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Trophic Ecology

Salmon may fuel a sky full of songbirds

Journal source: Wagner MA, Reynolds JD (2019) Salmon increase forest bird abundance and diversity. PLoS ONE 14(2): e0210031. https://doi.org/10.1371/journal.pone.0210031.


The ocean provides abundant resources—just a glimpse into its open waters on TV or in person rewards the viewer with scenes of abundant fish, different types of invertebrates, and the tiniest microbes and plants that form the foundation of marine food webs.

These various resources, while of oceanic origin, can be transported to far away ecosystems, such as streams and adjacent forested areas, where they can have profound impacts on the abundance and diversity of animals living there. Such linkages between ocean and land can create hotspots of biodiversity. These hotspots are described by the number of different organisms living together in a habitat. This is an important component of a healthy ecosystem: many organisms can carry out similar functions, so that if one species becomes less abundant, another one can jump in and keep the ecosystem going.

How can resources in the ocean move and be used by organisms of the land? Meet the Pacific Salmon (Oncorynchus spp.), a fish that has a very mobile life history.

Pacific Salmon have bright red coloration during their spawning period in coastal streams. Photo credit: R. Tabor, USFWS, via Wikimedia Commons.

Pacific salmon are born in coastal streams, but they don’t stay there for long; they travel to the sea to feed and mature. When they reach reproductive age, they return to spawn in the streams where they were born, and the cycle begins again.

Salmon swimming upstream to spawn. Photo credit: National Park Service via Wikimedia Commons.

A young salmon, also called an alevin when it first hatches, spends some time in the stream where it was born before heading out to the ocean to feed and grow. Photo credit: Wikipedia via Wikimedia Commons.

After spawning, the salmon soon die in the stream, but in this process, they leave behind nutrients from sea that can enhance productivity of the stream and nearby terrestrial habitats. For example, hungry bears, wolves, and other top consumers eat the salmon carcasses, and flooding can carry the salmon onto land, where they decompose and release nutrients that can support plant growth.

Increasing the number and types of plants can attract a variety of songbirds to these stream-woodland intersections. While research has shown that bears and wolves obtain much of their energy by eating salmon, little is known about the indirect ways in which salmon influence the surrounding ecosystem.

Bear hunting salmon in a coastal stream. Photo credit: National Park Service via Wikimedia Commons.

To address this knowledge gap, Wagner and Reynolds, researchers at Simon Fraser University in British Columbia, Canada, explored how salmon-derived nutrients affect birds in forests adjacent to streams in British Columbia. They conducted their study in the Heiltsuk First Nation along the Central Coast of British Columbia, Canada. They surveyed birds in 14 watersheds near coastal streams that supported a wide range of salmon densities. In each stream, they counted the number of salmon that were alive and dead before, during, and after peak spawning periods. The number of salmon counted at each stream was used to calculate biomass and density. They also measured the size and density of different trees and plants species growing near the streams to see if salmon had an effect on plant communities.

Zoomed-in map of the 14 streams surveyed in this study. Photo credit: Figure 1 from Wagner and Reynolds, used under Creative Commons License.

Map of the general study area of the Heiltsuk First Nation in British Columbia. Photo credit: Wikipedia via Wikimedia Commons.












What they found

Wagner and Reynolds found that streams with higher salmon abundance had a greater abundance of several species of birds. In fact, they found that salmon had a stronger influence on bird abundance and diversity than the abundance of different plants and the physical characteristics of the streams did.

These figures show the influence of A) salmon abundance , B) conifer tree abundance, and C) watershed size on bird abundance. Photo credit: Figure 2 from Wagner and Reynolds 2019 via Creative Commons License.

How can salmon have such a profound influence on the birds? When salmon carcasses are left behind after the spawning period, they begin to decompose. This releases nutrients such as nitrogen into the stream. These nutrients can be taken up by stream algae, an important food source for aquatic invertebrates.  Many of these aquatic invertebrates include insects, which eventually leave the stream to become flying adults and a food source for many birds. Additionally, carcasses can serve as a direct food source for aquatic and terrestrial invertebrates, such as pill bugs.

Because these carcasses attract lots of invertebrates, they provide a concentrated food source for insect-eating birds. Many invertebrates lay their eggs in the carcasses as well, and so by promoting enhanced abundances of insects and shrimp, salmon can help sustain a higher number of prey items for birds. This study also reported that compared to streams with little or no salmon, streams with salmon had higher densities of a shrub called salmonberry, which produces berries eaten by many types of birds. By enhancing local resources through the nutrients they leave behind, salmon have an indirect positive effect on birds.

The bigger picture

Increasingly, streams and adjacent woodlands experience destruction by human development. Protecting ocean resources, such as salmon and the parts of various marine food webs they are linked to, may help terrestrial animals that indirectly depend on the ocean. A major threat to these ecosystems is the diversion of water from coastal streams and dam construction—both of these could interfere with salmon spawning success as well as the surrounding terrestrial food webs. What would happen to the bears, wolves, aquatic insects, and birds if the salmon could no longer make their journey to these streams?

This study sheds light on the importance of considering linkages between seemingly distinct ecosystems to better understand how to protect organisms in both habitats. Although more studies are needed to measure salmon impacts on other stream-forest areas, it seems that by protecting the ocean’s resources, we may also help protect organisms in freshwater streams and forests.




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