Salmon Run - Keystone Species

Keystone Species

In the Pacific Northwest and Alaska, salmon is a keystone species, supporting wildlife from birds to bears and otters. The bodies of salmon represent a transfer of nutrients from the ocean, rich in nitrogen, sulfur, carbon and phosphorus, to the forest ecosystem.

Grizzly bears function as ecosystem engineers, capturing salmon and carrying them into adjacent wooded areas. There they deposit nutrient-rich urine and faeces and partially eaten carcasses. It has been estimated that bears leave up to half the salmon they harvest on the forest floor, in densities that can reach 4,000 kilograms per hectare, providing as much as 24% of the total nitrogen available to the riparian woodlands. The foliage of spruce trees up to 500 m (1,600 ft) from a stream where grizzlies fish salmon have been found to contain nitrogen originating from fished salmon.

Salmon continue to surprise us, showing us new ways in which their oceanic migrations eventually permeate entire terrestrial ecosystems. In terms of providing food and nutrients to a whole food web, we like to think of them as North America's answer to the Serengeti's wildebeest.

Wolves normally hunt for deer. However, a 2008 study shows that, when the salmon run starts, the wolves choose to fish for salmon, even if plenty of deer are still available. "Selecting benign prey such as salmon makes sense from a safety point of view. While hunting deer, wolves commonly incur serious and often fatal injuries. In addition to safety benefits we determined that salmon also provides enhanced nutrition in terms of fat and energy."

The upper reaches of the Chilkat River in Alaska has particularly good spawning grounds. Each year these attract a run of up to half a million chum salmon. As the salmon run up the river, bald eagles arrive in their thousands to feast at the spawning grounds. This results in some of the world's largest congregations of bald eagles. The number of participating eagles is directly correlated with the number of spawning salmon.

Residual nutrients from salmon can also accumulate downstream in estuaries. A 2010 study showed the density and diversity of many estuarine breeding birds in the summer "were strongly predicted by salmon biomass in the autumn." Anadromous salmon provide nutrients to these "diverse assemblages ... ecologically comparable to the migrating herds of wildebeest in the Serengeti.