Cutting off rivers to salmon changes everything for a songbird

An adult American Dipper gathers aquatic invertebrate to feed its’ nearby nest on Barnes Creek, Washington, USA. Photo: Christopher Tonra

By Christopher Tonra

Salmon have enormous impacts on the ecosystems they inhabit, particularly riverine systems. Their migrations from ocean to river are an amazing spectacle to behold, and they do something incredibly important for the river and beyond: they fertilize them. Salmon grow >95% of their biomass in the ocean, biomass that is filled with rich marine nutrients, such as nitrogen.  When they spawn and die in rivers, they contribute their own bodies and these nutrients, known as “marine derived nutrients” or “MDN”, in these often nutrient starved systems.  In this way they feed their own offspring, and the entire freshwater food web. This contribution to the ecosystem can even penetrate into the terrestrial biome. We sought to determine what the cost is of salmon being cut off from a river to a species at the aquatic-terrestrial interface, as they are by the thousands of dams across their range. We examined the breeding ecology and life history variation of the American Dipper (Cinclus mexicanus) in areas with and without salmon, either due to dams or natural barriers, on the Olympic Peninsula, Washington State, USA.

The removal of the Elwha Dam (pictured) opened up the middle section of the Elwha River to salmon for the first time in a century. American dippers almost immediately benefited by experiencing an increase in uptake of marine nutrients brought in by salmon. Photo: Olympic National Park.

Dippers in areas without salmon effectively represent a different population with an alternate life history strategy than those in areas with salmon, even within the same river. Dippers with salmon are year-round residents who often attempt multiple broods within a season, the biggest driver of dipper lifetime reproductive success. Dippers without salmon leave their breeding ranges in the fall/winter and rarely attempt a second brood. Annual survival is 11% higher in areas with salmon. The effects of salmon can be seen directly on individuals, especially females. Female dippers with salmon are in much better physical condition, and produce much larger female offspring. MDN, in addition to the added resource of juvenile salmon to dippers, appears to be responsible for these patterns, as stable isotope analysis showed that dippers in areas with salmon acquire almost 10% of their nitrogen and over 20% of their carbon from marine sources.

An adult American dipper is fitted with colored leg bands on the Sol Duc River, Washington USA. Bands allow for individual identification so that researchers can quantify breeding effort, seasonal movements and annual survival. Dippers that bred in areas open to salmon survived longer, were more likely to be year-round residents, and were more likely to attempt multiple broods than their counterparts behind salmon barriers. Photo: Christopher Tonra.

Our results demonstrate that the effects of barriers to migratory fish can reverberate beyond aquatic systems to impact life history and population dynamics of terrestrial consumers, at least those that utilize aquatic food resources. We hope that these findings further demonstrate the ecological cost of man-made dams, and the potential benefits of dam removal.  In a related study, we showed removal of the Elwha Dam resulted in an almost immediate increase in MDN in dippers.  While many dams are too important to human society to remove currently, many more have moved beyond their usefulness, and are old and unsafe. As we demonstrated here, removing them and restoring connections between biomes can have an enormous benefit to populations of river dependent species.

Fledgling American dipper await a food delivery from their parents on the Sol Duc River, Washington, USA.  Dippers fledge larger female offspring when breeding in areas open to salmon, compared to those behind salmon barriers. Photo: Christopher Tonra.

Press release here.

Videos: Two American dippers display the amazing locations they sometimes choose for their nests. Their nests are domes constructed of living moss. Videos by Christopher Tonra.

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