Most zombie fish die within days of spawning, but some can last up to a couple of weeks. Once the salmon die in the river, they are either scavenged by other animals, or they decompose and release inorganic nutrients to the planktons in the river and the riparian vegetations in the floodplains. The Pacific salmon are a classic example of a semelparous animal, which reproduce only once in their lifetime. Semelparity is sometimes called "big bang" reproduction, since the single reproductive event of semelparous organisms is usually large and fatal to the spawners. It is an evolutionary strategy that concentrates all available resources into maximizing reproduction, at the expense of individual organism's life, which is common among insects but rare among vertebrates. All six species of Pacific salmons live for many years in the ocean before swimming to the freshwater stream of its birth, spawning, and then dying. Most Atlantic salmon also die after spawning, but about 5 to 10% (mostly female) return to the ocean where they can recover and spawn again the next season. 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 from a stream where grizzlies fish salmon have been found to contain nitrogen originating from fished salmon. 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.