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Fish-eating trees sound like they’re straight out of science fiction.
But they’re a real thing—one that exists right here on Earth.
In many forests on the west coast of North America, the trees eat salmon.
And they show just how interconnected life on this planet is.
Now, when we say they “eat” fish, we don’t mean they’ve developed a mouth full of pointy
teeth or razor-sharp claws to pluck a swimming salmon from the water.
What we mean is that the trees obtain a large portion of their nutrients from fish.
And they can do that because bears that do have pointy teeth and sharp claws catch the
fish for them!
When salmon are ready to spawn, they make a one-way trip from the ocean up into streams
to lay their eggs.
This is an all you can eat buffet for bears that feast on the salmon to get ready for
their long winter hibernation.
But bears are messy eaters.
For example, in two months, on a single stream, they left over 5000 kg of salmon scraps along
the river banks and into the trees!
And those scraps have a lot of something trees need desperately: nitrogen.
Like any living thing, plants need a bunch of different nutrients to grow.
But whichever one is in short supply in a given area is deemed the limiting nutrient.
And along the northwest coast of North America, the limiting nutrient is often nitrogen.
Since fish carcasses have lots of nitrogen in them, scientists studying these ecosystems
suspected the trees were “eating” the bears’ leftovers, once the salmon decomposed
and their nutrients sank into the soil.
But to prove it, they needed to follow the nitrogen!
See, not all nitrogen is the same.
Some nitrogen atoms have more neutrons than others.
And it turns out nitrogen atoms can have either seven or eight neutrons and remain stable—so
nitrogen has what chemists call two stable isotopes.
Over 99% of the nitrogen on Earth is the lighter kind: nitrogen-14.
But salmon have a weirdly high amount of nitrogen-15, when compared with land-dwelling species.
That’s partly because most salmon spend nearly all of their adult life in the ocean,
and it just so happens that the chemical and biological processes that occur in seawater
make more nitrogen-15 available there than on land.
On top of that, though you may not think of salmon as fierce predators, they’re actually
pretty high up the food chain.
And animals use each nitrogen isotope slightly differently.
Because of quirks of biochemistry we don’t fully understand, nitrogen-14 tends to be
excreted more than nitrogen-15.
And since this happens at every level of the food chain, the higher up animals are, the
more nitrogen-15 they have in their diet — and therefore, in their bodies.
Starting in the late 1990s, research revealed that trees near salmon streams in Alaska have
impressively high levels of nitrogen-15.
The obvious conclusion: they get a lot of their nutrients from salmon!
That wasn’t all.
By looking at nitrogen in individual tree rings—those yearly layers you can see if
you cut a tree in half — the researchers were able to show how important salmon are
to the trees.
During and after years when more fish came upstream to spawn, the trees grew bigger
rings with more nitrogen in them.
And overall, trees that received nutrients from salmon grew 19% bigger than those that didn’t!
So, if you’re a tree who wants to grow nice and tall, you’d better not skip out on the sushi.
And if you’re a human who’s been inspired by this episode to dive deeper into biochemistry,
well, that’s something today’s sponsor—Brilliant.org—can help with.
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