Researchers in Antarctica have used 30 years of satellite data – combined with hand-scooped poop samples – to learn more about penguin populations. And what they’ve found has them worried.
In a new study published in Current Biology, the scientists suggest that Adélie penguins eat more fish when there is more sea ice and more krill when the sea ice is lacking.
This is concerning because penguin colonies that relied more heavily on krill were doing worse than those foraging fish – and global warming is causing sea ice to decline, pushing penguins towards a more krill-based diet.
The study came about because lead author Casey Youngflesh, assistant professor at Clemson University, wanted to understand the inner workings of the Antarctic ecosystem. “There are many processes that we just can't observe directly,” Youngflesh says.
So, he turned to penguins. These seabirds are what’s known as an indicator species. “They’re a really great bellwether of change in that system because they're integrating all of these different things that are going on in the environment,” he says.
For example, they nest on land and rely on different ocean resources – including shrimp-like krill and silverfish – for their food. That’s why studying penguins helps experts understand what’s going on in the wider system.
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Antarctic research is not easy. Getting to this remote, harsh continent is difficult and conducting science is expensive, not to mention logistically challenging. “But luckily we have satellites that we can actually see penguin colonies from,” says Youngflesh.
Adélie penguins – the species he was studying – nest in huge colonies. “You know, hundreds of thousands of pairs,” he says. “A lot of penguins in one concentrated area.”
And with a lot of penguins comes a lot of penguin poop.
That’s what Youngflesh and colleagues were looking for in the satellite images. “You can see those guano deposits from satellites,” he says. Against the bright snow, “it pops out at you.”
“It’s exactly what it sounds like,” says co-author Michael Polito, a professor of ocean sciences at the University of California, Santa Cruz, in a statement. “We spied on penguins from space by using satellite images to figure what they eat all around Antarctica to help explain their diet and population response to recent climate change.”
The idea of using faeces stains to find penguin colonies isn’t new – NASA scientist Mathew Schwaller came up with the idea many years ago – but Youngflesh and the team wanted to go a step further.
What if they could colour-match the poo to discover what the penguins have been eating, they wondered.
Penguins eat shrimp-like crustaceans called krill, which have bright pink exoskeletons that turn the penguins’ poop pink, as well as Antarctic silverfish, which produce whiter guano. The scientists wanted to measure the proportion of each in the birds’ diet based on the colour gradient of their excrement.
“We didn't just do this through Pantone colour cards, holding it up next to guano,” Youngflesh says. “That’s just not the way to do science.”
“Step one, go out on the penguin colony and scoop up little poop samples and put them in your bags with your plastic spoon,” he says. He then brought the samples back to the ship, spread them out and measured the colour.
Using a spectrometer – an instrument that measures light wavelengths – Youngflesh and the team quantified how much light was being reflected off the guano from each visible part of the electromagnetic spectrum (“That's the rainbow,” Youngflesh clarifies).
Once they’d measured the colour, they could link up the shade of scat with how much fish or krill the penguins were eating. They did this in their North American lab using a chemical technique called stable isotope analysis, which tells them what the birds had been eating.
Penguins aren’t the only animals eating krill and silverfish. Whales, seals and a whole bunch of other creatures also devour these key food items. “You have just these couple things that everything else is eating,” he says.
Seeing how their make-up in penguins’ diet changes can give the experts an inkling about what’s happening in the wider Antarctic food web. “These penguins are just eating what is available to them,” he explains. “When you have more ice, those penguins are eating more fish. Less ice during that year, penguins are eating more krill.”
Changes in the sea ice – which is declining – impact the abundance of these food sources. As the ice melts, there will be fewer silverfish and the Adélie penguins rely more heavily on krill.
This is a problem. “Penguin populations that are eating more krill are more likely to be in decline,” says Youngflesh. The silverfish are higher quality food because they are more calorie dense. “Studies have shown that chicks that are fed fish are generally in better body condition. They have higher survival,” he adds. “So, it seems likely that these penguins would prefer to be eating fish, but those fish are really no longer available in some regions or in some years, they've got to switch to krill.”
There’s also increasing competition for krill between all the Antarctic animals trying to eat them as well as from commercial krill fisheries. “This study does not paint a positive picture for long-term Adélie penguin population trends into the future with continued sea ice declines,” says Youngflesh.
There are things everyone can do day-to-day to help, Youngflesh adds: reducing carbon emissions through actions including flying less, driving less and eating less read meat, and avoiding products that contain krill (such as supplements).
Meanwhile, scientists are working through the huge swathes of data available from satellites, which have been capturing images from space for decades. “We need to find out how can we actually make use of this information, even if it wasn't originally intended and designed to do the things that we're interested in,” such as study penguin diets, he says.
To understand this complex ecosystem, “sometimes we need tens of thousands of data points,” Youngflesh adds. “And things like satellite-based sensors [camera traps, audio recorders and participatory science projects], they're really providing an opportunity for us to do this.”
Top image credit: Thomas Sayre-McCord/WHOI/MIT
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