Feed the Tree: Salmon’s Journey Through the Food Web
A salmon has an amazing life. It is hatched from an egg, one of 3,000 his mother has laid in a freshwater stream laden with oxygen. She stays there for anywhere from six months to three years. In general, the diet of a young salmon consists of insects, minnows and plankton. Then she begins a migration to the ocean, which can be a very long journey. During this trip she spends some time in brackish water where her body chemistry changes to permit the migration from freshwater to saltwater.
A salmon can spend from one to six years in the ocean, feeding on smaller fish, squid, eels and shrimp. After becoming sexually mature she returns, making the journey along with thousands of other salmon to the place of her birth to mate, lay eggs and die, exhausted.
The journey of life
Salmon make amazing journeys, sometimes swimming hundreds of miles upstream against strong currents and rapids, slipping between rocks and leaping over falls, just to make it back home to reproduce. For example the chinook and sockeye salmon from central Idaho travel over 1,400 km (900 mi) and climb nearly 2,100 m (7,000 ft) from the Pacific Ocean as they return to spawn. The journey is not only a grueling physical challenge, but animals such as bear and fox, seeing hundreds and hundreds of fresh fish leaping out of the waters, wait patiently on the river rocks to simply reach out and capture their dinner. Large birds also sweep down for the feast.
How do salmon bring nutrients to trees?
When salmon die, exhausted from mating after the last leg of their arduous journey when they quit eating, their carcasses line the river beds. And carrion birds eat what does not decompose into the earth.
Other birds and animals that fish along the stream for the salmon likewise play a part in bringing the ocean-rich diet into the woodlands. They too will deposit their feces to decompose into the earth. Or they may in turn become the prey for another predator who will reap the benefit of consuming the ocean nutrients and then passing on what is not digested and used in his feces.
In Alaska, grizzly bears are well known for their fishing abilities as they leave the evidence quite visible. They capture the salmon and carry them into the nearby woods. There they deposit their nutrient-rich feces. Bears also leave piles of partially eaten carcasses of the salmon. Photographs have shown salmon carcasses reaching 4,000 kilograms per hectare of forest floor. These carcasses decompose into the earth and the results are staggering. It is estimated that this transfer of nutrients from ocean to river to woodlands provides 24% of the total nitrogen available to these woodlands.
Aided by worms, bacteria, fungi and other microorganisms, the carcasses decompose into the earth. Their nutrients are then taken up along with groundwater by the trees. The foliage of spruce trees up to 500 meters (1600 ft) from a stream where grizzlies fish have been found to contain nitrogen from the salmon.
How does a food web function?
A food web is the natural interconnection of food chains. At the bottom of the web are the autotrophs or producers, which are life forms like algae, plants and trees that grow, develop and reproduce by producing organic matter from inorganic substances like minerals and gases such as carbon dioxide present in the surrounding area. The energy required for the chemical reactions to convert the energy into a usable form comes from the sun and photosynthesis in most species, though there are underwater plants that get their energy from hydrothermal vents and hot springs.
Above the autotrophs are the heterotrophs. Heterotrophs are not able to fix carbon from inorganic sources such as carbon dioxide and must get their nutrition from organic carbon. Ninety-five percent or more of all types of living organisms are heterotrophic, including all animals and fungi and most bacteria.
Both the salmon and the grizzly bear are autotrophs. The salmon has consumed nutrient-rich plankton and fish from both rivers and the ocean, thus bringing a rich dinner to the grizzly, the birds and other animals and subsequently to the earth again. It is then taken up into the tree by its roots.
Salmon has fed the tree.