Bleached Corals In Eastern Pacific Separated From Their Healthy Neighbors In The West By Darwin’s ‘Impassable’ Barrier

Across the world, coral reefs are currently struggling to recover from record bleaching events triggered by unusually warm ocean temperatures and a climate change-induced rise in ocean acidity. A lot of these imperilled reefs lie in the eastern Pacific region and are part of a population that, according to a new study, has been completely separated from the rest of the Pacific Ocean for at least the past two decades.

The study, carried out by a team of researchers who used supercomputers to simulate the movement of coral larvae in the region, supports the view expressed long back by Charles Darwin, who called the over 3,100 mile stretch separating the eastern and western Pacific an “impassable” barrier.

“Coral build the framework of tropical coral reefs, creating habitats which support one of the most diverse ecosystems on Earth,” lead author Sally Wood from the University of Bristol, said in a statement released Tuesday. “Whether coral reefs can survive the pressure of climate change as well as local stresses will depend to a large extent on the ability of coral to reproduce and disperse; to replenish damaged populations, migrate from deteriorating conditions and colonise new frontiers. So it's important to map where coral are able to get to.”

In order to do this, the researchers created a computer model to simulate the movement of over five billion coral larvae — each smaller than a poppy seed — from over 630 reefs throughout the central and eastern Pacific from 1997 to 2011. They found that even during the extreme El Niño event of 1997-1998, when shifting ocean currents promoted long-distance dispersal, the larvae could not survive long enough to cover the vast expanse separating coral colonies in the east and the west.

They then compared the results obtained through the simulation with genetic data obtained from coral populations in the eastern and western Pacific Ocean. Based on this, the researchers concluded that the eastern and western populations of some species have been isolated for at least the previous few generations, and maybe even thousands of years for long-lived corals.

“Coral larvae are tiny and can survive for a maximum of about 120 days. The larvae travel mainly by ocean currents to establish new colonies, but because of their small size it is currently impossible to track them across the vast distances of the Pacific Ocean to know if healthy populations of corals in the western Pacific could help to rejuvenate decimated populations of corals in the eastern Pacific,” co-author Iliana Baums, an associate professor of biology at the Pennsylvania State University, said in a statement. “For the first time, our computer simulations combined with genetic data allowed us to test whether the larvae could survive this journey.”

What this means is that the vulnerable reef populations in the eastern Pacific, which are witnessing massive bleaching, are on their own, and that only local efforts to protect them are likely to have any mitigative impact.

“Reefs provide habitats for one of the most diverse ecosystems in the world and they are extremely economically important for fisheries, coastal protection, tourism, the aquarium trade, and as sources for new pharmaceuticals,” Baums said. “The reefs in the eastern Pacific that we study are particularly important because they survive in inhospitable conditions, and understanding how they do this could be critical when designing strategies for reef conservation as the climate continues to change.”