Climate Change Threatens One Of The World’s Great Migrations



CREDIT: Max Orchard, Parks Australia

Beer, football and coffee aren’t the only things that could fall victim to climate change. Changes in weather patterns could mean one of nature’s most spectacular migrations slows or stops completely some years, according to a new study.

The study, published in Global Change Biology, found the Christmas Island red crab is sensitive to changes in timing and amount of precipitation during their migration time. Each year in November, millions of red crabs travel en masse from their forest homes on Christmas Island to the beaches of the Indian Ocean, where they spawn and lay their eggs. The trek takes two weeks, and the crabs must have meticulous timing: by the morning of the high tide that comes before the December new moon, the females must lay their eggs. A month later, if ocean conditions are right and they don’t get devoured by fish, the eggs hatch and babies emerge from the ocean, travelling back to land to live in the forests.

The study analyzed 36 years of migration data for the red crab and found that, except in three cases, the crabs did not start their migration if there had not been at least .87 inches of rainfall, meaning that an abnormally light or late arrival of the rainy season could push the crabs’ migrations back or forward months. In 1997, a strong El Niño made for an especially dry rainy season, which caused the crabs to forgo migration and mating, which means more El Niños could be bad news for crabs in the future.

“We know that 1997 was a very big El Niño event and we can predict changes in migration patterns by using climate models that suggest that El Niño frequencies will potentially increase in the future,” Lead author Allison Shaw said. “So, years like this could potentially become more common. If the crabs’ response is to not migrate in El Niño years, that’s going to be a very big problem.”


CREDIT: Max Orchard, Parks Australia

Any effects on the red crabs’ migration will be felt by the entire Christmas Island ecosystem. The crabs eat vegetation along their migration route, helping prevent overgrowth, and their droppings act as in important fertilizer. Their larvae are a key food source for fish and other marine life — whale sharks migrate to the shores of Christmas Island each November to feed on red crab larvae.

The study is one of the first to examine climate change’s potential effects on migratory tropical species — most other studies up until this point have focused on migratory species that live in temperate areas. Those studies have found similar examples of climate change’s potential to throw off the delicate timing involved in many migrations. The red knot, for example, relies on horseshoe crab eggs during its stopover in Delaware Bay during its migration from the southern tip of South America to the Arctic. But changes in weather patterns could push the crabs to spawn early or late, leaving the red knot without a key food source on its long migration. Other birds are running into changes in the timing of bud bursts and insect hatchings once they reach their spring habitat, which makes it difficult for the birds to feed their young.