Why should not individual rare or endangered species be protected, but integral biogeocenoses, which they are part of?

When they try to save a certain species from extinction, they usually conditionally fence off the territory where this species lives. This is how protected areas are created where human activity is prohibited. This is done due to the fact that a certain species needs a sufficient amount of food, the normal natural conditions to which it is accustomed, and so on. This method helps when it was the person who destroyed the species. If a species is not dying out because of a person, then it is much more difficult to save it, since it is the surrounding factors that destroy it.