What If Gone

Species → Pink salmon

Pink salmon

Oncorhynchus gorbuscha

About 650 million individuals in peak odd years, across the North…
Extinction risk Least Concern
Nutrient cyclingMarine environmentWaterwayAnadromous fish
Pink salmon (Oncorhynchus gorbuscha)
Photo: Timothy Knepp, U.S. Fish and Wildlife Service, Wikimedia Commons (Public domain)

What if it disappeared?

Impact High
1 year

Without salmon to carry marine-derived nitrogen into the rivers where they spawn and then die, leaves of trees near spawning streams would lose, from the very first season without them, up to 22 to 24% of the nitrogen that today comes directly from them.

Documented
Detail

Helfield, J.M. et Naiman, R.J. (2001). Effects of Salmon-Derived Nitrogen on Riparian Forest Growth and Implications for Stream Productivity. Ecology, 82(9), 2403–2409.

Using isotopic analysis, the authors trace the marine-derived nitrogen that salmon carry back into the streams where they were born and then die after spawning. Sitka spruce trees near salmon-bearing streams grow significantly faster than those near comparable streams without salmon.

10 years

Over a decade without this yearly influx, the growth gap measured today between spruce near salmon streams and those near comparable salmon-free streams would gradually close — all the faster because bears, with no more carcasses to drag away from the water, would stop fertilizing the wide band of riparian forest they currently reach by feeding under cover.

Documented
Detail

Helfield, J.M. et Naiman, R.J. (2001). Effects of Salmon-Derived Nitrogen on Riparian Forest Growth and Implications for Stream Productivity. Ecology, 82(9), 2403–2409 ; Helfield, J.M. et Naiman, R.J. (2006). Keystone Interactions: Salmon and Bear in Riparian Forests of Alaska. Ecosystems, 9(2), 167–180.

Sitka spruce near salmon-frequented streams grow significantly faster than those near comparable salmon-free streams (Helfield & Naiman 2001) — a gap that builds season after season. Bears amplify this effect by dragging carcasses away from the water to eat them under cover, fertilizing a much wider band of forest than dead salmon alone would reach in the water: nitrogen input is markedly higher where bears and salmon are present together than where either species is absent (Helfield & Naiman 2006).

100 years

Over the lifespan of a forest stand — a century rather than a decade — this sustained nitrogen input could durably alter the composition and structure of the riparian forest itself, not just the growth rate of a few trees: a plausible link, but one never tracked as a complete forest-scale phenomenon over such a long timeframe.

Extrapolated
Detail

Extrapolation à partir de Helfield & Naiman (2001, 2006) : le raisonnement combine deux effets mesurés séparément à l'échelle de quelques années à quelques décennies, sans étude de suivi forestier sur l'échelle de vie complète d'un peuplement (environ un siècle).

If the extra growth and fertilization measured over a few years to a few decades persists over the full lifespan of a forest stand, it could change the composition and structure of the riparian forest itself — but no study has tracked a forest over such a long horizon to confirm this.

What's missing to confirm it: No one has tracked a riparian forest over the full lifespan of a stand to confirm that the extra growth measured in the short and medium term translates into a lasting structural change to the forest itself.

1000 years

Over a thousand years, this riparian forest would likely have had time to adjust to the absence of that annual marine-nitrogen input: tree species less dependent on that seasonal fertilizer would probably come to dominate over the ones that benefited most from it, and even a return of the salmon would not recreate exactly the same forest, since the composition that took hold in the meantime would have its own momentum.

Hypothesis

Documentation

Last verified: August 16, 2026

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