Of stars and stripes: NASA satellites used to predict
zebra migrations
Zebra in the Makgadikgadi grasslands.
Image Credit:
Hattie Bartlam-Brooks
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One of the
world's longest migrations of zebras occurs in the African nation of Botswana,
but predicting when and where zebras will move has not been possible until now.
Using NASA rain and vegetation data, researchers can track when and where arid
lands begin to green, and for the first time anticipate if zebras will make the
trek or, if the animals find poor conditions en route, understand why they will
turn back.
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Researchers used cues gleaned from GPS tracking of
the zebras and satellite data to predict when the zebras will be on the move, a
powerful tool for conservation.
Image Credit: NASA
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Covering an area of approximately 8,500 square miles (22,000 square
kilometers), Botswana’s Okavango Delta is one end of the second-longest zebra
migration on Earth, a 360-mile (580-kilometer) round trip to the Makgadikgadi
Salt Pans—the largest salt pan system on the planet. Zebras walk an unmarked
route that takes them to the next best place for grazing, while overhead
thundering cloudbursts of late October rains drive new plant growth, filling
pockmarks across this largest inland delta in the world. In a matter of weeks,
the flooded landscape could yield ecosystems flush with forage for the muscled
movers.
High above, Earth-orbiting satellites capture images of the zebras' movements
on this epic trek, as well as the daily change in environmental conditions.
Zebras don’t need data to know when it’s time to find better forage: The surge
of rain-coaxed grasses greening is their prompt to depart. But now, researchers
are able to take that data and predict when the zebras will move.
Pieter Beck, research associate with the Woods Hole Research Center in
Falmouth, Mass., and three collaborators studied animal migration in a novel
way, which they described in a paper published in the Journal of Geophysical
Research--Biogeoscences, a publication of the American Geophysical Union. While
tracking animal movement with satellites has been accomplished many times, Beck
said, he and his team combined that information with in-depth use of
environmental satellite data, using a series of images of vegetation growth and
rainfall taken over days and weeks. This sheds unprecedented light on what
drives animals to migrate, he said, what cues they use, and how animal
migrations respond to environmental change.
Dr. Hattie Bartlam-Brooks collared a zebra with GPS
to track their location along the migration route.
Image Credit: Botswana Herbivore Research
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The Zebra Migration Research Project began in 2008 after Hattie
Bartlam-Brooks and her team discovered the migration during field work for
Okavango Herbivore Research. Anecdotal evidence—unverified stories—prior to the
1970s described a zebra migration from the Okavango Delta to the Makgadikgadi
Salt Pans at the start of the rainy season in September and continuing through
April, but from 1968 to 2004, veterinary fences prevented zebras from making the
migration. The veterinary fences—which had been built to keep wild buffalo from
transferring diseases to cattle—were taken down in 2004. Within three years of
the removal of the veterinary fences, zebra began making movements on the
migration path toward the Makgadikgadi Salt Pans. These movements were recorded
by GPS collars that were fitted to zebra mares, allowing researchers to
accurately record their movements.
Zebras in the wild live for about 12 years, so the migration path could not
have been learned from previous generations, said Bartlam-Brooks. She and her
team in the field observed that the zebras began their migration at the onset of
the rains so she joined forces with Beck to see how extensive the environment's
influence is on the timing of the zebras' journey.
Beck combined this GPS movement data with satellite imagery taken over the
months of the migration. This allowed the researchers to see how environmental
conditions changed over time and across the landscape. To track the greening of
leaves, the researchers relied on the Normalized Difference Vegetation Index
data acquired by the Moderate Resolution Imaging Spectroradiometer on board
NASA's Terra and Aqua satellites. The MODIS sensors capture growing conditions
by measuring the reflectance of near-infrared light from plants. The team also
used NASA’s Tropical Rainfall Measuring Mission data to map daily rainfall,
which gave the researchers an idea of how much rain was falling in three-hour
intervals. The scientists converted the rainfall measurements to daily rates,
and cumulative weekly amounts, and checked the accuracy by comparing them with
ground-based rain gauges.
Beck and his team learned that zebras do not follow an internal clock, nor do
they migrate at a steady pace. By examining daily rainfall and weekly vegetation
data from satellite images and entering the data into migration models, the
researchers were amazed at how well they could predict when zebras started
migrating and how fast they migrated.
The Okavango Delta in Botswana.
