Marine iguanas are most vulnerable before birth or when juvenile. However introduced cats, rats, pigs, dogs, and humans also prey upon iguanas of any age. The greatest impact on marine iguana populations has been El Nino. The warm water that arises off the Galapagos Islands from El Nino causes the algae that the iguanas eat to die. They are left with only brown algae to eat.
Brown algae is toxic to marine iguanas.
Additional threats to the marine iguana include oil spills and other marine pollution. These could destroy the algae on which the iguana depend as well as their coastal habitat. This endemic species has sparked a great deal of academic conversation about its' origin. There are land iguanas scattered throughout the world and even in the Galapagos Islands. However the marine iguana is the only iguana that feeds exclusively underwater.