The world’s only sea-going lizards, marine iguanas are endemic to the Galapagos Islands and found on all 13 of the major islands, with the largest numbers on Fernandina and Isabela. In 2004, scientists made “very rough estimates” for the populations of these islands as 120,000 and 40,000 respectively. Other islands have substantially smaller populations – San Cristobal, for example, has fewer than 400. While there are natural limits on population due to the availability of the red and green algae it almost exclusively feeds on, habitat for courtship and breeding and suitable locations to lay eggs, it is the human impacts that conservationists are more concerned with.

The presence of non-native domestic cats throughout the Galapagos is believed to be a significant threat on some islands, while feral dogs can be a problem too, but are generally more visible and easier to eradicate. Marine pollution has been shown to have a huge impact on marine iguanas: an oil spill off San Cristobal in 2001 caused a 6 o per cent crash in iguana numbers on Santa Fe island where the species was being monitored. It’s drought the oil poisoned the gut bacteria that enables the iguanas to digest algae. Finally, El Nino events -where there are temporary increases in sea surface temperatures – have even greater impacts, causing population crashes of 90 per cent by killing the algae the iguanas eat. Though the process is natural, and numbers rebound, climate change may be increasing the severity of these episodes. It is feared a severe El Nino could completely kill off small populations.