Paradigm Shift On How To Restore Kelp Forests

Divers smash sea urchins with rock hammer.Paradigm shifts in science are rare, and it looks like one is now happening around kelp restoration science and Marine Protected Areas.

In the past few weeks, several notable scientific papers have been published, which point to a paradigm shift on how kelp forests can be restored and protected.

Satellite photo showing dramatic decline of Northern California kelp forests in the last 10 years.

California’s kelp forests are in decline and something must be done soon

California has experienced an extraordinary loss of kelp forests. Over 90% of bull kelp has been lost in the past 10 years in Northern California, and Southern California has experienced a similar decline over a longer period. This story has been know for a while, but it made national news when satellite images were published. 

Something must be done to reverse this decline, before the kelp loss becomes permanent.

Read: “Large-scale shift in the structure of a kelp forest ecosystem co-occurs with an epizootic and marine heatwave”, by McPherson et al.
Read an explanatory article in UCSC news.

Marine Protected Areas provide no benefit for kelp restoration

The concept of Marine Protected Areas (MPAs) is a core tenet of ocean protection. MPAs have been proven to protect and sustain biodiversity and resilience of fish, shellfish, and many other species.

However the ocean is full of surprises, and a long-term study in California’s Channel Islands has shown that MPAs do not provide a benefit for kelp forests. The study “found no evidence that giant kelp is positively affected by reserves,” because urchin populations increased and urchin barrens expanded within the MPAs observed over 15 years.

Read “After 15 years, no evidence for trophic cascades in marine protected areas”, by Malakohff and Miller.
Read an explanatory article in

This study was published in the prestigious Proceedings of the Royal Society. It was soon followed by a second paper, published in the equally prestigious, Proceedings of the National Academy of Sciences, which helped explain why urchin barrens persist inside MPAs.

Sea otter eats a healthy purple sea urchin.

Urchin Barrens tend to expand because zombie urchins taste bad

A core principle of MPAs is that protecting top predators keeps other elements of the ecosystem in balance. However sea urchins are a unique animal that has radically different behaviors in a healthy kelp forest vs. an urchin barren.

This new study explores the relationship between sea otters and urchins. Otters are top predators in a kelp forest, and keep urchin populations in check. However sea otters will not eat urchins in a barren, despite the extreme abundance of urchins and ease of capture.

In a healthy kelp forest, urchins hide in cracks to avoid predators, and play an essential role in the health of an ecosystem by processing kelp detritus. They are well fed and quite tasty for predators, which keeps urchin populations low.

However when urchins increase to a density of around 5-10 per square meter, there is not enough detritus available, and they start to eat the kelp holdfast. Which results in the disappearance of the kelp forest and its urchin predators, resulting in the formation of an urchin barren. These urchins go into a zombie-like state, where they can live for 50+ years.

Zombie urchins are essentially starving, and lose all nutritional value for predators. There is no reason for an otter to expend energy catching an urchin in a barren, and this applies to all predators of urchins. Thus urchin barrens become a “metastable state”, that will persist even in the presence of urchin predators.

Read, “Behavioral responses across a mosaic of ecosystem states restructure a sea otter–urchin trophic cascade”, by Smith et al.
Read an explantory article in

The solution is to reset the ecosystem balance with a hammer

The practice of smashing urchins in a barren with a hammer was first studied in the 1980s and has been shown to restore healthy kelp forests. However prior studies have been small scale and isolated, and policy makers have resisted the idea as a large scale restoration technique. Particularly in MPAs which are meant to be undisturbed by humans.

The recent papers described above have shown conclusively that kelp forests are unlikely to recover on their own, therefore we must look at human intervention as a strategy to restore kelp forests. However, the question remains on the fate of that restored forest. Would it become stable and healthy?

A third and final paper is about to be published, which details a ten year study of active kelp forest restoration at Palos Verdes, California. This paper compares stable healthy kelp forests with restored forests in the SeaTrees / Bay Foundation kelp restoration project. The restored forests quickly regained full health and became indistinguishable from a healthy forest.

The study also examined a small natural urchin mortality event. Where a disease killed a large number of urchins in a section of reef in the area. Kelp forest soon returned to the reef, and this forest also was identical to nearby healthy forests. Thus it doesn’t matter how urchin barrens are cleared, whether by natural causes or by human intervention. Both mortality events resulted in the restoration of kelp forests.

This 10 year study examines one of the largest and most successful kelp restoration projects in the world. It conclusively shows that it is possible to restore an urchin barren back to a healthy kelp forest on a timescale of a few years. Then existing MPAs can do their job to protect urchin predators.

Read, “Sea urchin mass mortality rapidly restores kelp forests”, by Williams et al. The full paper will be published in Q2 2021.

Scientists monitor restored kelp forest.

SeaTrees is proud to support kelp restoration

As part of our mission to turn the tide on climate change, SeaTrees is proud to support the restoration of blue carbon ecosystems around the world. This includes kelp forests in California, with the Bay Foundation and California Reef Check.

Both individuals and brands can donate to support the restoration of kelp forests.

Together, our community will help kickstart the active restoration of kelp forests around the world.