The Crozet Islands were discovered by French explorers in 1772, and while the little archipelago between the southern tip of Africa and Antarctica was not inhabited by people, it had plenty of other residents, including seals, whales and, most spectacularly, penguins. In fact, the Morne du Tamaris colony on Île aux Cochons was found to be almost wall-to-wall king penguins—the largest king penguin colony anywhere and the second largest penguin colony in the world. Over the centuries, whalers hit the Crozets hard and hunters almost wiped out the island's seals, but the penguins seemed to be doing fine.
That’s not the case anymore. Agence France-Presse reports that a new study shows the colony has collapsed in the last 30 years, dropping by 85 percent from 2 million animals to just 200,000.
According to the paper, which appears in the journal Antarctic Science, the number of breeding pairs on the island has also fallen from 500,000 in 1988 to 60,000 in 2015. Getting to the extremely remote outpost is difficult, so the researchers estimated penguin numbers using images gathered from helicopter and satellite surveys of the colony conducted between 1962 and 2016.
The researchers found that after reaching their maximum concentration of 2 million penguins between 1982 and 1988, the colony has rapidly shrunk, with vegetation taking over many areas where breeding penguins once stood flipper to flipper. “It is completely unexpected, and particularly significant since this colony represented nearly one third of the king penguins in the world,” lead author Henri Weimerskirch, ecologist at the Centre for Biological Studies in Chize, France, tells AFP.
Why the penguins are in decline is mystery. According to the paper, smaller colonies of king penguins on other islands in the archipelago have remained stable. And in general, over the last half century, king penguins, which are not currently classified as endangered, have increased throughout the Southern Ocean as fish stocks and habitats recover from human exploitation. But that is not happening on Cochon.
AFP reports that the decline likely began around 1997, when a major El Niño event temporarily warmed the southern Indian Ocean, displacing the abundant fish and squid the penguins rely on. As a non-migratory species, the king penguins were stuck on their foodless island. “This resulted in population decline and poor breeding success for all the king penguin colonies in the region,” Weimerskirch says. However, unlike peguins on other islands, Cochons birds continued to decline instead of recovering.
According to a press release, there are other possibilities as well. The colony may have simply grown too large, leading to something called density-dependent effects. In that case, the population gets too large, making it harder for individuals to find food. Eventually, that fierce competition for resources leads to a rapid and drastic collapse. It’s also possible that some sort of disease, like the avian cholera which has affected seabirds on other nearby islands, is to blame. Finally, there's a chance that numbers of invasive species like cats, rats or mice have exploded, leading to the failure of the colony. Invasive mammals, which eat eggs and attack nesting birds, can have huge impacts on large seabirds which did not evolve in their presence.
However, the researchers say none of the theories alone can adequately explain the massive drop in numbers at Cochon. The only way to figure it out is to do field studies on the colony, which they hope will begin in 2019.
News of the collapse of this major colony comes just few months after a another study revealed that climate change will have an enormous impact on the non-migratory king penguins. Changes in ocean temperatures will push their preferred seafood stocks further away from their breeding colonies, which are located on a tiny handful of ice-free islands near Antarctica. According to the research, up to 70 percent of the 1.6 million king penguins will either have to find new homes or will die by the end of the century.