It is a valid question, especially in light of my primary specialization in reconstructing past environments of arid lands in the desert Southwest. But the tools we use as paleoecologists can come in many surprising forms, and it turns out those choices the penguins make concerning where and where not to locate maternity colonies is packed with environmental information that can be interpreted to understand changes through time. To decipher that signal, however, requires knowledge of Adélie penguins and their biogeography.
Adélie penguins (Pygoscelis adeliae) is one of the two penguin species that are distributed the farthest south in Antarctica, the other being the emperor penguin (Aptenodytes forsteri), made famous in film March of the Penguins. Both species have an obligate relationship with sea ice- in the case of the emperor they form their maternity colonies on sea ice in fall and early winter, with the males left to tend to the eggs over the long night as the females head to the ice edge to feed until spring. The large size of the emperor penguin demands that incubation and early chick growth is prolonged, essentially taking most of the winter. In the case of the smaller Adélie, they have evolved to jam their entire reproductive cycle into a short summer season, with breeding birds arriving at the maternity colony in late October or November, laying eggs in late November, chicks hatching in December, and parents feeding the rapidly growing chicks through January until they fledge and are ready to go to sea on their own in early to mid-February. This demanding schedule requires a readily accessible source of highly nutritious food to supply the ballooning chicks, and this is where sea ice is crucial to Adélie penguins.
As counterintuitive as it may sound, the most productive parts of the ocean are places that are periodically covered with sea ice. The thickness is critical- more than a meter thick and light is impeded in penetrating to fuel photosynthesis. But in ice up to a meter thick tiny cryophyllic algae grow and thrive in brine channels within the subsurface of the ice. When the ice begins to melt in the spring, vast amounts of this primary production is released into the ocean for the taking. And this massive volume of nutrition provides the base for all the upper levels of the trophic system. Many species eat the algae, but one noteworthy primary consumer is the Antarctic krill- a tiny crustacean that resembles a miniature shrimp and is thought to be the most abundant animal on Earth. And like the algae, many species eat krill (baleen whales being notable), but krill also provides a critical resource for Adélie penguins.
So now we have set of environmental requirements for breeding success in Adélies: snow-free beaches and terraces where nests (in the form of pebble mounds) can be made, a rich supply of krill nearby, made possible by sea ice disintegrating at just the right time (fish and squid are also part of the Adélie diet, but krill makes up almost all of what the bird biologists find in their stomachs), and finally open water nearby that breeding parents can reach quickly to keep the gravy train moving fast enough for the growing chick (so sea ice staying around too long in the summer season causes problems). The combination of these parameters greatly limit where Adélies can form colonies and breed successfully (for instance, there are only around a dozen large colonies in the entire Ross Sea region). Finally, Adélies have high site fidelity, meaning they are very consistent in returning to the same breeding sites year after year, and very resistant to moving somewhere new. Thus, when we find evidence of abandoned sites, where the birds were once successful but had to leave, we can interpret that something in the environment must have changed to force them to go elsewhere. The nature of that change, however, may be a little more complicated than we once thought.
It would be simplest to think of sea ice persistence as a proxy for temperature: if it’s warmer, the sea ice goes away (or melts earlier in the season). If it’s colder, the sea ice persists, forming a cap on the ocean that remains throughout the summer and prevents access to the sea for foraging parents. But events in the Ross Sea in the middle of this decade have added some complexity to this issue. Since 2000, large tabular icebergs have been calving off the front edge of the Ross Ice Sheet, and drifting around the Ross Sea, periodically getting stuck on shallows surrounding Ross and Beaufort Islands (see this Earth Observatory Image of the Day for an example). When these bergs run aground, they block the wind and hold the sea ice in place, which is especially effective in regions prone to polynyas (open water patches) from katabatic (gravity-driven) winds. Several Adélie colonies such as the ones at Cape Bird and Cape Crozier have been severely impacted by the persistence of sea ice- breeding adults are forced to march 10’s of kilometers across the ice to reach open water to feed, which forces them to burn much of the food they acquire in turn starving the chicks. Reproductive success during these episodes is low, and if the condition persisted it is likely modern-day abandonment would take place.
In summary, we feel that details on several important issues can be illuminated by the work that we are doing here, and that Steve is doing around the rest of the continent. For one thing, very little has been known about the past distributions of Adélie penguins: where they were in the past, how long they have been at the modern colonies, and how they react when environments change are all important details to understand in a polar landscape that may change rapidly in the future. Second, the pattern of occupation and abandonment can preserve a proxy record of sea ice extent and past environments in a poorly known region of the planet where no other record exists. Finally, human impacts on the natural world are often subtle and difficult to discern in the vastness of Antarctic environments. These data are just now revealing some unexpected impacts we humans have had on polar ecosystems.
From our three seasons of work in the Ross Sea, combined with Steve’s work on the Antarctic Peninsula and in East Antarctica, it appears that the location of most modern colonies is a relatively recent event. The large colonies on Ross Island and further north along the Victoria Land Coast have only been occupied for the past 1000 to 2000 years. Before then, especially between 2000 and around 4000 years ago, Adélies were occupying sites much farther south along the coast than where they currently live during a “penguin optimum”. So it is clear the birds have the ability to adapt to significant environmental change as long as there is somewhere more suitable to go and they can reach it (the million dollar question in a warming world is whether both of those factors will again be fulfilled). We also found evidence that Adélies have been around the Ross Sea for a very long time- two of the sites we discovered date to before the peak of the last glaciation, before the Ross Ice Shelf extended out to almost cover the Ross Sea (in fact they date to much earlier, to the very limits of radiocarbon dating at around 50,000 years ago). Isotope work Steve has done on eggshell and bone from the abandoned and modern colonies led to an unexpected discovery concerning Adélie diet. Isotopic signatures preserved in these materials leave a record of where in the food chain an organism feeds- in the case of the Adélies today it is relatively low, mostly exploiting the tiny grazers, krill. But this lower trophic level diet apparently only began 150 years ago, prior to that the penguins were apparently feeding mostly higher up the food chain on fish and squid, with krill making up a much smaller part of the diet. What could cause such a dramatic shift in diet? Steve’s current hypothesis is that krill became abundant after the much larger, much more efficient krill eaters of the southern oceans were removed- in the mid- to late-1800s whale populations were decimated around Antarctica (and in all the world’s oceans). It is likely that a “krill surplus” encouraged the Adélies to shift to reliance on this resource once their larger competitors were gone.
Emslie et al., 2007. A 45,000 yr record of Adélie penguins and climate change in the Ross Sea, Antarctica. Geology 35, 61-64.
David Ainley’s Penguin Science website.
