Galápagos marine iguanas: El Niño, algae and climate risk

The marine iguana is the only living lizard that routinely forages in the sea. It grazes on algae along Galápagos shores, so its fortunes are tied to ocean conditions and the productivity of coastal food webs. During strong El Niño events, warmer water and weaker nutrient upwelling can change which algae grow and how much food is available. Starvation and population decline are documented consequences, but the pattern is not identical on every island.

A lizard that depends on the sea

Marine iguanas bask on land but feed mostly on marine algae in the intertidal and shallow subtidal zones. The algae available to them depend in part on cold, nutrient-rich water brought toward the surface by ocean circulation. During El Niño, sea-surface temperatures rise and upwelling weakens. The resulting change can reduce preferred food and favour algae that are less useful to iguanas. In severe events, animals lose body condition, growth slows and breeding may be reduced.

A long-term study on Santa Fe tracked the 1982–83 El Niño, one of the most severe events recorded at the time. The study estimated that about 60 percent of that island’s monitored population died between March and August 1983, with similarly severe mortality reported on other islands. Those figures describe a historic event and a particular study, not a permanent annual death rate. After conditions improved, survivors regained body condition and reproduction recovered.

Marine iguanas can also shrink in body length during food shortage and later grow again when food becomes plentiful. This unusual response can improve survival by reducing maintenance costs, but it is not evidence that the animals are unaffected by famine. It is a physiological adjustment within a highly stressful period.

One archipelago, different population responses

Island populations differ in body size, colour and other traits, and they do not experience identical conditions during an ENSO event. A 2015–16 study found that body-condition responses varied among sampled populations. Some sites showed poorer condition, while others were less affected. Local algae, exposure to currents, population density and access to feeding areas help explain why a single archipelago-wide average can hide important differences.

That variation changes how monitoring should work. A survey at one accessible beach cannot stand in for every island. Researchers need repeated counts and condition measurements at multiple sites, ideally linked to sea temperature, algal communities and the timing of breeding. Counts can also be difficult to interpret when iguanas move among nearby feeding and resting areas. The 2004 conservation review described a very wide range of total abundance estimates; because that range is old and broad, it should not be presented as a current census.

Natural ENSO variability is not the same thing as anthropogenic climate change. El Niño occurred before industrial warming and will remain part of the climate system. Human-driven warming can alter the background ocean conditions in which those events unfold, but attribution requires climate and ecological evidence. It is more accurate to describe warming as a factor that may change the frequency, strength or consequences of marine heat stress than to call every iguana decline a direct result of climate change.

Conservation beyond a single climate headline

Food stress is only one part of the picture. Introduced cats, dogs, rats and pigs can prey on iguanas or their eggs; oil pollution can harm shoreline populations; and coastal activity may disturb nesting or feeding sites. Galápagos Conservancy lists the marine iguana as Vulnerable and identifies invasive species, pollution and changing climate conditions among the concerns. Protection therefore includes biosecurity, response planning for spills, habitat management and consistent island-level monitoring.

Past events also show why recovery matters. After a severe natural crash, marine iguanas can rebound when algae return, although recovery may differ among islands and can be complicated by other pressures. This does not make an event harmless: a second stressor may arrive before a population recovers. Restoration projects elsewhere in the archipelago illustrate how targeted management can address a specific ecological bottleneck; see the Floreana giant-tortoise restoration story for a different Galápagos case.

The most useful question is not whether climate is “the new enemy,” but which conditions affect which population, how the effect was measured and whether the cause is natural variability, human pressure or both. That framing keeps the striking biology of marine iguanas connected to evidence that can guide conservation.