Species → Brown Bear
Brown Bear
Ursus arctos
What if it disappeared?
Impact ModerateWithout bears to catch salmon and drag carcasses away from the water, the riparian forest would lose much of the 37.2 kg of marine-derived nitrogen an adult female bear redistributes there on average each year — 96% excreted directly through urine, well away from the bank. Nearby spruce foliar nitrogen already measurably decreases with distance from salmon streams, direct proof of this transport.
DocumentedDetail
Hilderbrand, G.V., Hanley, T.A., Robbins, C.T. & Schwartz, C.C. (1999). Role of brown bears (Ursus arctos) in the flow of marine nitrogen into a terrestrial ecosystem. Oecologia, 121, 546–550.
The net input of marine nitrogen to the riparian forest rises markedly when salmon and bears are present together, but not with either alone: the bear isn't merely a witness to salmon abundance, it's the vector carrying it inland. Helfield & Naiman (2006, Ecosystems, 9, 167–180) estimate this salmon-bear interaction can supply up to 24% of the riparian nitrogen budget.
Depriving a salmon stream of its bears wouldn't stop marine nitrogen from entering the ecosystem, but it would concentrate it near the bank rather than dispersing it deep into the riparian forest — an effect on the spatial extent of forest fertilization rather than on its existence.
ExtrapolatedDetail
Extrapolation à partir du modèle de bilan de masse de Hilderbrand et al. 1999 et de Helfield & Naiman 2006 : aucune de ces études ne compare directement un cours d'eau à saumons avec et sans ours, faute d'un tel site naturel.
Both studies measure what bears actually transport, but neither has a bear-free control site to isolate their specific effect from that of other vectors (wolves, eagles, human harvest). The inference — narrower, more concentrated fertilization near the stream in the bears' absence — follows logically from the documented mechanism, without having been tested as such in the field.
What's missing to confirm it: A direct comparison between salmon streams with and without bears, controlling for other marine-nitrogen transport vectors, would confirm the true scale of the bear-specific loss rather than inferring it from a mass-balance model.
Documentation
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Hilderbrand et al. (1999) — Role of brown bears (Ursus arctos) in the flow of marine nitrogen into a terrestrial ecosystem, Oecologia 121
Accessed August 7, 2026
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Helfield & Naiman (2006) — Keystone Interactions: Salmon and Bear in Riparian Forests of Alaska, Ecosystems 9
Accessed August 7, 2026
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IUCN Red List — Brown Bear (Ursus arctos), statut et estimation mondiale
Accessed August 7, 2026
Last verified: August 7, 2026