We researched kelp forests and the issues they face due to increased grazing pressures by sea urchins. Kelp forests are composed of two types of brown algae, giant kelp and bull kelp. They grow best in areas where the water is cold and nutrient-rich, either due to upwelling or ocean gyre movement up continental coasts from the poles. Kelps require a strong, rocky substrate on which to anchor themselves with their holdfasts, and enough light to photosynthesize. 

Kelp forests are recognized as one of the most productive ecosystems on earth. Under the right conditions the giant kelp can grow up to 2 feet per day. With this amount of productivity and plant biomass, kelp forests support a huge variety of marine life and provide considerable ecosystem services to humans. As we discuss in our project however, kelp forests have been facing threats and declines due to an increase in sea urchin herbivory as a result of a trophic cascade. 


Although kelp forests are found in multiple places around the world, we will be narrowing our focus to a regional scale and focusing on the forests along the west coast of North America, specifically the California coast. The ecosystems affected by the issue are coastal ecosystems which depend on the kelp forests for buffering, and the marine biome surrounding kelp forests which are home to many different organisms.
Global distribution of kelp forests 
Harbor seal in a kelp forest near San Diego, California

Specifics of the Problem

History of the Issue

The problem of kelp forest decline was first noticed in the early 20th century when otter hunting along the western coast of North America was very common and unrestricted. With the loss of otters for their pelts, people began to notice that the California kelp forests were also beginning to decline, and the entire ecosystem that the kelps supported were suffering in turn. Although hunting by humans was significantly reduced over the course of the century, sea otters began to face a new threat in the 1990s when killer whales turned their attention to the otters and began to feed on them more than ever before. 

Efforts have been made to support increases in the California sea otter populations and reduce sea urchin populations, which in turn helps kelp forest health return to previous levels. Some methods of these efforts are detailed in the Solutions, and indicate that the situation has been improving. 

Underlying Causes of Kelp Forest Decline

Sea urchins pose a great threat to kelp forests in large numbers. The main predator of sea urchins is the sea otter. In the early 20th century, sea otters were greatly threatened by human hunting along the North American west coast. Humans wanted the otters for their pelts, but an increase in awareness of sea otter decline led to a great reduction in overhunting over the decades due to the International Fur Seal Treaty. 

However, sea otters became the focus of another deadly predator in the 1990s. Pacific killer whales were not finding enough sea lions to support themselves, and began to turn to otters as a main source of prey. The first recorded attack on a sea otter by a killer whale was in 1991, after sea lion populations had reached minimum levels along the California coast in the 1980s. The sea lion population decline was due to a decrease in the abundance of their own prey. Killer whales require the energy contained in as many as 1,825 otters a year, so only a few killer whales can do a lot of damage to the sea otter population. 

killer whale:

Otters as a Keystone Predator

The loss of otters set into motion a trophic cascade -- when the loss of a top predator has a disproportionately large effect relative to their numbers on lower trophic levels. 

With the decline of sea otters, sea urchin populations soared. Usually kept in check by otter predation, the sea urchins were now free to travel in huge masses and decimate kelp forests. Rather than hide from otters in rock crevices and only feast on scraps, they would attack the holdfast of the kelps, graze it away from the substrate, and the kelp would be left to float away and die. 

With their numbers left unchecked, sea urchins can travel in herds large enough to destroy kelp forests at a rate of 30 feet per month. An area where the urchins have wiped out the kelps is called an urchin barren, and can be difficult to restore if the urchins remain at their high population levels. 
otter eating: 
urchins eating:
urchin barren:

Consequences and Impacts of Kelp Forest Decline


Many species suffer with the loss of California kelp forests. They are an important home for many organisms at different stages of their life. Herring and Atka mackerel spawn in them, and salmon fry use kelp forests as their nursery. Sea birds find food and a resting place amongst the kelp fronds. For California sea lions, harbor seals, and whales, the kelp forests are not only an important site to find food, but a place of shelter during rough storms and waves, as well as from predators. 

Mackerel near San Clemente Island, California

Ecosystem Services and Impact on Humans

The California kelp forests' massive size allows them to provide a huge buffer for the coastline from big storms and wave action. Not only do they prevent a lot of erosion to the natural coastline, they save people considerable amounts of money by protecting homes and other coastal developments as well. 

