Are Keystone species always predators?

What is or are the keystone species of New Zealand?

• A Keystone species is:...A species whose presence and role within an ecosystem has a disproportionate effect on other organisms within the system. A keystone species is often a ...show more

• Answer:

  I think I would go with the possum (not the same as the north American opossum) that has established itself here. It has no known natural enemies (only man) but does impact native species, especially birds.