How salmon get into trees

By Liam Nash

Everything in nature is connected. The sun’s power is captured and converted into food for plants. Herbivores eat these plants and are in turn eaten by predators. These predators die and decompose, turning back into nutrients for plants and continuing the cycle. In wise words from The Lion King:

“When we die, our bodies become with the grass. And the antelope eat the grass. And so, we are all connected in the great Circle of Life.” – Mufasa, 1994.

As straightforward as this all sounds, it is, in a sense, the central tenet of ecological science – that everything is connected to everything else by the complex chains of interactions between species and their environment. Energy and matter flow through ecosystems by being continuously eaten, excreted, moved around and absorbed.

Organisms are connected by flows of energy and matter
Image by Liam Nash

Even completely distinct and separate habitats, such as oceans and forests, deserts and rivers, can be tightly interconnected in unexpected ways. Leaf litter from trees is washed into rivers and lakes where bacteria and invertebrates break it down. Thus, these aquatic creatures are fueled by the energy produced in an entirely different ecosystem, on land. Conversely, many aquatic animals, such as mosquitoes, dragonflies and frogs, undergo metamorphosis, transforming from water-living juveniles into flying or hopping, land-dwelling adults. So, after developing on energy from aquatic food, these adults transfer that energy to the land when they die, decompose or are eaten by land predators such as spiders, bats and birds^1,2. Some of these animals may have grown up in the water on leaves from the land whilst in their aquatic life stage; their eventual death on land merely completing the cycle of exchange between these two ecosystems.

Resources transfer from land to water in a cyclical manner as material moves from one to the other
Image by Liam Nash

In fact, the evidence of the unexpected interconnections between land and water are evident almost everywhere we look, even when vast distances separate the two. In the tree trunks of the Pacific Northwest of Canada and US, a thousand kilometers inland and two thousand above sea level, dark bands reveal the tight links between these distant ecosystems³. When analysed, these tree rings were found to have a chemical signature more like a marine environment than a forested one. The nitrogen atoms contained within were heavier than expected, something typical of nitrogen-containing compounds produced by life in the sea.

But, how did these heavy nitrogen atoms find their way deep into the forest? Believe it or not, this unusual chemical signature comes from salmon, millions of them, which migrate upstream from the ocean every year to spawn. After this, they die. Their decomposing carcasses provide a suite of land animals, from flies to boars, with food derived originally from the sea. Brown bears, famous for their taste for salmon, carry fish many kilometers inland, where their uneaten (or defecated) nutrients are taken up by the forest. Here, energy from the sun, trapped by marine algae and transferred from little fish to big fish, ultimately finds its way to the top of an Alaskan conifer, thousands of kilometers away.

Fish can end up in trees through a series of cascading ecological interactions
Image by Liam Nash

Whilst these connections are facilitated by the movement of animals or material from one habitat to another, separate ecosystems can be linked, indirectly, through more abstract chains of interactions. Famously, wolves have been shown to change the course of rivers⁴. By hunting deer around riverbanks, wolves gave tree saplings some respite from constant grazing, allowing them to grow much larger. In turn, their roots held the soil together, preventing erosion, and forced the water to flow around them, permanently altering the course of whole rivers.

Fish have been shown to increase the pollination rate of flowers⁵. By consuming juvenile dragonflies before they emerge as deadly aerial predators, fish give pollinating insects, such as flies and bees, a better chance of survival, to the benefit of the surrounding plants.

Fish pollinate flowers, wolves meander rivers, everything affects everything else. As wondrous examples of nature as these are, these complex, interconnected pathways also have a dark side – they allow the impact of human activities to spread from one ecosystem to an entirely different one. For example, overfishing of salmon in the Pacific will have knock-on effects on inland forests, kilometres from the sea by reducing the amounts of fish-fertiliser available to them. Climate change caused by industrialised nations in the Northern hemisphere melts sea ice in Antarctica. As organisms themselves don’t obey country borders and legislation is often separate between the land, rivers and ocean, their movement can transport environmental problems far beyond the source of the issue to completely pristine and distant ecosystems⁶.

Aquatic insects which develop into flying adults are known to import gigatons of anthropogenic pollutants, such as polychlorinated biphenyls (PCBs), mercury, and more, from freshwater to land, polluting pristine habitats and poisoning agricultural land⁷. More recently, microplastics have been found to stay in the bodies of adult mosquitos, which ingested them as aquatic larvae⁸. Whilst generally thought of as an aquatic problem, these metamorphosing creatures may make a connection for microplastics to return to their source, on land.

Chains of connections between species means that our impact in one area can have unintended consequences in another
Image by Liam Nash

Ecology is often philosophically thought of as a holistic science as it must consider habitats as whole interconnected systems. This is especially true in conservation. The interconnectedness of nature means that efforts to clean-up an environment may be hampered by sources coming from an entirely separate ecosystem, potentially far, far away. Considering habitats as separate entities is dangerous for restoration and conservation efforts as human effects may cascade through to protected areas through unexpected ecological avenues. A protected area is only as strong as the landscape it connects to, only as strong as the stream which flows through it, which connects it to the nearest city and, ultimately, the ocean. Fewer salmon in the ocean, less salmon in the trees.

References

1. Richardson, J.S., Zhang, Y. and Marczak, L.B., 2010. Resource subsidies across the land–freshwater interface and responses in recipient communities. River Research and Applications, 26(1), pp.55-66. doi:1002/rra.1283
2. Nakano, S. and Murakami, M., 2001. Reciprocal subsidies: dynamic interdependence between terrestrial and aquatic food webs. Proceedings of the National Academy of Sciences, 98(1), pp.166-170. Doi:10.1073/pnas.98.1.166
3. Reimchen, T.E., Fox, C.H., 2013. Fine-scale spatiotemporal influences of salmon on growth and nitrogen signatures of Sitka spruce tree rings. BMC Ecology 13, 38. https://doi.org/10.1186/1472-6785-13-38
4. Beschta, R.L. and Ripple, W.J., 2006. River channel dynamics following extirpation of wolves in northwestern Yellowstone National Park, USA. Earth Surface Processes and Landforms: The Journal of the British Geomorphological Research Group, 31(12), pp.1525-1539. doi:1002/esp.1362
5. Knight, T.M., McCoy, M.W., Chase, J.M., McCoy, K.A. and Holt, R.D., 2005. Trophic cascades across ecosystems. Nature, 437(7060), p.880. doi:10.1038/nature03962
6. Gende, S.M., Edwards, R.T., Willson, M.F. and Wipfli, M.S., 2002. Pacific salmon in aquatic and terrestrial ecosystems: Pacific salmon subsidize freshwater and terrestrial ecosystems through several pathways, which generates unique management and conservation issues but also provides valuable research opportunities. AIBS Bulletin, 52(10), pp.917-928. https://doi.org/10.1641/0006-3568(2002)052[0917:PSIAAT]2.0.CO;2
7. Walters, D.M., Fritz, K.M. and Otter, R.R., 2008. The dark side of subsidies: adult stream insects export organic contaminants to riparian predators. Ecological Applications, 18(8), pp.1835-1841. doi:1890/08-0354.1
8. Al-Jaibachi, R., Cuthbert, R.N. and Callaghan, A., 2018. Up and away: ontogenic transference as a pathway for aerial dispersal of microplastics. Biology letters, 14(9), p.20180479. doi:10.1098/rsbl.2018.0479