A group of geographers and ecologists from three continents has taken an unprecedented look at Antarctica's emperor penguins. Using very high resolution (VHR) images from satellites 450 kilometers above Earth, the team has come up with the first total population count for an entire species. With a whopping 595,000 penguins, they found nearly twice as many emperor penguins as did previous studies, and they counted 46 colonies, up from the earlier total of 38. Their results were published today in PLoS One.
"We were very surprised by the results," says geographer Peter Fretwell of the British Antarctic Survey, one of the study's authors. In 2009 Fretwell's group, in collaboration with the University of Minnesota's Polar Geospatial Center (PGC), Scripps Institution of Oceanography and the Australian Antarctic Division, took satellite images of emperor penguins during their September-through-December breeding season. Using a sharpening technique, a method of image editing commonly used to improve map quality, they adjusted their images to differentiate between adult penguins, their droppings and shadows, details that had confounded earlier efforts to survey the species. Using ground-based counts for reference, they developed an algorithm to identify which pixels in an image represented penguins, as opposed to the surrounding environment, and counted the hundreds of thousands of birds.
Previous measures have relied on either ground tallies or aircraft surveys, which require scientists to observe and count a group of penguins, then estimate the total number. The latter method can be costly and the former fraught with challenges. An emperor penguin colony packs thousands of penguins together, and the animals congregate in areas that hover at –50 degrees Celsius. Many of the colony locations are inaccessible by land or sea.
"There's literally no other way to do this," says ecologist and research fellow Michelle LaRue of the PGC, also a co-author. "The sea ice is way too extensive, it would be way too dangerous. I think this is probably the only cost-effective and efficient way to do it."
Monitoring the animals inhabiting both polar regions has taken on new importance as humans try to determine the impact climate change is having on various species. A team of scientists from a number of U.S. and Russian institutions have launched an aircraft-based survey of Bering Sea ice seals to evaluate how diminishing sea ice affects them.
LaRue is working on similar satellite projects to study Antarctica's Weddell seals, and biologist Heather Lynch of Stony Brook University in New York State is using satellite imaging to study crested and brushtail penguins on the White Continent. Satellite studies are a particularly good match for dark colored species that live on the ice as the animal is easy to spot on a white backdrop. LaRue also observes that given the high resolution of satellite cameras, other kinds of surveys—such as of deserts or savannas—are possible. To illustrate the strength of the satellite imaging, she explained that despite being read from hundreds of kilometers above Earth, a single pixel within an image represents a 61- by 61-centimeter area, or an area as wide as a grown man's shoulders.
Whereas the emperor penguin's environment may be too harsh for a human, the flightless bird is well adapted to its home. The hostile landscape ensures minimal competition and predation, and the waters are rich with scrumptious sea life. The penguins are facing new perils, however. The sea ice on which they breed is among the most susceptible habitats to climate change.
Last year Fretwell and Trathan, with Bernard Stonehouse of the Scott Polar Research Institute in England, announced the disappearance of an emperor colony first spotted in 1948. Its distance from other colonies, gradual population decline and the region's record loss of sea ice suggest that the colony's collapse is related to rising temperatures. The case illustrates why regular satellite imaging of the world's penguin population is needed, so scientists can monitor these species made vulnerable by a warming planet.