Because of their huge degree of productivity and photosynthesis, kelp forests are huge carbon sinks which take in the greenhouse CO2 from the atmosphere. When enough otters are around to keep urchin populations under control, kelp forests can take in as much as 12 times the amount of CO2 they are able to when under pressure by urchins. 

Kelp forests are important to humans in recreational ways as well. Tourism and activities such as scuba diving and kayaking to observe the diverse marine habitat will be affected by a loss in the biodiversity kelp forests create. 

Larger Consequences of the Problem

Other Locations

This issue of kelp forests being largely affected by other factors in the ecosystem are occurring in several other places around the world. These locations include the coasts of New Zealand, Tasmania, Australia, South Africa, Chile and Japan. Urchin barrens are such a global phenomenon that Japanese divers have come to the Palos Verdes coast in order to study the methods that California is utilizing to fix the issue. This way they can go back to their country and implement the successful practices in the project. Since this is an issue facing several different parts of the world, it can result in a potential for global consequences.  

Global Consequences

Another similar issue is the balancing act that must be maintained between the urchin population, kelp forest, and sea otters. Sea otters are larger than river otters and have special adaptations that make them perfect for the ocean. These include thicker fur, flattened tail, and skeletal modification to their hind legs and pelvis that allow for better underwater movement. They also have a very high metabolism, causing them to be a key predator and reduce the populations of sea urchins. This in turn causes more kelp forests to be sustained and leads to other effects as well, including higher net productivity, more nursery habitat for fin fish and other invertebrates, reduced wave energy and coastal erosion, and even increased fixation of atmospheric carbon.

Since most life on the planet requires oxygen to live, this is a great issue. Most marine life including salmon, crabs, shellfish and more need oxygen and because of this many of them are not able to survive in low oxygen seawater. These areas have been called “dead zones.” These areas have also been known to host specialized urchins that result in urchin barrens. Since so many species are not able to properly live in these conditions, this greatly damages the commercial fishermen’s success in the economy. Modern fisheries, conservation and resource management will have to grapple with these changes in the near future. A living, breathing ocean is necessary to sustain the rest of the world’s population.


Organizations Acting to Address the Issue

One of the biggest projects currently undergoing along the California coastline is the Kelp Forest Restoration Project by The Bay Foundation. The goal they have is to reduce the number of purple urchins from 40 per meter to just two. As of now, the Foundation has accomplished restoring 21 acres of kelp forest in four separate coves with the assistance of four commercial fisherman and 35 volunteer divers spending 3400 hours underwater restoring the kelp. They have now reduced the average of 36 purple urchins per square meter to only 2 per square meter.  

One of the main points they make is that volunteer divers go out and must hammer the urchin very thoroughly. This is so divers behind them won’t waste their time trying to hammer an urchin that has already been taken care of by another diver. Time and efficiency are vital when it comes to this. Another important tip is to make sure you take the ones out at the bottom of the rock first as opposed to the top. This is due to the fact that if you hammer the ones at the top, the debris from them will obscure the ones still living on the rock. Another project sponsored by the Coast Keeper Alliance did restoration work from Ventura County to San Diego County utilizing a different technique. In Orange County, kelp forests have definitely rebounded but there is still a lot of work that needs to be done.

These efforts are already beginning to reverse the effects of urchin population surges and hopefully will reestablish healthy kelp forests that provide food and habitat for many species. This project is truly making advancements in order to maintain biodiversity and taking action on urchin populations.


Since there has been so much progress in the area of kelp forest restoration already in the area, I feel like the issue could definitely be resolved in due time. The algae is one of the quickest growing species in the world. They are known to rise as much as two feet per day. As the kelp continues to grow, the fish will then come to populate the kelp forests. Along with the fish will come other ocean creatures. The biodiversity will be rich and restored to what it should be. 

One of the overarching lessons to be learned from this issue would be that time is definitely vital. Handling a situation soon enough before it gets too out of hand is extremely important when it comes to taking back what is needed.

Kelp forest in the Channel Islands


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