A forest ecosystem can support a limited number of bears because bears are the supreme predators of their ecosystem and very few animals can hunt bears so nature by supporting a limited number of bear populations supports a balanced number of population of its prey like insects, fishes, deer.
Another reason for the limited population of bear is that bear occupies the highest trophic level in the ecosystem so according to 10% law of energy transfer, it will get very less energy from its food so it needs to eat more.
Therefore the population of bears is limited because the environment can not meet the energy requirement of many bears so a forest ecosystem can support a limited number of bears.
If their are to many bears all the bears will eat all the plants and smaller animals and keep multiplying so there would be almost no natural vegetation and less and less smaller animals which would eventually destroy the ecosystem and in the end everything dies including the bears.
