If coral reefs are like tropical rainforests, then kelp beds are the ocean equivalent of oak forests. It is estimated that some 175 species live on or among the holdfasts alone, polychaete worms and seastars among them. Kelp forests support a huge range of animals that depend on the giant algae and on each other. They form part of a complicated relationship web and one that changes as the seasons pass.

Microscopic animal life forms live on kelp fronds. These include bryozoans that grow to form white lacy mats. They can be a problem for the kelp because they stop the sunlight getting to the surface of the plant. Another kelp eater is the snail, or rather 14 species of snail, including the black turban snail. Young kelp blades are protected by a layer of slippery mucous but this is lost over time so older fronds become vulnerable. Fish eat kelp plants too. Opaleyes leave characteristic holes in the blades, just like caterpillars on cabbages. They eat the kelp and any invertebrates that happen to be living in it at the time.

Sea urchins and abalone eat drift kelp that sinks to the sea floor. They congregate in crevices where drift kelp accumulates. Urchins, particularly red ones, have a potentially damaging habit. If there isn’t enough drift kelp around, they can survive for a while, but eventually they come out of hiding and move across the sea floor. They start to eat living kelp plants, climbing up the stipes and eating the fronds or nibbling the holdfasts and setting the plants adrift. The urchins can be present in such large numbers, and can reproduce so successfully, that new kelp plants are eaten before they have a chance to establish themselves. Whole kelp forests can disappear, leaving what’s called an urchin barren.

Sheephead wrasse and sea otters feed on hard-shelled prey. They have an important role in the kelp community because they keep urchin numbers down. Sea otters were nearly hunted to extinction for their thick fur, resulting in a sea urchin population explosion and consequent kelp forest destruction.

Many fish shelter in the kelp, mounting raiding expeditions into the open sea to find certain foods that are there at specific times of day and night. Blacksmith fish head out to the open ocean to find plankton but retreat to the forest at the first sign of danger. Señorita wrasse are also niche feeders. They have a specific role to play in the forest. They are cleaner fish that pick parasites from the skin of other fish. They are good for the kelp because they also eat the lacy bryozoans, keeping the fronds healthy. Kelp forests provide vital nursery areas. Many fish species arrive there specifically to spawn.

Seals and sea lions are frequent visitors to kelp beds, but sea otters live there. They wrap kelp fronds near the surface around their legs to stop themselves drifting away on the current. Once secure, they bask in the sunshine. They dive to the bottom to find shell fish and can stay underwater for 10 minutes at a time. Kelp forests offer cover for marine mammals keeping them safe from great white sharks that prefer to hunt in open water where they won’t become entangled in the plants. Sea birds also roost on the fronds and use them as a base for hunting raids.

Grey whales pass through the Californian kelp forests on their migrations between summer feeding grounds in the north and winter mating and calving areas off the coast of Mexico. They feed in the kelp forests, stripping tiny crustaceans from the surface of the blades. They also use the kelp to shelter the newly born calves from killer whales. It’s estimated that between ten and twenty thousand whales pass the California coast each year.