Ecological Responses to Climate Change on the Antarctic Peninsula
The peninsula is an icy world that's warming faster than anywhere else on Earth, threatening a rich but delicate biological community
Ice, Krill and Penguins
Much remains to be understood about the food web that culminates with penguins and other avian and mammalian predators. Some two decades ago, the prevailing model for the dynamics of these populations was the "krill surplus hypothesis." According to this view, krill populations had long been held in check by baleen whales, but when those whales were nearly exterminated in the 20th century, krill were released from predation; the ensuing surplus led to significant population growth in other krill-dependent predators. This explanation no longer seems fully adequate. Although krill are indeed a key component of the food web and critical to the diets of many top predators, the surplus hypothesis cannot explain the population trends in brush-tailed penguins, for example. Because all three species have diets dominated by krill, a surplus would be expected to produce similar trends in all of them.
An analogous situation has been observed among seals. Like Adélie penguins, Weddell seals are ice-dependent, and their populations have plummeted, while the populations of ice-avoiding fur and elephant seals have increased significantly. Of the three species, only fur seals are krill-dependent. An obvious inference is that sea ice, rather than diet, is the dominant factor governing the animals' response to climate change.
In the years since the krill-surplus hypothesis was first put forward and then challenged by the ice-reduction hypothesis, we have learned that krill themselves are an ice-dependent species. Without sea ice, krill do not reproduce successfully because the larvae need to graze on phytoplankton on the underside of the ice to survive winter. Along the northern half of the western Antarctic Peninsula, and in much of the Atlantic sector of the Southern Ocean, there is no krill surplus; on the contrary, this area has experienced an 80 percent decrease in krill abundance over the past 30 years, attributed to loss of winter sea ice.
Theories of top-predator population dynamics are necessarily shifting to take into account changes in both krill abundance and winter ice cover. Sea ice is increasingly regarded as the variable that mediates food-web interactions. The extent of the ice has a direct impact on krill reproductive success and abundance. Then the presence or absence of ice determines which predators are best able to reach the prey populations. This double effect of sea ice may explain why some krill-dependent but ice-avoiding predators, such as gentoo penguins, have continued to increase even as krill abundance has decreased.
Our collective observations, spanning microbiota to megafauna and encompassing the many complex relationships between taxa, paint a picture of the Antarctic Peninsula as an environment undergoing unprecedented ecological shifts in response to climate change. We would not claim to fully understand all the mechanisms driving the patterns we have observed; we are acutely aware that much more research is needed. But we have learned enough to conclude that this unique ecosystem is in peril. We sound an urgent call to mitigate all the factors under human control that are contributing to global climate change.
- Amsler, C. D., K. B. Iken, J. B. McClintock, M. O. Amsler, K. J. Peters, J. M. Hubbard, F. B. Furrow and B. J. Baker. 2005. A comprehensive evaluation of the palatability and chemical defenses of subtidal macroalgae from the Antarctic Peninsula. Marine Ecology Progress Series 294:141–159.
- Aronson, R. B., S. Thatje, A. Clarke, L. S. Peck, D. B. Blake, C. D. Wilga and B. A. Seibel. 2007. Climate change and invasability of the Antarctic benthos. Annual Review of Ecology, Evolution, and Systematics 38:129–154.
- Atkinson, A., V. Siegel, E. A. Pakhomov and P. Rothery. 2004. Long-term decline in krill stock and increase in salps within the Southern Ocean. Nature 432:100–103.
- Clarke A., E. J. Murphy, M. P. Meredith, J. C. King, L. S. Peck, D. K. A. Barnes and R. C. Smith. 2007. Climate change and the marine ecosystem of the western Antarctic Peninsula. Philosophical Transactions of the Royal Society B 362:149–166.
- Ducklow, H. W., K. Baker, D. G. Martinson, L. B. Quetin, R. M. Ross, R. C. Smith, S. E. Stammerjohn, M. Vernet and W. Fraser. 2007. Marine pelagic ecosystems: the West Antarctic Peninsula. Philosophical Transactions of the Royal Society B 362:67–94.
- Fraser, W. R., W. Z. Trivelpiece, D. G. Ainley, and S. G. Trivelpiece. 1992. Increases in Antarctic penguin populations: Reduced competition with whales or a loss of sea ice due to environmental warming? Polar Biology 11:525–531.
- Meredith, M. P., and J. C. King. 2005. Rapid climate change in the ocean west of the Antarctic Peninsula during the second half of the 20th century. Geophysical Research Letters 32:L19604.
- Orr, J. C., et al. 2005. Anthropogenic ocean acidification and its impact on calcifying organisms. Nature 437:681–686.
- Patterson, D. L., A. Easter-Pilcher and W. R. Fraser. 2003. The effects of human activity and environmental variability on long-term changes in Adélie penguin populations at Palmer Station, Antarctica. In Antarctic Biology in a Global Context , eds. A. H. L. Huiskes, W. W. C. Gieskes, J. Rozema, R. M. L. Schorno, S. M. van der Vies and W. J. Wolff, 301–307. Leiden: Backhuys Publishers.
- Pearse, J. S., J. B. McClintock and I. Bosch. 1990. Reproduction of Antarctic benthic marine invertebrates: Tempos, modes and timing. American Zoologist 31:65–80.
- Peck, L. S. 2005. Prospects for surviving climate change in Antarctic aquatic species. Frontiers in Zoology 2:9.
- Siniff, D. B., R. A. Garrott, J. J. Rotella, W. R. Fraser and D. G. Ainley. 2008. Projecting the effects of environmental change on Antarctic seals. Antarctic Science (in press).
- Thatje, S., K. Anger, J. A. Calcagno, G. A. Lovrich, H. O. Pörtner and W. E. Arntz. 2005. Challenging the cold: Crabs reconquer the Antarctic. Ecology 86:619–625.
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