Clemson University researcher Casey Youngflesh and collaborators are using satellite images of Adélie penguin colonies and their guano to determine what they eat— and how changes in sea ice are affecting the Antarctic’s food web.
Based on the color of the guano, the researchers reconstructed Adélie penguin diets across nearly the species’ entire global range over a 30-year period, from 1984 to 2013. Changes in diet were strongly linked to both changes in Antarctic sea ice and long-term penguin population trends.
“Satellites enabled us to do something that would otherwise be impossible,” Youngflesh said. “The innovation wasn’t the satellite technology itself, but the ability to leverage these decades of satellite imagery with modern statistical and computational tools. No one intended for these satellites to be used to monitor penguins, but now we’re able to use them in these novel ways.”
The wide distribution of Adélie penguins and the relative ease with which they can be monitored from satellites make them useful indicators of broader Antarctic ecosystem change.
But studying that process across the entirety of Antarctica has been difficult because the continent is vast, remote and logistically challenging to work in. Researchers can visit some penguin colonies, collect samples and monitor populations, but it is not feasible to sample every colony repeatedly across decades.
Satellites changed that.
In the study, the researchers used the spectral signature, essentially a quantitative measure of color across visible and infrared wavelengths, of guano to infer what the penguins had been eating.
Youngflesh collected guano samples at penguin colonies in the field and then analyzed the spectral properties in a lab. The researchers then used stable isotope analysis to determine where the penguins’ diet fell on a spectrum from krill to fish. Those data allowed the team to build a model connecting guano spectra to diet and apply that model to satellite imagery from Landsat satellites, a long-running NASA earth-observing satellite program.
Youngflesh said the study is the first use of satellite observations to capture food web dynamics at continental and decadal scales.
He said the study points to a broader future for satellite-based wildlife monitoring.
Sea ice important
Adélie penguins are one of the most numerous predators in Antarctica, but they depend on a relatively small number of prey species to survive. During breeding season, their diet consists primarily of Antarctic silverfish and krill, a small, shrimp-like crustacean. In years and places with more sea ice, Adélie penguins ate more fish. In years and places with less sea ice, they relied more heavily on krill.
“As sea ice dynamics change through time, we expect those changes to cascade through the food web,” said Youngflesh, an assistant professor in the Department of Biological Sciences.
The study also found that diet was linked to long-term changes in penguin populations. Colonies with more krill-based diets were more likely to be declining than colonies with more fish-based diets. Previous research has shown that chicks fed more fish tend to be larger and have better survival prospects than chicks fed more krill. In areas where fish are less available because of a decline in sea ice, penguins may be forced to rely more heavily on krill.
“Penguins can certainly eat krill,” Youngflesh said. “But there are likely fewer krill available now than there were previously, partially because there are more things competing with penguins for that resource, including humans.”
Large-scale declines
In the years since the end of this study period, large-scale declines and record lows in Antarctic sea ice have been observed. If those declines continue, Adélie penguins may have to shift toward more krill-dominated diets across larger parts of their range.
“The fact that we can figure out what something is eating from a satellite is pretty incredible,” Youngflesh said. “It illustrates the power, and to some degree the necessity, of these types of data resources for understanding how natural systems work and how they are changing over time.”
The study, “Space-based monitoring of penguin diet links sea ice, food webs and population change,” was published in the journal Current Biology.
The research was supported by NASA NESSF fellowship NNX16AO27H, a Sigma Xi Grant-in-Aid of Research, an Analytical Spectral Devices Goetz Award, NASA grant NNX14AC32G, National Science Foundation grant ANT-1443585 and a Stony Brook University Institute for Advanced Computational Science Postdoctoral Fellowship.
