They are related to the land iguana, which also lives on the Galapagos Islands, as well as the green iguana which inhabits the mainland of South America. There are probably more marine iguanas on the Galapagos Islands than any other creature. Researchers believe that none of the native predators relied greatly on the marine iguana for food. This hypothesis comes from the fact that marine iguanas are very docile and don't usually leave an area when molested. They give every indication that they have no predators.
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.