Image Credit: Teo Gomez
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“By comparing the results of the models, it was possible to determine which
environmental variables are the most effective in predicting zebra movement, and
then use this knowledge to try and infer as to how the zebra make their
decisions,” said Gil Bohrer, assistant professor in the Department of Civil,
Environmental, and Geodetic Engineering at The Ohio State University, who
collaborated on the project. “It shows we can figure out very closely what
‘makes the zebra move.’”
Bill Fagan, professor of biology at the University of Maryland, finds hope in
the team’s discoveries. “Their discussion," he said, "was particularly
intriguing as a demonstration of how important the consistency and strength of
the rainfall cues were for migration success." He said that it may be possible
for species that have had their migration patterns disrupted to relearn them
from "exploratory walks" driven by environmental cues. "With so many ungulate
migrations declining worldwide, it is nice to have an optimistic result about
migration for a change."
Satellite safari: Guiding light among the stars
Having access to NASA's free satellite images that shed light on the
environmental conditions migratory animals face is something that Beck finds
invaluable. The models provided the team with the means to think like a zebra,
which has practical applications in management issues that concern humans.
“We’re getting close to the stage where for some organisms, we can use
satellite data in management,” he said.
Satellite image of Botswana's Okavango Delta and
Makgadikgadi Salt Pans.
Image Credit: Terra MODIS/ NASA
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He sees the capability of using the team’s research in the future to design
models that will help game managers, conservation managers, farmers and tour
operators predict animal migration, whether it’s zebras or other migratory
animals. Understanding the mechanisms that drive migratory behavior is
increasingly important, Beck said, in terms of climate change, as migrating
animals rely on multiple habitats.
If migrating animals lose any of the habitats they rely on because the timing
of their food—insect hatches, greening plants, for example—no longer coincides
with their travel, this can have serious consequences for their continued
survival. Under climate change, things are likely to accelerate, Beck said. Many
of the major migrations on Earth, especially on land, have already been lost, he
explains, and few landscapes are left on Earth where the migratory animals do
not have to share land resources with agriculture and other human
activities.
"We need to know what the fate of those migrations is under climate change,"
Beck said. "Understanding when animals might come through, what drives them,
what they’re looking for sometimes. Being able to predict that into the future
is very useful information to managing those landscapes so that migratory
animals and humans can coexist." Helping zebras continue a journey—newly
discovered by the animals and their observers—may allow them to cope with
changes in their environment, an outcome that is not so black-and-white.
The environmental data sets from the Tropical Rainfall Measuring Mission and
the MODIS sensors will continue into the next decade with data provided by their
follow-on missions: the Global Precipitation Measurement mission to launch in
early 2014, and the Visible Infrared Imaging Radiometer Suite on the Suomi
National Polar-orbiting Partnership satellite currently in
orbit.
Lisa-Natalie Anjozian
NASA's Goddard Space Flight Center
NASA's Goddard Space Flight Center
Media Invited to NASA Google+ Hangout on Wildfire and
Climate Change
NASA will host a Google+ Hangout at 1 p.m. EDT Friday, Aug. 9, about wildfire
research and what a changing climate could mean for future fire activity in the
United States.
A decades-long record from ground surveys and NASA satellites shows the fire
season in the western United States is starting earlier in the spring and
producing larger and more intense fires throughout the summer. Is this a result
of climate change, or are other factors involved? How do scientists anticipate a
continued increase in global temperatures will influence the number and strength
of wildfires?
Panelists for the Google+ Hangout are:
- Doug Morton, research scientist, NASA's Goddard Space Flight Center, Greenbelt, Md.
- Bill Patzert, research scientist, NASA's Jet Propulsion Laboratory, Pasadena, Calif.
- Elizabeth Reinhardt, national program leader for fire research, research and development, Office of the Climate Change Advisor, U.S. Forest Service, Washington
The panelists will discuss the 2013 fire season so far, recent trends in U.S.
and global wildfires, and what climate projections reveal about potential fire
activity in the future.
Journalists who want to ask questions by phone during this Google+ Hangout
must provide their media affiliation information to Aries Keck at 301-286-4435
or aries.keck@nasa.gov by 10 a.m. Aug.
9. The Hangout will be broadcast publicly via NASA Goddard’s YouTube and Google+
pages. The Hangout also will be carried live on NASA Television and the agency's
website.
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Guillermo Gonzalo Sánchez Achutegui
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