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Changing rainfall patterns spell catastrophe for Christmas crabs

Disruptions to mating patterns of Christmas Island red crabs will disrupt multiple ecosystems across the tropics

Source: Allison Shaw, Department of Ecology and Evolutionary Biology

Source: Allison Shaw, Department of Ecology and Evolutionary Biology

By Sophie Yeo

Every year, as the November rain starts to fall across the Christmas Islands, swarms of native red crabs scuttle down to the sea, where mating season begins.

It is a meticulously timed pilgrimage, with the land dwelling species undertaking a two-week dash to the ocean, where the male crab sets up a mating burrow, in which the female will then sit and incubate the batch of baby crabs for a further two weeks.

They have a strict deadline: by the morning of the high tide that precedes the December new moon, the eggs must be released into the ocean. A month later, a new generation of crabs emerge from the sea.

But changing patterns of rainfall could disrupt this delicate process, say scientists at Princeton University, with impacts that extend beyond the survival of the Christmas Island red crab.

As temperatures rise across the planet, the previously reliable cycle of the wet and dry seasons in the tropics is shifting. For the red crab, whose mating patterns hinge on the timing and the amount of rainfall, this could prove catastrophic.

One ecosystem to another

Allison Shaw, the lead author of the paper, which was published in Global Change Biology, says that climate change poses a particular threat to animals which have to migrate before they reproduce. Since they do not live and breed in the same area, any obstacle which threatens their transition from one area to another threatens the survival of the entire species.

To measure the effect that rainfall has on the crabs, Shaw and co-author Kathryn Kelly, an oceanography professor at the University of Washington, examined the migration data from between 1919 and 1939, and 1976 to 2011.They found that, except in three years, the crabs did not make their seaward journey unless there had been at least 22 millimetres of rainfall.

Source: Allison Shaw, Department of Ecology and Evolutionary Biology

In 1989, a November dry spell followed by heavy rain in late December meant crabs didn’t migrate until January. (Source: Allison Shaw, Department of Ecology and Evolutionary Biology)

Meanwhile, a light or late rainy season could push their trip backwards or forwards by months. With the El Niño in 1997, which caused an especially dry season, the crabs never migrated at all.  El Niño occurrences are expected to increase as the planet warms.

“If they don’t migrate, they can’t reproduce,” says Shaw. “That’s true for a subset of migratory species that have to breed in a specialized area, but spend most of their adult lives in a different area. They rely on migration to bring them between the two areas that they need.

“On the other hand, species like many temperate birds migrate to avoid harsh winters, but if winters become less harsh they can still survive even if they don’t migrate.”

Turtles, wildebeest and sharks

The Christmas Island red crabs are not the only animals whose yearly migration is prompted by the rains. Sea turtles, she adds, could also be in danger. In Africa, the migratory patterns of wildebeest and gazelles are also driven by the cycles of the wet and dry seasons.

It also impacts upon the animals that depend upon the migration patterns of others for their own survival. If the numbers of the red crabs dwindled, this would affect both the marine and the terrestrial ecosystems that they occupy in turn throughout the year.

For instance, says Shaw, whale sharks migrate to the Christmas Islands to eat the larvae of the red crabs, while the crabs themselves help keep the island from becoming overgrown by munching on plants and saplings as they make their journey to the coast and back.

“Migratory species by definition are traveling either long distances or spanning across different ecosystems,” says Shaw. “The crabs migrate from terrestrial areas to drop their eggs in marine environments.

“Because they’re spanning these ecosystems, they have the potential to impact not only marine environments and species such as the whale sharks, but also terrestrial species and forest dynamics.

“If you took away migratory species you could potentially be affecting multiple ecosystems.”

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