Sunday, August 12, 2007

Penguin Interlude Part II

A paper in the Proceedings of the National Academy of Sciences by Steve Emslie and William Patterson recently prompted a New York Times article and mention in the Querencia blog.
This paper discusses changes in Adelie Penguin diets over the last 38000 years by examining eggshell remains found in detritus of historic and present Adelie Penguin colonies. The stable isotope values of carbon and nitrogen in over 270 eggshells suggested that there was an abrupt shift in the diet of Adelie Penguins throughout their range from fish to krill about 200 years ago. The change in diet coincides with the discovery of the Antarctic Peninsula and subsequent exploitation of the Antarctic Fur Seals in the area.
I will provide a bit of the back-story on this paper. Sealing began in the sub-Antarctic islands in 1784 and on the Antarctic peninsula in 1819 although sealers were probably working the peninsula before that but not giving away the location of their hunting grounds (you thought it was an untouched, pristine continent didn't you - just wait). During the 1820-21 austral summer 55 to 60 sealing ships were in the South Shetland Islands and they took an estimated 1/4 million seals in 3 months. Millions of seals had already been killed in the Sub-Antarctic islands and by 1825 the seals were gone and the Antarctic Fur Seal was believed extinct.
Antarctic Fur Seal on whaler's cross. South Georgia.

Whalers came next but the harvest of whales and whale-like seals was limited to the slower more accessible species such as the Southern Right Whale and Elephant Seals. By the early 1900's however, with the advent of the explosive harpoon and faster ships, the rorqual whales which were previously too fast to catch became the preferred target. Shore stations for processing whales were established in 1904 on South Georgia and by 1910 about 12,000 whales were harvested. In 1926 the first floating factory ship began working in the Antarctic waters. Nearly 30,000 Blue Whales alone were killed during the 1930-31 season. Blue Whales, Fin Whales, Sei Whales and Minke Whales were harvested in turn and in 1940, 40,000 whales were killed. (See here for more history and here and here for more historical whaling information).

The above graph shows the numbers and species of whales taken in Antarctica from 1904-05 to 1980-81. See here.

In 1985, Richard M. Laws, then the head of the British Antarctic Survey, published a paper in American Scientist entitled "The Ecology of the Southern Ocean". In his paper he estimated that whales, a pre-harvest biomass of nearly 45 million tons, had probably consumed 190 million tons of krill in addition to 14 million tons of squid and 5 million tons of fish each year. He further estimated that my 1973 the whale biomass was about 9 million tons and they consumed 43 million tons of krill. This left a "surplus" of 150 million tons of krill annually (italics are mine). He surmised that this surplus had a profound affect on the ecology of Antarctic animals and in particular had increased populations of Adelie, Chinstrap and Macaroni Penguins. In addition, during this time the Antarctic Fur Seal reappeared from the mist of extinction and prospered in the absence of the large whales. They probably now exceed their pre-exploitation population. His premise certainly seemed logical.

However, in 1992, a host of Antarctic researchers then affiliated with the Point Reyes Bird Observatory, published an article in Polar Biology suggesting that the krill surplus theory was not responsible for changes in penguin populations at all. The title of their paper, "Increases in Antarctic penguin populations: reduced competition with whales or a loss of sea ice due to environmental change" suggested it was an either/or proposition and that they believed that much of the observed changes in penguin populations were not due to reduced competition for food but were instead a result of changes in the amount and duration of Antarctic sea ice. They provided very compelling evidence that the amount and duration of sea ice had more influence on the relative populations of Chinstrap and Adelie Penguins than the surplus of krill.

So where does this recent evidence of a shift in Adelie Penguin diet from fish to krill during the time of heavy exploitation of krill eating animals leave us?

I began my Antarctic penguin studies during the time of the sea ice theory and in fact I had two of the authors as advisers at one time or another. But I was never comfortable ruling out the impact of the removal of that many whales. I just couldn't believe that the loss of that much krill-eating biomass didn't have some affect on the ecology of the animals I was studying. In a sense, Emslie and Patterson's work vindicates my uneasiness. They noted that the switch from fish to krill coincided with the presumed krill surplus and supports the krill surplus theory. They also noted the proponderance of krill in the modern diet of Adelie Penguins is relatively recent phenomenon.

Although this work does support a switch in diet from fish to krill and suggests that the krill surplus may be responsible for this shift, it does not address the fundamental premise in either the krill surplus or sea ice theories for changes in penguin population- that there was a change in penguin populations as a result of the shift from fish to krill.

I believe that all of these factors have impacted penguin populations in one way or another. The physical environment and changes in sea ice extent do have an impact on penguin populations through the availablity of krill to both adult and recently fledged penguins and they affect species differently depending on their dependence on the sea ice environment. I also believe that there was a profound affect on penguins and other Antarctic animals as a result of the loss of the Antarctic Fur Seals and great whales. These factors have interacted and continue to interact in subtle and dramatic ways to influence the distribution and abundance of Antarctic and Sub-Antarctic penguins today.

Steven D. Emslie and William P. Patterson. 2007. Abrupt recent shift in δ13C and δ15N values in Adélie penguin eggshell in Antarctica. Proceedings of the National Academy of Sciences 104(28)pp 11666-11669.

William R. Fraser, Wayne Z. Trivelpiece, David G. Ainley, and Susan G. Trivelpiece. 1991. Increases in Antarctic penguin populations: reduced competition with whales or a loss of sea ice due to environmental warming? Polar Biology 11:525-531

Adelie Penguin colonies from the air. Note the nice pink color of the colonies. Photo by Ron Naveen (Oceanites).

This is where the color comes from - lots and lots of penguins in one spot all digesting large amounts of krill and depositing the remains.

Over many years this forms a layer of ornithogenic soil (old bird shit). Within this layer are old egg shells and concentrated organic pollutants - see here.

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