Competitive Exclusion Principle: no two species can coexist if they occupy the same niche.
The example of this in the temperate deciduous forest is; a bear and a couger. They cannot coexist if they occupy the same niche because they would both be competing for fish and deer and one of the two species would be less capable and become extinct.
Speices diversity can affect a ecosystem"s stability because if there are so many different species they start to compete for the same resources. This can cause some species to occupy the same niche and end with a secies becoming extinct.