Saturday, April 14, 2012

Scientists double their estimates of Antarctic emperor penguin populations after careful satellite study

Posted on by Bob Berwyn

Some populations — especially along the Antarctic Peninsula — still considered vulnerable to global warming impacts
Emperor penguin colony near Halley Bay. IMAGE COURTESY DIGITALGLOBE.


By Bob Berwyn
SUMMIT COUNTY — After carefully studying satellite images of emperor penguin colonies and comparing the numbers to sites with ground-truthed data, biologists estimate there may be twice as many emperor penguins in Antarctica than previously thought.

The ground-breaking study also discovered four new colonies and confirmed the location of three other colonies.

The research is published  this week in the journal PLoS ONE. It provides an important benchmark for monitoring the impact of environmental change on the population of this iconic bird.

“We are delighted to be able to locate and identify such a large number of emperor penguins,” said lead author and geographer Peter Fretwell, of the British Antarctic Survey. “We counted 595,000 birds, which is almost double the previous estimates of 270,000 – 350,000 birds. This is the first comprehensive census of a species taken from space.”

The latest count of 238,000 breeding pairs far exceeds the last estimate from 1992 of 135,000-175,000 breeding pairs.

The total population size was estimated from the number of breeding pairs based on 80 percent of penguins breeding in any one year in all colonies. Uncertainties stem from remnant difficulties in differentiating penguins from guano and shadow, which will be resolved as the resolution of satellite imagery improves further. Forty three of the forty four images used in the analysis were from one breeding season (2009), so the estimate can be considered a single year census.
Emperor penguin colonies show up as dark splotches against the white ice near Halley Bay. PHOTO COURTESY DIGITALGLOBE.

On the ice, the black-and-white feathered emperor penguins  stand out against the snow and colonies are clearly visible on satellite imagery. This allowed the team to analyze 44 emperor penguin colonies around the coast of Antarctica.

Despite the new population estimates, biologists said emperor penguins remain vulnerable to global warming impacts — especially the colonies close to the Antarctic Peninsula, where the annual sea ice duration is less now than 2 decades ago and where large portions of coastal ice shelves are disintegrating.

An emperor penguin with chick. PHOTO COURTESY BRITISH ANTARCTIC SURVEY.“Current research suggests that emperor penguin colonies will be seriously affected by climate change," said British Antarctic Survey biologist Dr. Phil Trathan, who previously documented the disappearance of an emperor penguin colony from the west coast of the Antarctic Peninsula. "An accurate continent-wide census that can be easily repeated on a regular basis will help us monitor more accurately the impacts of future change on this iconic species.”
Emperor penguins depend on stable sea ice during the breeding and fledging cycle.
“They need  a stable platform the whole time the eggs are being incubated, and during chick-rearing and fledging,” Trathan said.

Shifts in the timing of sea ice formation and duration — as observed close to the Antarctic Peninsula — could drastically affect the reproductive cycle.

For example, if the ice melts earlier, or there’s more snow, the chicks could get wet, leading to direct mortality.

“The colonies that are most vulnerable are those close to the peninsula, where climate change is happening,” he said, adding that there are regional variations in observed climate changes in Antarctica. “We know the eastern side of the Antartic Peninsula is vulnerable,” he said, referring to the disintegration of parts of the Larsen ice shelf.

Other studies have shown a direct link between changes in sea ice, abundance of krill and a decline and shift in other penguin populations along the western Antarctic Peninsula, where the number of both Adelies and chinstrap penguins may have dropped by as much as half since the 1980s. In that same span, mean winter air temperatures in the region have climbed by up to 10 degrees fahrenheit, leading to a huge loss of sea ice.

A satellite image shows the emperor penguin colony at Smyley Island. PHOTO COURTESY DIGITALGLOBE.

Without the sea ice in the mix, the ocean is not nearly as productive, NOAA and Scripps Institute of Oceanography scientists said, adding that an increase in the number of whales and seals, as well as a developing commercial krill fishery may also be factors in declining krill populations.

Trathan said followup studies may focus on the condition of the sea ice where emperor penguins breed.

“One of the things I’d really like to do is look at fast ice conditions … to try and see which are the most vulnerable colonies, and which are more stable than others. If there are good stable ice conditions it means that maybe a colony will be successful each year” he said.

The research

The scientists used images from three different satellites (Quickbird2, Worldview2 and Ikonos) to identify locations of emperor penguin colonies. Then they acquired more detailed imagery from Boulder-based DigitalGlobe and pansharpened the shots to differentiate between penguins, shadow and guano.

Pansharpening involves the merging of a higher-resolution panchromatic image (black and white, but sensitive to all wavelengths of light) and a lower-resolution color image to produce a single high-resolution color image. This process lowered statistical deviations between known and predicted penguin counts considerably.

University of Minnesota biologist Michelle LaRue explained how the team used the images for their count.

“We created an algorhithm, training the computer to be able to tell the differecne between guano, ice and penguins,” she said, adding that she previously tested the technique by counting Weddell seals to show that satellite imagery is a reliable way to count populations.

“Now that we know we can do this, and how many emperor penguins there are, we can hone in a little bit,” she added.

“The methods we used are an enormous step forward in Antarctic ecology because we can conduct research safely and efficiently with little environmental impact, and determine estimates of an entire penguin population,” LaRue said.

“The implications of this study are far-reaching: we now have a cost-effective way to apply our methods to other poorly-understood species in the Antarctic, to strengthen on-going field research, and to provide accurate information for international conservation efforts.”

Previous counts of emperor penguins have been challenging because they live in remote terrain and the best time to count them is when weather conditions are incredibly severe and when extensive sea ice makes it difficult to reach the colonies.


Emperor penguins

The emperor is the giant of the penguin world and one of the largest of all birds. Gold patches on their ears and on the top of their chest brighten up their black heads.

Emperors are the only penguin species to breed through the Antarctic winter, with temperatures as low at  minus 50 degrees Celsius and winds of up to 200km (or 124 miles) per hour.

They form large colonies on the sea-ice, with the female laying a single egg and passing it to the male for incubation. The eggs are balanced on the penguins’ feet, which are then covered by a thick roll of skin and feathers. This keeps the egg some 70 degrees Celsius warmer than the outside temperature.

The females will then go to the sea to feed, and return around the time when the chicks are due to hatch. She then takes over brooding and feeding the chick while the male, after a nine-week fast during which it may lose 45 percent of its body weight, treks up to 100 kilometers over the ice to find food.

To survive the Antarctic winter, adult emperors have a special combination of adaptations, with a dense double layer of feathers and a large fat reserve. Afterwards, both adults rear the chick.
The survey found four new colonies, at Brownson Islands, Dolleman Island, Dibble Glacier and Rupert Coast, and three previously suspected colonies, at the Shackleton Ice Shelf, Bowman Island and the Lazarev Ice Shelf. There are 46 emperor penguin colonies locations around the coast of Antarctica.


(click on images for larger size)

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