Inhabitants of the Bull Kelp Forest

A web of connections
Photo: Steve Clabuesch

The trophic food web shows the complexity of who eats whom. At the bottom are the photosynthesizers: the kelps, seaweeds, and microalgae or diatoms. They are considered primary producers and are crucial for the health of the web above them.

What Eats What

The bull kelp forest as a term signifies a web of interconnections between primary producers (seaweeds, kelps and single-celled microalgae) and a host of organisms both big and small that get eaten by other organisms. There are also the interactions between the foundational kelp and abiotic factors such as the ocean water itself (source of nutrients, more or less acidic, warmer or cooler), what the benthos or bottom is made of (rocky or sandy), the dynamics of wave action in a given cove or range of coastline, what humans have been doing along that coast, and complex interactions us humans can only guess at. A trophic cascade is when the effect of displacing one of the trophic (food) levels ripples throughout the ecosystem. Sea otter, sea urchin, and kelp form a classic trophic cascade.

We have chosen a cast of characters to learn more about because they are important to know about in their own right, but also because they illustrate some of the connectivity of the kelp forest food web. By diving deeper into the life history and historical ecology of a few of these primary characters of the bull kelp forest we can learn how humans, both ancient and contemporary, have affected the balance of the kelp forest over time, and learn about ocean systems and how changes are affecting these very different kinds of beings. The drama in the kelp forest is ever changing and always fascinating!

Bull kelp illustration

Bull Kelp

The bull kelp forests of the North Pacific are some of the most productive and biodiverse habitats on earth. Bull kelp is the brown algae that holds it all together, creating habitat for countless other organisms and forming the base of the vast ocean food web.

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Sea otter illustration

Sea Otter

Sea otters are the smallest marine mammal and native to the North Pacific Ocean’s edge from Northern Japan through the Aleutians, south to Mexico. Its range intersects completely with the bull kelp forests. Nearly driven to extinction because of their luxuriant fur, sea otters play an outsize role keeping urchin populations in check so that kelp forests can thrive. 

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Sunflower sea star illustration

Sunflower Sea Star

Known as the wolves of the sea, these large, twenty-armed sea stars can move fast and eat small to mid-sized urchin. Their populations were decimated in 2014–2016 by a sea star wasting disease.

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Sea urchin illustration

Sea Urchin

Sea urchins populate all the world oceans, living on algal detritus that drifts down to them. If the kelp forest balance is upset, urchin populations can grow too large and overgraze the kelp, leaving what is known as an urchin barren.

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Abalone illustration


Abalone are sea snails with a large fleshy foot that grows inside a nacred shell. The shells of abalone have been important to indigeous communities for centuries as decorative elements in regalia as well as a trading item. Bits of abalone shell are a tidepooling treasure to find.

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Rockfish illustration


The North Pacific has the highest diversity of rockfish in the world. They thrive in the rocky habitat of the bull kelp forest, some living to over 200 years old. Their colors can range from orange to yellow to black.

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Humans illustration


Humans populated the Pacific Coast of North America during the last ice age by following ‘the kelp highway’ along the coastlines, a region of abundance. Human brains developed and societies evolved because homo sapiens could access the vast protein sources of oyster, fish, and seaweed as well as the minerals of the ocean such as iodine and potassium. From continuous Indigenous relationships to bull kelp to 20th century fisheries management, to 21st century kelp restoration projects, humans have been part of the kelp forest since their arrival.

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Giant kelp illustration

Giant Kelp

Known for its beauty and prodigious growth, giant kelp mingles with bull kelp in some regions, but is the signature species that creates the great kelp forests of Southern California, Peru, Chile, and Argentina and can be found in New Zealand, Southern Australia, and South Africa.

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Understory kelp illustration

Understory Kelp

Pterygophora californica (walking kelp) and other lower growing kelps thrive under the canopy of bull kelp. These less recognized kelps are vitally important in supporting the rich diversity of life in the kelp forest. Pterygophora is the oldest growing of North Pacific seaweeds and has the toughest, most upright stipe.

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Kelp wrack illustration

Kelp Wrack

Bull kelp wrack is the piles of bull kelp found on beaches or washed up in coves. All kelp and other seaweeds wash up onshore as wrack, but bull kelp wrack is particularly astounding in its shear biomass. Bull kelp grows into a massive mature alga in only a few months, the stipes often entangled with each other. As an annual, it reproduces and then the whole entanglement of kelps dislodges from the ocean bottom. In healthy years of cold water, strong winter storms and abundant kelp growth, there should be piles of bull kelp wrack on the shore.

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