The Impact of El Niño on Local Galapagos Species

People usually associate El Niño with floods, mudslides, and roof damage, yet there are actually positive side effects of this unique weather phenomenon. Indeed, every El Niño occurrence provides an excellent template for ecologists to study environmental effects on genetic diversity.

El Niño, shorthand for El Niño Southern Oscillation, is a recurring weather event caused by rising water temperatures in the eastern tropical Pacific. During non-El Niño conditions, trade winds blow from the eastern to western Pacific (from the Americas to Indonesia). Normally, water blown towards Asia is warmer because it is exposed to the sun for longer durations, while water near the South American coast is colder and even shallower (by 0.5 meters).

The cold water in the Eastern Pacific cools the air and makes it denser, reducing cloud-formation and precipitation. Conversely, the warmer water towards Southeast Asia heats up the lower troposphere, drastically increasing precipitation.

Every 3 to 7 years, the trade winds slacken or even reverse direction, blowing from the western to eastern Pacific. Weather conditions reverse: the American Pacific coast experiences torrential rainfall and eastern Asia suffers from drought. This event is what is called an El Niño.

The 1997 occurrence allowed Yale biologists to study the before and after effects of El Niño on the genetic diversity of marine iguanas living on the Galapagos Islands. Their series of ongoing research in local biodiversity has implications concerning the effects of global warming, blamed for the increasing frequency of El Niño events.

These studies are crucial in determining how such weather aberrations alter the gene pool of species indigenous to the Galapagos Islands and other isolated niches, especially if mankind wishes to preserve nature’s diversity. According to Geology and Geophysics Professor Jeffrey Park, damages in a species’ gene pool make it vulnerable to extinctions.

Park explains, “Any population confined to an isolated area like an island is susceptible to the bottleneck effect, which is when a natural disaster kills large numbers of the organism, leaving a smaller variety of genes for future reproduction.”

One such critical study was conducted by Gisella Caccone, a Yale senior researcher in evolutionary biology, before and after the 1997/ 1998 El Niño. Working with German scientists, Caccone compared more than 800 samples of iguanas taken in 1993 and 2004 to determine the influence of El Niño on the genotypes and phenotypes of organisms and species.

The scientists closely examined the nuclear microstatelllite and mitochondrial DNA of iguana samples taken during before and after the El Niño for genetic variation. Changing mitochondrial markers may reveal a decrease in genetic diversity and signs of a dwindling gene pool. Microsatellite frequencies allow researchers to predict the direction of population growth.

The marine iguana, native to the Galapagos Islands of the Pacific.

What Caccone found was that only one island population showed strong statistical evidence that the El Niño had indeed caused a genetic bottleneck, but that different populations within the marine iguana species responded to the storm in different ways.

Graduate student co-author Scott Glaberman explains, “Our study points out that there was a low population size on the island of Marchena during the same period in which there was serious volcanic activity as well as El Niño.” He emphasizes the importance to differentiating between the two disasters when interpreting their effects on genetic diversity.

Another important result was that while some iguana populations suffered high mortality rates, genetic ramifications were lacking, a positive conclusion suggesting that localized species may be able to adjust efficiently and rapidly to sudden environmental fluctuations.

The surveying of the marine iguanas is by no means complete. Caccone makes it clear that current samples may provide indispensable for future, longer-term comparisons of iguana genetic material. Further research may one day correlate the demise of particular populations to specific weather or climate patterns.