Marine iguanas, unique to the Galápagos Islands, are the world’s only marine lizards. These reptiles have developed extraordinary adaptations to forage for marine algae, which constitutes almost their entire diet. While their populations can be locally abundant, the species is classified as Vulnerable by the IUCN Red List, with 10 of its 11 subspecies listed as Endangered or Critically Endangered.
Inherent Vulnerabilities
Marine iguanas possess specific biological and geographical traits that contribute to their susceptibility. Their existence is limited solely to the Galápagos Islands, making them endemic and highly vulnerable to localized threats. They have a specialized diet, feeding almost exclusively on marine algae, which ties their survival to the health of coastal and intertidal ecosystems.
Their physiological adaptations, while unique, also present vulnerabilities. Marine iguanas are cold-blooded and must bask in the sun for several hours daily to raise their body temperature after foraging in cool ocean waters. They also possess specialized salt glands that allow them to expel excess salt ingested with their food. Their slow metabolism and reproductive rates mean populations recover slowly from significant declines.
Ecological Climate Shifts
Large-scale environmental changes, particularly climate phenomena, pose substantial threats. El Niño events, characterized by warmer ocean temperatures, significantly impact their primary food source. During strong El Niño periods, the preferred red and green algae marine iguanas consume disappear or are replaced by brown algae, which they cannot digest. This can lead to widespread starvation, with mortality rates reaching as high as 90% in some populations.
Rising sea temperatures and changes in ocean currents, influenced by global climate change, further exacerbate food scarcity. The iguanas have developed an adaptation to shrink their body size, including their vertebrae, by up to 20% during food scarcity, allowing them to survive on fewer resources. Increased frequency and intensity of these events are predicted, which could overwhelm their ability to recover. Sea level rise also poses a threat to their nesting sites and coastal habitats, which are typically located close to the ocean.
Non-Native Biological Threats
The introduction of non-native species to the Galápagos Islands presents significant biological threats. Feral animals such as cats, dogs, and rats, brought by human activity, prey on marine iguana eggs, hatchlings, and even adults. These introduced predators disrupt the natural balance of the ecosystem, as native iguanas lack effective anti-predator strategies due to their historical isolation.
Feral dogs can prey on adult iguanas, while cats and rats often target vulnerable eggs and young. The presence of these invasive species can lead to very few young iguanas surviving, threatening the long-term viability of populations. Additionally, the introduction of diseases and pathogens, to which native species have no immunity, represents another significant biological risk.
Direct Human Pressures
Direct human activities also contribute to the endangered status. Marine pollution, particularly plastic waste, is a growing concern in the Galápagos. Studies have found plastic in all marine habitats, including beaches where marine iguanas reside. Marine iguanas are at high risk of ingesting plastics, which can lead to internal injuries or starvation, and entanglement in fishing lines and other debris.
Oil spills represent a severe threat. The 2001 MV Jessica oil spill resulted in a 62% mortality rate in the Santa Fe marine iguana colony, likely due to oil killing beneficial gut bacteria essential for digestion. Habitat degradation from human development and tourism infrastructure can reduce available nesting areas and disturb basking sites. While tourism is managed, increased human presence can still lead to accidental harm from boats or disturbance in sensitive nesting and foraging areas.