Final answer:
Population ecology is the theory that suggests organizations cannot easily adapt to changing conditions once they have established themselves in a certain environmental niche.
Step-by-step explanation:
The theory that proposes that once an organization is successfully established in a particular environmental niche, it is unable to adapt to changing conditions is population ecology. This theory suggests that entities, like biological species, go through a life cycle of birth, growth, maturity, decline, and death. A key tenet is that structural inertia and environmental selection pressures limit an organization's ability to adapt to change once it has found its fit or niche within its environment.