The Black Bear and the Salmon: Mighty Ecosystem Engineers

Source: Food and Agriculture Organization of the United Nations, 2013, Christine Gibb, CBD and FAO, "The Youth Guide to Biodiversity" 1st Edition (Chapter 1) Youth and United Nations Global Alliance. Reproduced with permission.

Chapter 1. Verbatim

Nutrients such as carbon, nitrogen and phosphorus generally flow downstream - from land to rivers then out to sea. But not always. In riparian forests (forests next to a body of water like a river, lake or marsh) in British Columbia, Canada, black bears help to transfer nutrients from the ocean back to the forest!

To understand how this nutrient transfer works, we need to know a little about the life cycle of Pacific salmon. Pacific salmon are born in freshwater streams, where they feed and grow for several weeks. Once they’re ready, they swim downstream and undergo physiological changes that allow them to survive in marine conditions. 

The salmon spend up to several years in the ocean eating lots of crustaceans, fish and other marine animals (i.e. acquiring lots of nutrients from the ocean). Once they reach sexual maturity, the salmon leave the ocean and swim back to the exact freshwater stream where they were born. There, they spawn and die. 

During the annual salmon run, black bears catch spawning salmon and carry them into the woods to eat. The nutrient transfer is significant. Each salmon offers two to 20 kg (sometimes even 50 kg) of essential nutrients and energy. One study in Gwaii Haanas, Canada found that each bear took 1 600 kg of salmon into the forest, eating about half. Scavengers and insects dined on the remains. The decaying salmon also released nutrients into the soil, feeding forest plants, trees and soil organisms.  

In this way, vital nutrients are transferred from one ecosystem to another first by the salmon, then by the black bear.

Sources: ring.uvic.ca/99jan22/bears.html and www.sciencecases.org/salmon_forest/case.asp