Lack Of Rain Halts Migration Patterns Of Christmas Island Crabs
October 11, 2013

Lack Of Rain Halts Migration Patterns Of Christmas Island Crabs

April Flowers for - Your Universe Online

The Christmas Island red crab makes a hard and amazingly-precise journey every year from its earthen burrow to the shores of the Indian Ocean for weeks of mating and egg-laying.

The crabs are native to the Australian territories of Christmas Island and the Cocos (Keeling) Islands. When the November rains begin, millions of the crabs begin rolling across the island roads and landscape in crimson waves. Crabs scuttle for two weeks before each males of the species set up and defend a mating burrow for themselves and a female. The female will incubate her clutch for another two weeks in this burrow before emerging the morning of the high tide that precedes the December new moon to release her millions of eggs into the ocean. The next generation of crabs comes ashore a month later.

A new study led by Princeton University reveals that this meticulous process can be delayed or entirely cancelled by a lack of rain. The findings, published in Global Change Biology, could help researchers understand the consequences of climate change for the millions of migratory animals in Earth's tropical zones.

The researchers found that that the crabs' reproductive cycle tracked closely with the amount and timing of precipitation, which suggests that erratic rainfall could be harmful to animals that migrate with the dry-wet seasonal cycle that breaks up the tropical year. If climate change causes those rainfall fluctuations to become more extreme and frequent, then scores of animals could be in trouble — not just the migrators themselves, but also the creatures that rely on them for food.

Allison Shaw led the research while she was a Princeton doctoral student in ecology and evolutionary biology. She explained that the knowledge scientists currently have about the possible impact of a warming planet on animal movement is dominated by studies of how creatures that migrate with the summer-to-winter seasonal shifts of Europe and North America will be affected by changes such as the severity and duration of those seasons.

For other animals, such as the Christmas Island red crab, the wildebeests and gazelles, their regular migratory quests for safety, food and reproduction are driven by wet and dry seasons.

However, Shaw said that how the erratic rainfall that is expected to accompany an altered climate will affect these animals is not well understood.

"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," said Shaw. She is currently 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 said.


Shaw worked with the University of Washington oceanography professor Kathryn Kelly to obtain migration data for 36 years that fell between 1919 and 1939, and 1976 to 2011. This data allowed them to predict the egg-release date for each year, after which they compared the later figures to actual rainfall measurements from 1973 to 2011.

The crabs did not launch their migration if there had not been at least 0.87 inches of rainfall, except for three years during the entire study period. The start of their journey can be moved back or forward by months if there is a light or late rainy season. For example, a November dry spell in 1989 was followed by a heavy rain in late December that kept the crabs in their burrows until early January. The crabs neither migrated nor mated during the especially dry season of 1997, which was the result of a strong El Nino. El Nino is projected to become more common as the planet becomes warmer.

"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 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."

For animals such as Christmas Island red crabs or sea turtles that migrate to specific locations to reproduce, climate change could be a special challenge as these animals do not live and breed in the same ecosystem. Any obstacle between one location and the other threatens their survival as a species, Shaw said.

"If they don't migrate, they can't reproduce," Shaw said. "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 migration of the bright red crabs benefits more than their own species, according to Shaw. Migratory animals play a role in every ecosystem they cross. Whale sharks, for example, migrate to Christmas Island to feast on the larvae of the red crabs. The parental crabs keep the island from becoming overgrown by feeding on plants and saplings as they travel to the coast and back.

"Migratory species by definition are traveling either long distances or spanning across different ecosystems," Shaw said. "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 said. "If you took away migratory species you could potentially be affecting multiple ecosystems."


David Sims, from the behavioral ecology group at the Marine Biological Association in England, said that the majority of climate change studies concern temperate species because the most observable outcome of climate change has been seen outside of the tropics.

"I suspect most studies have been on temperate species because the long-term data sets needed to support robust analysis are more available for species in these regions," said Sims, who is familiar with the research but had no role in it. "In addition, some of the largest changes in sea temperatures seen globally have been recorded in temperate regions such as the North Sea, so biological signals have been clearer."

The current study may provide a basis for applying previous research on temperate species to counterparts along the equator, according to Sims.

"The paper exemplifies well that the migration timing of tropical species is perhaps more similar to the responses of temperate species than previously realized," Sims said. "Generally it seems that ectothermic [cold-blooded] species' migrations [such as that of the red crabs] correlate with environmental temperature changes, however, the picture for individual species can be complex."

Shaw began the study at Princeton while planning a research trip to Christmas Island. Her overall research focuses on modeling migratory patterns to understand why animals migrate in the first place. She gained an interest in the red crabs in 2008 as a doctoral student funded by the National Geographic Society/Waitt Grants Program.

Shaw studied the influence of size, sex and other factors on the migration of the crab, using global climate data to time her second visit to the island with the crabs' 2009 migration. She was disappointed when climate alone did not seem to precisely predict crab movement. After reading more about climate change and migration, Shaw realized that her findings about climate-influencing migration via rainfall were valuable .

"In the process of trying to ask this question about timing we were able to link migration patterns to rainfall, and rainfall to El Niño and global climate patterns," Shaw said. "I realized that, seen in a different light, the analysis we had done on the crabs was quite valuable — it demonstrated a connection between climate and migration through rainfall, which hadn't been done for many species."


Image Below: The researchers found that the crabs largely migrate only if there has been at least 22 millimeters (0.87 inches) of rainfall. The crabs never migrated or mated during the especially dry 1997 season because of a strong El Niño, the warm-water climate pattern that creates dry conditions in the Indian Ocean. The occurrence of El Niño is projected to become more common as the planet gets hotter. Credit: Princeton University/Shaw