Galapagos Marine Iguana's Impact from El Nino 2015-16
Marine iguanas face hard times ahead as El Nino conditions strenghten
The time is ripe for Darwin's theory of natural selection to again be put to the test, this time with the Galapagos marine iguana and other ocean-dependent species in the Galapagos. As the 2015 super El Nino heats up waters in the Galapagos archipelago, food is becoming scarce, and the population of 200,000+ marine iguanas will be competing for a limited supply of nutrients. As scientists have noted in past strong El Nino events, this will likely result in serious losses and a critical hit to the species.
Typically, marine iguanas feed on red and green algae that thrives in the shallows of the Galapagos shoreline. Their unique ability to dive down to 30 feet (or deeper) into the water and also be able to digest the algae found there has helped them to live for millions of years on these volcano-born islands. However, during El Nino years, the nutrient-rich currents and upwelling weaken significantly, suffocating the algae's ability to use photosynthesis. When it dies, there isn't enough food for the booming marine iguana population, and thousands die.
According to NOAA climatologist Michael McPhaden, "The warm sea surface temperatures are caused by a reduction in the upwelling of cold water from below the surface. This reduction of upwelling also shuts off the supply of nutrients (think of this as ocean fertilizer) that enables the growth of one celled plants called phytoplankton. Phytoplankton form the base of the marine food chain so that when there are fewer of them, all higher marine organism suffer."
While some species, like birds or sharks, have migratory mobility and can leave the Galapagos in search of better food sources during El Niño years, the impact is especially felt with the species that cannot leave the islands, like the marine iguanas. For them, it is a case of adapt or die.
As a testament to their incredible adaptability, though, many of the iguanas can actually shrink their bodies to require less food. This increases their likelihood of survival and their ability to pass on these genes to the next generation.
Dr. Martin Wikelski, director of the Max Planck Institute of Ornithology and a professor at the University of Konstanz, has been studying the Galapagos Islands extensively since 1987 and focused on the evolution of body size in marine iguanas.
"We don't know yet how marine iguana shrink but they shrink by up to 25% in body length," he says. But how big an iguana is definitely correlates to its chances of surviving an El Nino. "The size of an iguana is indicative of its survival […] The largest individuals on each island are the first to die during an El Nino."
He notes that these evolutionary changes are an adaptation that has evolved over millions of years, thanks to the recurring nature of El Niño. "Marine Iguanas are very resilient towards environmental changes and El Nino conditions. They have gone through these conditions presumably for their entire history in the Galapagos," he remarks. "Otherwise they would not live in the Galapagos anymore."
Although the El Niño phenomenon happens every 2-7 years along the tropical coasts of the Pacific, the biggest effects tend to follow the super-El Niño events, which only occur every 15-20 years.
In recent history, the super-El Niños of 1982-83 and 1997-98 had major impacts on the marine iguana populations.
Galapagos National Park marine biologist and conservationist Eduardo Espinoza explains that in 1982, when the red and green algae of the intertidal zone died and were replaced by an opportunistic, invasive brown algae species, thousands of marine iguanas starved.
"Due to the lack of food and therefore energy, the majority of individual [iguanas] that tried [to dive deeper in the open water for food] died in their attempt to find conditions a bit more favorable," says Espinoza.
This was seen again in 1997, when a Yale study found that certain marine iguana populations suffered losses of up to 90 percent.
However, in both of these cases, the populations had rebounded quickly in the following years, and population numbers again stabilized.
Researcher Andrew Laurie, of the Max Planck Institute, noted this resilience in a 1987 report: Despite the starvation of close to 70 percent of the Galapagos marine iguana population, the clearly depressed growth rates during the 1982-83 El Niño were followed by growth rates that had almost doubled in juveniles in the years immediately after and had decreased again towards pre-El Niño levels by the time the report was published.
In the Yale study, postdoctoral fellow Sebastian Steinfartz says that the most remarkable finding was that "natural populations may be able to balance even severe short term climatic disturbances, and that such fluctuations will not necessarily have long-term negative consequences on the population structure."
Wikelski confirms that how the marine iguanas are affected depends largely on where they are and how currents impact their island. "Not all marine iguana populations are equally affected. It depends on the currents – the ocean currents, the upwelling currents and the sea surface temperatures on the very islands they live on," he explains. "Depending on the population history on a specific island El Nino may kill up to 90% of iguanas per island. However, if a population has gone through a bottleneck before, even a strong El Nino may not kill more than 10-15 % of a population."
Although it's clear that this year's super-El Niño, which climatologists have been tracking since the beginning of the year, will have severe consequences for the current marine iguana populations, it is unlikely that even a "Godzilla El Niño" (as it has been dubbed) will have a lasting impact on the survival of the ancient Galapagos marine iguana.