Tropical animals migrate in response to wet-dry seasons. If fluctuations in rainfall become more extreme and frequent with climate change, then scores of animals could be in trouble. Migratory animals have a role in the ecosystems spanning the territory they traverse. For instance, whale sharks migrate to Christmas Island to feast on the red crabs' larvae. As the parental crabs journey to the coast and back, they feed on plants and saplings that keep the island from being overgrown.