When humans arrived and cleared a lot of the forests, it changed a huge number of ecosystems. Land and water creatures had to adapt to their new environment, move on, or die. There are lots of real life examples of how ecosystems can be affected by our changing world. The sea is warming up because humans all over the world are pumping different types of gases into the atmosphere around the Earth. This is an example of how some ecosystems can affect other ecosystems, even if they're on the other side of the world.
Humans can also do good things to fix ecosystems.
If you want to find out more about a New Zealand ecosystem that humans have helped to fix, look for information on the Chatham Islands and the black robin species. To make sure the black robin survived, humans had to remove the pests and set up a breeding programme.