Why One Missing Animal Can Empty a Whole Forest

There are forests under the sea.

Off the cold rocky coasts of the Pacific, giant seaweed grows up from the seabed in ropes as tall as a fifteen-storey building, swaying in the current with fish moving through it. Divers go into it the way you would walk into a wood. It is properly a forest, with a floor and a roof and shade.

And in other places along the same coast, at the same depth, in the same water, with the same light coming down, there is nothing. Bare rock, from edge to edge.

The difference between the two is an animal that does not eat seaweed at all.

First, the forest

Giant kelp is a seaweed, not a plant — it has no roots and it takes nothing out of the rock. It grips on with a tangle called a holdfast, which is a hand rather than a root, and everything it needs it takes from the water.

What it does have is speed. In good conditions giant kelp can grow as much as half a metre in a single day. That is a plant you could nearly watch move.

Second, the animal that eats it

Sea urchins live on that same seabed. They are round, spiny, slow, and they graze seaweed.

They do not nibble away at the top of the kelp, though, which is what you might picture. They chew through the holdfast at the very bottom — and when the holdfast lets go, the entire thirty-metre rope of seaweed floats away and is lost. A small animal at the bottom takes out the whole thing.

Third, the animal that eats that

Sea otters eat urchins. They also eat crabs and clams and snails, but they eat a great many urchins.

And they eat an astonishing amount, for a reason worth knowing. Seals and whales keep warm in cold water with a thick layer of blubber. Sea otters have none. They keep warm with fur, and by burning food — which means a sea otter has to eat about a quarter of its own body weight, every single day, for its whole life.

That is a hungry animal. Where there are otters, there are not many urchins.

Now take the otters away

In the 1700s and 1800s, sea otters were hunted for their fur across almost their whole range. From something like a couple of hundred thousand animals, the population fell to a couple of thousand. Along long stretches of coast there were none left at all.

Nothing was done to the kelp. Nobody cut it, nobody poisoned it, the water did not change.

The urchins multiplied. And the forest went.

What is left when it goes has a name: an urchin barren. Bare rock, thick with urchins, and no seaweed above it — which means no shelter, no shade, and none of the fish and crabs and small things that lived in the forest. Take out an animal that ate urchins, and you lose a forest and everything living in it.

The link that skips a step

Here is the part that matters most, and it is easy to read straight past.

An otter never touches kelp. Not once, in its whole life. It is not tending the forest, or protecting it, or planting it. It is just eating urchins, for its own reasons, because it is hungry.

The otter's effect on the seaweed travels through the urchin. It is a link with a gap in the middle — and that is exactly why nobody saw it coming. If you had been asked in 1800 what hunting otters would do to seaweed, the honest answer would have been "nothing at all, they are not connected."

They were connected. Just not directly.

Why the bare rock stays bare

You would expect this to fix itself. Fewer kelp means less food, less food means fewer urchins, fewer urchins means the kelp comes back. A dip, then a recovery.

It does not go that way, and the reason is quietly remarkable.

A starving urchin does not die. It waits. It shrinks itself, drops its appetite to almost nothing, and sits on the bare rock — for years, if it has to. A barren is not a place with too little food for urchins. It is a place full of urchins that have stopped needing much.

So the bare rock stays bare, and it can stay that way for decades. Breaking it takes something from outside: otters coming back, or a disease running through the urchins, or a storm. The system does not tip back on its own, because there is no longer anything pushing.

Nobody is certain how far it reaches

One honest note to finish on, because it belongs to the story.

That otters, urchins and kelp are linked is not in doubt — it has been watched for fifty years, in places with otters and places without, and it comes out the same way. But how far the knock-ons travel beyond that, and how much else is going on at the same time, is still argued about by the people who study it. Real systems have more than three animals in them.

We also know the pattern is not only about otters. Along the coast of California from 2013, a wasting disease killed enormous numbers of sunflower sea stars, another animal that ate urchins. The urchins boomed. The kelp went. Same shape, different animal, and this time nobody was hunting anything.

Which is the useful lesson, and it is not a comfortable one. When you take one animal out of a place, you cannot always work out beforehand what will change — because the things that will change may be things that animal never touched.