Dry conditions can delay or even cancel red crabs’ annual mating process on Christmas Island and the Cocos (Keeling) Island, both Australian territories.
The findings could help scientists understand the consequences of climate change for the millions of migratory animals in Earth’s tropical zones.
Each year, the land-dwelling Christmas Island red crab takes an arduous and shockingly precise journey from its earthen burrow to the shores of the Indian Ocean where weeks of mating and egg laying await.
Millions of the crabs start rolling across the island roads and landscape in crimson waves when the November rains begin.
After a two-week scuttle to the sea, the male crab sets up and defends a mating burrow for himself and a female of his kind, the place where she will incubate their clutch for another two weeks.
Before the morning of the high tide that precedes the December new moon, the females must emerge to release their millions of eggs into the ocean. A month later, the next generation of crabs comes ashore.
In the journal Global Change Biology, researchers report that the crabs’ reproductive cycle tracked closely with the amount and timing of precipitation. Writ large, these findings suggest that erratic rainfall could be detrimental to animals that migrate with the dry-wet seasonal cycle that breaks up the tropical year, the researchers report.
If fluctuations in rainfall become more extreme and frequent with climate change, then scores of animals could be in trouble—not just the migrators themselves, but also the creatures reliant on them for food.
For tropical creatures such as the Christmas Island red crabs, or the wildebeests and gazelles of Africa, however, the regular quest for safety, food, and reproduction is driven by wet and dry seasons.
Yet how the erratic rainfall expected to accompany an altered climate will affect these animals is not well understood, says lead author Allison Shaw, who conducted the work as a Princeton University doctoral student in ecology and evolutionary biology.
“Potentially there’s been a perspective bias in how migratory species are studied, and this particular species represents two perspectives that have not been well documented—species that are migrating because they have to breed in a certain area, and species that are migrating in response to rainfall,” says Shaw, a postdoctoral researcher at the Australia National University who will start as a University of Minnesota assistant professor of ecology in 2014.
“Targeting those two types of migration patterns that have so far been understudied is really what we’re hoping to accomplish with this paper, and to encourage more studies in those areas,” Shaw says.
Messed up schedules
Shaw and her co-author Kathryn Kelly, an oceanography professor at the University of Washington, obtained migration data for 36 years that fell between 1919 and 1939, and 1976 to 2011. They predicted the egg-release date for each year, and compared the later figures to actual rainfall measurements from 1973 to 2011.
Except in three years, the crabs did not launch their procreative journey if there had not been at least 22 millimeters (0.87 inches) of rainfall. A light or late rainy season could push their journey forward or back months—in 1989, for instance, a November dry spell followed by heavy rain in late December kept the crabs in their burrows until early January.
During the especially dry 1997 season, the crabs never migrated or mated. The culprit was a strong El Niño, the warm-water climate pattern that creates dry conditions in the Indian Ocean. The occurrence of the El Niño is projected to become more common as the planet gets hotter.
“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,” Shaw says. “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.”
Climate change could be a special challenge to species such as Christmas Island red crabs or sea turtles that migrate to specific locations to reproduce, Shaw says. These animals do not live and breed in the same ecosystem, so any obstacle between one location and the other threatens their survival as a species.
“If they don’t migrate, they can’t reproduce,” Shaw says. “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.”
The movement of little red crabs does not only benefit the species itself, Shaw says. Migratory animals have a role in the ecosystems spanning the territory they traverse. For instance, whale sharks migrate to Christmas Island to feast on the red crabs’ larvae. As the parental crabs journey to the coast and back, they feed on plants and saplings that keep the island from being overgrown, Shaw says.
“Migratory species by definition are traveling either long distances or spanning across different ecosystems,” Shaw says. “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,” she says. “If you took away migratory species you could potentially be affecting multiple ecosystems.”
The National Science Foundation Graduate Research Fellowships program, the National Geographic/Waitt Institute for Discovery, NASA, and the University of Washington supported the study.
Source: Princeton University