5. What species took over the desert ecosystem when the hummingbirds were
removed?

Hummingbirds pollinate native cactus. In areas of the Sonoran Desert with few hummingbirds, (5)invasive species such as buffelgrass have taken over the ecosystem. The theory that the balance of ecosystems can rely on one keystone species was first established in 1969 by American zoology professor Robert T. Paine.