system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How are beavers benificial for ecological systems?

Beavers were recently reintroduced in my country to help the ecological system develop. All I know is that they build dams, which I would assume isn't very good for river fish. I also just learned they tend to eat/use young trees, which can't be good for the forest growth. How do they actually benifit the ecological system?
gpt
They store water above sea level. Among other things, this makes them one of the most valuable creatures nature has ever created. They are classified as a keystone species like salmon. This means without them, other parts of the ecosystem will collapse. 

You could rid all the humans off the planet and leave the beavers. In a century they can reforest what we deforested in Canada, which is a lot. They can turn deserts into rainforests if you give them enough time.