From kelp forests to barrens

Kelp Loss | Sea Urchin Barrens | The Process

Kelp forests are highly diverse and productive ecosystems that provide food and shelter for a plethora of marine life including endangered and threatened species, marine mammals, birds, and key fisheries species such as red abalone and red sea urchins. When kelp washes onto shore during winter storms known as beach wrack. Invertebrates in beach wrack are an important food source for many shore birds.

There are over 30 species of kelp worldwide and they typically grow close to shore in cool, nutrient rich water. In California, kelp forests are dominated by two of the tallest kelp species in the world: Giant kelp and Bull kelp. Giant kelp is the tallest, growing up to 2 feet a day in ideal conditions to 175 feet and bull kelp isn’t far behind reaching heights of 115 feet. In the ocean they form towering underwater forests that bustle with marine life like underwater cities.

Photos of Bull Kelp by Steve Lonhart, NOAA

Bull kelp is the more dominant canopy forming species from central California and north to Alaska while giant kelp is more common in southern California. The root-like holdfast anchors kelp to boulders and rocky formations at depths of 6 to 60 feet. It can be seen on the surface where it grows.

Sea urchin barrens

Where there was once kelp from the ocean floor to the surface, coastal waters are now dominated by sea urchin barrens. Urchin barrens that lack kelp and other macroalgae occur when sea urchins are so abundant, they eat the algae faster than it can regrow.

The loss of kelp forests has had significant impacts, affecting the many animals that depend on kelp as well as the fishing industry. Compared to kelp forests, barrens have significantly lower diversity and productivity.

Video by Paul Chetirkin, NOAA

The Purple Sea Urchin

Sea urchins are free-moving marine invertebrates that walk along the sea floor using tube-feet. They are characterized by round, globular bodies with five-fold symmetry and a cover of spines. They are found throughout global oceans from the tropics to temperate and polar regions where they live in rock pools in the intertidal zone to the depths of the ocean.

The purple sea urchin, Strongylocentrotus purpuratus, is native to the California Coast. They can reach up to 3 inches (7 cm) in diameter. The adults are entirely purple while juveniles <0.5 inches in diameter have green spines. They eat brown, red, and green algae including algae growing on the surface of rocks and macroalgae like kelp.

Purple sea urchins are tough and opportunistic with an incredible capacity to survive when food is scarce and capitalize when it is abundant.

Images (from left to right):  A purple sea urchin adult (D. Deaker, FRC), a juvenile purple sea urchins (Alex Heyman, source:iNaturalist), and purple sea urchins in the tide pools in Sandy Cove (D. Deaker, FRC), Fort Ross State Historic Park

Are population booms unexpected?

Sea urchins are a class of the phylum Echinodermata that includes sea stars, brittle stars, feather stars, sea cucumbers and sand dollars.

Population booms are not unusual among echinoderm species. When there is a change in an ecosystem, the food web changes and the effects radiate out to other species.

Example: Crown of thorns sea stars, Acanthaster sp. 

In tropical coral reefs around the world, there are outbreaks of coral-eating crown of thorns sea stars that reduce coral cover and impact the health of reefs.

Similar to kelp forest restoration in California with physically removing sea urchins, management of crown of thorns includes culling but it has remained a challenge for coral reefs since the 1960s.

Processes and traits that are likely to have a positive (+) or negative (−) effect on the success of crown of thorns starfish [3].

Click here for a free publication by Dr. Dione Deaker, FRC

It is incredible that, out of the many echinoderm species living on the coast, just a few like crown of thorns sea stars and the purple sea urchin can capitalize on changed conditions and experience such rapid population growth.

Sources:

[1] Allen, J.D., Richardson, E.L., Deaker, D., Agüera, A., and Byrne, M. (2019) Larval cloning in the crown-of-thorns sea star, a keystone coral predator. Mar Ecol Prog Ser 609: 271-276.
[2] Babcock R.C., Milton D.A., and Pratchett, M.S. (2016) Relationships between size and reproductive output in the Crown-of-Thorns starfish. Mar Biol 163: 234. https://doi.org/10.1007/s00227-016-3009-5
[3]Deaker D.J., Byrne M., (2022) Crown of thorns starfish life-history traits contribute to outbreaks, a continuing concern for coral reefs. Emerg Top Life Sci. 14;6(1):67-79. https://doi.org/10.1042/ETLS20210239