Time To Read
Date Last Modified
Place 02 · Monterey Bay · Chapter 4 of 7
Otters, Urchins and Kelp
Take the sea otters out of a kelp forest and the forest disappears, even though otters never eat kelp.
About 15 minutes · 1 video · 1 live otter cam · 3 quick activities
Start here
The animal that holds the forest up
Purple sea urchins are spiny pincushions with a jaw built for scraping, and their favorite food is kelp. Sea otters’ favorite food includes sea urchins, which they crack open while floating on their backs, sometimes using a rock as a tool. Take the otters away and the urchins march across the seafloor, mowing the forest down to bare rock. One furry predator decides whether there is a forest at all.
Watch this
See it move
Monterey Bay Aquarium — The secret life of sea otters: Guardians of the kelp forest. Three minutes with a biologist whose job is watching wild otters, which is a real job and yes, we checked.
While you watch, look for
- Which grazers the otters eat, besides sea urchins
- What happens to the kelp when those grazers are kept in check
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The comeback
From 50 to 3,000
Sea otters have the densest fur on Earth, over a million hairs per square inch, and fur traders nearly hunted them out of existence. In 1938, about 50 were found surviving off Big Sur. Today California has around 3,000, and the stretch of coast where they came back has kept its kelp better than places without them.
The keystone
Small numbers, huge effect
A keystone species matters far more than its numbers suggest, like the top stone that holds an arch together. The idea came from a tidepool: in the 1960s, ecologist Robert Paine pried ochre sea stars off the rocks in Washington and watched mussels take over. Sea otters are the textbook keystone of the kelp forest.
The barren
When the urchins win
More otters means fewer urchins and more kelp; fewer otters means the reverse. That chain reaction is a trophic cascade, and its bad ending is an urchin barren: rock grazed bare, where starving urchins can hang on for years. After a wasting disease killed over 90% of sunflower sea stars, another big urchin-eater, barrens spread along northern California.
Photos: Beth Fitzpatrick, Veronika Andrews, Derek Keats / Pexels
Play with it
Watch the otters right now
This is the Monterey Bay Aquarium’s live sea otter cam; these otters were rescued and cannot survive in the wild. Watch for grooming, eating, and pounding food against things, the same move wild otters use with rocks.
Live cam: Monterey Bay Aquarium, Live Sea Otter Cam (YouTube). Feeding times are the best show; if the otters are asleep, check back later.
Two questions to answer while you are in there
- How long can an otter go without grooming? Why would an animal with no blubber groom that much?
- If you catch an otter eating, what is it doing with its paws and chest, and how would that help with a spiny urchin?
Try it yourself
Check yourself
Try it
Who Eats Whom
Flip each card, guess before you look, and meet the cast of the kelp forest.
Try it · interactive
Otter Cascade
Choose how many sea otters patrol the coast, switch sea star wasting disease on or off, and run 20 years to watch the urchins and the kelp answer back.
Open the Otter Cascade in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Food web
The whole tangle of who eats whom
Keystone species
A species with an outsized effect
Trophic cascade
A chain reaction down the food web
Urchin barren
Seafloor grazed bare by too many urchins
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: Otters, Urchins and Kelp
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- MiraCosta 13.2 — Trophic Pyramids (food chains and food webs) — LibreTexts · 5 minutes
- MiraCosta 15.14 — Kelp and Kelp Forests — LibreTexts · mostly photos · 5 minutes
- Monterey Bay Aquarium — Just add otter — Keystone species and the kelp comeback · 8 minutes
Photo credits (Pexels): Sergey Guk, Kindel Media, Beth Fitzpatrick, Veronika Andrews, Derek Keats.