Answer:
An invasive species enters the environment.
Step-by-step explanation:
All other answers already naturally happen in an environment so they are stable. But if an invasive species enters, the ecosystem will be less stable because invasive species dominate ecosystems which isn't natural to the animals that live there. So for example, if someone introduced an invasive species to the ecosystem, it can cause many changes like if it was a predator, there'll be less primary consumers (the animals that eat plants) because the invasive species isn't natural to them.