An ecosystem is an important idea. If we understand that a certain group of plants and animals all live together in a place, then we can see what changes to that place might have. For example, if we think about the plants and animals that live in a winding river, we will find out that there are deep and shallow parts to a river, and that some parts have muddy beds, while others have sandy beds. Each part of this ecosystem is home to a certain range of plants and animals that cannot live in other parts. So, if we were to dredge and straighten a river, all of the conditions that many plants and animals need to thrive would be lost. We would have destroyed the ecosystem.

People worry about Global Warming for the same reason. If a place becomes hotter, then many living things will not be able to survive. The Polar bear lives in a frozen waste ecosystem. It depends on ice floes to walk on to make a base for catching food. If Global Warming causes the ice to melt the Polar bear may not be able to catch enough food, and so die out.