INSIGHT REVIEW NATURE|Vol 437|15 September 2005|doi:10.1038/nature041613621Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany; 2Australian Antarctic Division, Department of the Environment and Heritage,Channel Highway, Kingston, Tasmania 7050, Australia; 3Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Hobart, Australia.Polar ocean ecosystems in a changing worldVictor Smetacek1and Stephen Nicol2,3Polar organisms have adapted their seasonal cycles to the dynamic interface between ice and water. Thisinterface ranges from the micrometre-sized brine channels within sea ice to the planetary-scale advance andretreat of sea ice. Polar marine ecosystems are particularly sensitive to climate change because smalltemperature differences can have large effects on the extent and thickness of sea ice. Little is known aboutthe interactions between large, long-lived organisms and their planktonic food supply. Disentangling theeffects of human exploitation of upper trophic levels from basin-wide, decade-scale climate cycles to identifylong-term, global trends is a daunting challenge facing polar bio-oceanography.elling the effects of bottom-up and top-down forcing on pelagic ecosys-tems is an immediate task facing polar bio-oceanographers.Polar marine predatorsIt is widely assumed that marine ecosystems are controlled by bottom-up processes: the supply of nutrients by physical forcing and their con-version to organic matter by phytoplankton photosynthesis. Highertrophic levels simply harvest the levels below them. However, there isno simple relationship between the magnitude of primary productionand the biomass and composition of higher trophic levels. That ecosys-tem interactions are more complex is demonstrated by the marked,decade-scale fluctuations in upper-level predator populations of allhigh-latitude systems that seem to be linked to corresponding changesin local climate and hydrography. Because the effects of climate changeare difficult to disentangle from those of human exploitation, andbecause the sea-ice environment can directly affect both productivityand the population dynamics of birds and mammals, the mechanismsby which environmental changes are translated up the polar food webhave remained elusive. An example from terrestrial ecology is instruc-tive in this context.The mammalian fauna of northern Eurasia and America, until theend of the last ice age, was morphologically and functionally similar tothe extant megafauna of Africa and India. Herds of mammoth, deer,horses and bison, preyed upon by lions and wolves, roamed a land-scape covered with a vegetation called mammoth steppe that becameextinct together with the megafauna. The megafauna is likely to haveconditioned the vegetation at the landscape level by promoting thegrowth of grasses on which they depended6,7. It seems increasinglylikely that human exploitation rather than climate change was respon-sible for the global demise of terrestrial megafauna8and consequentlyits characteristic vegetation6. It seems that the wave of extinction pro-ceeded from temperate to Arctic regions. Dwarf mammoths survivedon Wrangel Island in the high Arctic and were hunted by humans tillonly 5,500 years ago9.This insight from terrestrial ecology can assist our understandingof marine ecosystems and the effect of past and ongoing human depre-dation10. First, the wave of megafaunal extinction associated withhuman expansion in Europe will have also affected the accessiblePolar marine ecosystems are located at the ends of planetary gradientsin daily and annual solar radiation and are ice-covered for varyinglengths of the year. They harbour, or have until recently, large stocks ofconspicuous animal life — birds, seals and whales — which led to theconclusion that polar ecosystems channel a greater proportion of pri-mary production to warm-blooded predators than those at lower lat-itudes1. This early view was explained by short, low-diversity foodchains in polar regions, evoking images of simple systems dominatedby a few key organisms. Research conducted over the past two decades has shown that theconcept of short, low-diversity polar food chains is overly simplistic.Although comparatively few species do provide the bulk of food to polarmarine predators, the planktonic base of their food supply is equivalentin phylogenetic diversity to the planktonic base in temperate climatezones2,3, implying that the structure and functioning of pelagic (open-water) food webs are broadly similar across all latitudes. But the key preyorganisms for vertebrates vary between polar ecosystems: benthos andfish are the main prey in the north whereas crustaceans are in the south.This indicates that cold adaptation has not favoured a specific foodchain. If air-breathing predators play a greater role in polar-ecosystemfunctioning than they do in lower latitudes, then a decrease in theirstocks should have cascading effects down the food chain and lead tomarked shifts in ecosystem structure. The evidence for such cascadingeffects is controversial, however, because of both the absence of baselinesagainst which to assess change4and our poor understanding of the car-rying capacity of pelagic food webs for higher trophic levels.Are seasonally ice-covered pelagic ecosystems fundamentally dif-ferent from their counterparts in adjacent ice-free waters and how willthey be affected by the retreat of sea ice in a warming world? Clearlythe organisms that live in the sea ice or are dependent on it to completetheir life cycles will be most severely affected, but some organisms mayactually benefit from sea-ice retreat and overall productivity mightactually increase5.In this review we examine the possible effects of a warming world onpolar ecosystems and consider only the seas and oceans directly influ-enced by sea ice and its melting. Because human-mediated change hasinfluenced polar ecosystems at both ends — thinning and retreat of theice cover and heavy exploitation of top predator populations — unrav-05 Smetacek 28-34 NEW 6/9/05 11:23 AM Page 28Nature PublishingGroup© 2005© 2005 Nature Publishing GroupNATURE|Vol 437|15 September 2005 INSIGHT REVIEW363from the land to the sea4. The polar regions may be the last refuge forthe marine megafauna — the Serengetis of the sea — implying thattheir teeming animal life is but a reminder of what the temperateoceans were like before human