From Princeton University
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.
Each year, millions of Christmas Island red crabs make a two-week
journey to the coast to mate.
The migration begins in November at the
start of the rainy season, and female crabs must release their eggs into
the ocean before the morning of the high tide that precedes the
December new moon.
The researchers found that a late or light rainy
season can delay or entirely cancel this meticulous process.
Native to the Australian territories of Christmas Island and the
Cocos (Keeling) Islands, 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.
But a lack of rain can delay or entirely cancel this meticulous
process, according to research conducted through Princeton University
that could help scientists understand the consequences of climate change
for the millions of migratory animals in Earth's tropical zones.
The
researchers report in the journal
Global Change Biology 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.
Great Migrations: Millions of Crab Babies
Lead author Allison Shaw, who conducted the work as a Princeton doctoral student in
ecology and evolutionary biology,
explained that what scientists understand 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 summers and winters.
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, Shaw said.
"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, 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.
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 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 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."
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 said.
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 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 researchers found that the crabs largely only migrate 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 the El Niño is
projected to become more common as the planet gets hotter.
The movement of little red crabs does not only benefit the species
itself, Shaw said.
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
said.
"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, who heads the behavioral ecology group at the Marine
Biological Association in England, said that most climate-change
studies relate to temperate species because the outcome of climate
change has often been more clearly observed outside of the tropics.
"I suspect most studies have been on temperate species because the
long-term datasets 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 work by Shaw and Kelly might help provide a basis for applying
research on temperate species to their counterparts along the
equator, Sims said.
"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, who received her Ph.D. from Princeton in 2012, began the study
at the University while planning a research trip to Christmas
Island.
Her research focuses on modeling migratory patterns and
determining why animals migrate in the first place.
Her interest in
the red crabs began in 2008 when, as a Princeton doctoral student
funded by the National Geographic Society/Waitt Grants Program,
Shaw studied how size, sex and other factors influence if a red
crab migrates.
She used global climate data to time a second visit
in 2009 with the crabs' migration, but was at first disappointed
when climate alone did not seem to precisely predict crab
movement.
After reading more about climate change and migration,
Shaw realized that her finding that climate generally influenced
migration via rainfall was 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."
The paper, "Linking El Niño, local rainfall and migration timing in a
tropical migratory species," was published in the November 2013
edition of Global Change Biology.
This work was supported by grants
from the National Science Foundation Graduate Research Fellowships
program (grant number DGE-0646086), the National Geographic/Waitt
Institute for Discovery, NASA (grant number 61-7449) and the
University of Washington.
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