by Jaume Forcada for Planet Earth Online
Penguins are one of the most common seabirds in the Southern Ocean. Their size, shape and other traits make them well adapted to the extreme conditions of sea and ice they live in, and some species live in huge colonies. Their environment determines the quantity and quality of their food and the availability of their preferred breeding and resting places. So a penguin’s way of life is restricted to a closely defined set of conditions, and as a group this makes them particularly sensitive to climate change.
When changes in climate affect the penguins’ environment – for better or worse – they have to adapt, especially when their critical habitats are affected. But penguins don’t all have the same lifestyles, so does this mean some species will respond differently to others as the Antarctic environment changes? Together with colleagues at the British Antarctic Survey, I have been trying to find out.
Eight of the world’s 17 penguin species live in the Southern Ocean. All but two of these 17 are ice intolerants, which means they live on ice-free land and ocean. Adélie and emperor penguins are ice-obligates: they rely on the sea ice and can live in the most extreme environments. Emperors in particular can survive some of the harshest weather on the planet. The species differ in other aspects too, for example, their breeding chronologies – in other words the timing of arrival at their breeding site, egg-laying, chick fledging and moulting in adults.
To find out how these different lifestyles might influence the penguins’ response to climate change, we gathered almost 30 years of continuous records of the lives of penguins across the Scotia Sea, in the south-west Atlantic sector of the Southern Ocean. We used historical records and information gathered by our own colleagues in the field, to take a closer look at the macaroni and gentoo penguins of South Georgia, and the gentoo, chinstrap and Adélie penguins of the South Orkney Islands.
Conditions in the Southern Ocean vary distinctly with the seasons, and this seasonality is crucial for the penguins. It defines the spring/summer window when there is enough food and the right habitat for breeding.
But this seasonal window is becoming less reliable, as global warming affects the complex interaction of ocean and air currents that influence the temperature and sea-ice conditions of the Southern Ocean. If the window changes, it can have a profound effect on the ability of the birds to hatch and rear healthy young – but it will have different effects depending on the particular lifestyle of the species.
For example, Adélie penguins live on sea ice but need ice-free land to breed. They migrate thousands of miles to reach their breeding grounds then return to the sea ice to moult. They arrive in the breeding grounds in October or November, at the start of spring – any later and their journey becomes increasingly dangerous as the melting sea ice increases the distance they have to travel to their moulting locations on the solid pack ice. In contrast, gentoo penguins do not migrate as far as the Adélies, and because they are closer to their breeding areas year round they can be more flexible about when they breed.
In years of increased snow storms in the South Orkneys and the West Antarctic Peninsula, incubating birds can literally be covered up to the neck in snow. When the snow melts in summer the nests are flooded and the eggs die. If the penguins can rebuild their nests, as gentoo penguins seem able to do, they will have another chance to breed. For Adélie penguins, which are more constrained by the ice cycles, this will be a real problem and they will invariably fail.
Over our long study period, the chinstrap and Adélie populations of the South Orkneys have declined significantly, but gentoo penguins have increased in number. These trends appear to go hand in hand with warmer, more variable weather and a more variable climate.
These same factors also impact on the penguins through their effect on the birds’ main source of food – krill. These small, shrimp-like creatures sustain millions of Southern Ocean predators, from fish to whales. Many penguins depend directly on krill, or on fish species that feed on them, and if they don’t get enough they will be under nourished at breeding time and more likely to produce weak chicks that don’t survive.
A big reduction in krill could also mean the birds don’t have enough resources to be able to moult their feathers. The moult is a critical period because all penguins need to shed their old, worn-out feathers and get a new coat to survive the winter, and this usually happens right after breeding. The moult lasts for a few weeks and the birds fast while it’s happening, so they must build up fat before the moult starts. If they haven’t found enough food they’ll be in very poor condition and can literally starve during the moult – as we have seen for ourselves in years when krill is in short supply.
For the ice-obligate Adélie penguins it’s even worse, because they need to reach the hard pack ice for their moult. In bad years not only is food scarce, but the pack ice retreats farther south in the Weddell Sea, very far away from the Adélies’ breeding colonies in the South Orkneys. The penguins have to swim much farther to reach the ice in time to start moulting, and if they haven’t been able to find enough food beforehand they will be very weak when, or if, they arrive.
Studies of penguin remains – some thousands of years old – from abandoned or old penguin colonies, suggest that the birds have responded to climate change in the past by temporarily adopting new habitats, or by permanent migration. We think we’re going to see similar responses in modern penguin populations that are living at the edge of their current geographic range – and therefore at the limits of their tolerance to change.
Our work shows that some penguin species are already feeling the effects of changes to their habitats, and while some suffer others appear to be much more adaptable. With increasing variability in regional climate, particularly rapid warming, penguins may continue to live across their current geographic range if they can adapt to the new environment. So those penguin species that can find alternative food, and are flexible enough to breed in different habitats and at different times, are going to be the winners in a warmer Southern Ocean. Over the longer term they might even evolve in response to these changes.
Given their location and the types of penguins that live there, our sites will give us an excellent opportunity to test this theory.
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