The remainder of paradise

How important are whales and seals for polar ecosystems?

Polar regions are among the most inhospitable on earth; however, they harbour the largest animals , albeit in the ocean. Until recently, a seemingly endless food supply in the Arctic and Antarctic appeared to explain the large stocks of whales and seals. Now there is increasing evidence that the large mammals may have survived as a consequence of the polar regions’ harshness and inaccessibility to humans, and that their distribution may have been much wider in the past. Furthermore, it is not unlikely that the disappearance of large marine mammals from temperate oceans resulted in profound changes to the whole ecosystem.

According to Professor Dr Victor Smetacek of the Alfred Wegener Institute for Polar and Marine Research and Dr Stephen Nicol from the University of Tasmania in Hobart, Australia, the ability to predict changes in polar ecosystems within the context of global climate change requires a better understanding of the ecological role of whales and seals. In their contribution now published in the scientific journal Nature, the two researchers present the hypothesis that large marine mammals have a central and stabilising role in marine ecosystems and were once widely distributed across all oceans.

In analogy to the large fish stocks in African lakes being dependent on hippos, large mammals may also control their environment in the coldest oceans on earth. It is already clear that the notion of short food chains with few organisms represents an oversimplified view of polar ecosystems. in the productivity of the Arctic and Antarctic is comparable to temperate oceans. However, large differences exist between the Arctic and Antarctic with regard to nutrient availability and key species in the food chain. In the Antarctic, availability of iron is the primary growth-limiting factor in the system. Krill, a shrimp-like crustacean occurring in enormous amounts, is the major food source for larger animals. In the Arctic, the krill’s ecological role is played by fish, and productivity in Arctic oceans is, more frequently, limited by the availability of nutrients. The significance of large mammals in the coldest oceans on earth, however, remains largely unresolved. Similarly, the effect of their feeding activity and their excretory products on the stability of the ecosystem continues to be unknown.

In comparison, the structuring role of large mammals on land ecosystems is better understood. Prior to the expansion of modern man, large herbivores, similar to those surviving in east Africa, were distributed across the entire northern continents. With the extinction of mammoth and nearly all large terrestrial animals in Europe, Asia and America, the landscapes changed as well. Today, a programme for the re-establishment of large herbivores in Siberia aims to restore the original mammoth steppe; similar suggestions have been made by US-researchers for North America.

With the invention of boats, the terrestrial animal man extended wholesale annihilation of large animals to the oceans as well. The extinction of the European Grey Whale and of the Steller’s Sea Cow in the North Pacific represent recent examples of the dramatic decline of almost all other large animals in the oceans of temperate latitudes. In regions of the earth inaccessible to humans, the giants of the animal kingdom were able to persist longest. Hence, the Arctic and Antarctic – marine “Serengetis” - have sustained largely intact ecosystems and remnants of former diversity. Above all, these are places where the investigation of interactions between large marine mammals and their environment is still possible - and necessary - today.

The global rise in temperatures now threatens these polar refuges. These changes are likely to differ between Arctic and Antarctic because of differences in the two regions’ geography and ecosystem function. In order to implement meaningful protective measures for the conservation of these regions, a better understanding of the ecological significance of large animals in the oceans is required.

Bremerhaven, September 15th, 2005

Reference: Nature 437, 362-368 (15 September 2005)