A volcanic eruption would likely create a new type of ecosystem. This is because volcanic eruptions can dramatically alter the landscape, creating new landforms and changing soil composition, which can lead to the establishment of new habitats and ecological communities.
In contrast, an introduced species might alter an existing ecosystem but does not typically create a new one on its own. A yearly migration is a behavioral pattern and, while it influences ecosystems, it doesn't create a new type of ecosystem. Therefore, the best answer is:
a volcanic eruption.