They reproduce resulting in an increase in their numbers. They add nutrients to their ecosystem by excreting wastes, dying and from the decomposition of the trees they cut down. They act as a habitat for other species like the fleas and ticks that live on their bodies. This is not even a complete picture of the beaver's niche. Two different species cannot occupy the same niche at the same time.
All species have a slightly different role in the ecosystem.
If a new one in introduced competition will occur resulting in the removal of one species. It will either die out, move out of the area or adapt to a new niche.