural predator. Without a natural predator this species would populate quickly and eat the food sources of other animals in the ecosystem forcing the the original animals to adapt to survive.
I agree with every new thing that is introduced into the world, something is changed. So it is obvious that if people are poisioning the world that it will change the ecosystem