Keystone species: how marine mammals can tell us about ocean health

By Asia Lie

The ocean can tell us a lot about the health of our environment, namely in terms of global warming. The world relies on the ocean for a large amount of industry and products. According to the Organisation for Economic Co-operation and Development, the ocean contributes to the economy at a conservative estimate of USD 1.5 trillion and offers about 31 million full-time employment jobs.1 Many anthropogenic, or human-caused, hazards coming from the ocean have been listed in the past couple of years2, exemplifying the link between human and ocean health. Not only do we affect our ocean, but our ocean has impacts on our food supplies, transportation, jobs, and habitat. One way we can monitor the health of our oceans is the monitoring of keystone species. According to National Geographic, keystone species are organisms that help to define an ecosystem with no functional redundancy, meaning that without them, no other species would be able to fill their niche and the ecosystem would change dramatically.3

The ocean is central to life on Earth. It produces over 50% of the world’s oxygen and stores over 50 times more carbon dioxide than the atmosphere, also transporting heat to regulate our climate.4 The biggest threats to it today include climate change, plastic pollution, overfishing, and maritime transport.5 As human action continues to contribute to the threats against the ocean, it is essential that we monitor the ocean’s health. 

Sea otters (Enhydra lutris) are one of the most important marine mammals that can tell us about our ocean’s health. Sea otters spend most of their time in the water, using kelp forests to find food and anchor themselves in the waves to take care of their young or sleep6. The kelp forests that they live in are one of the ocean’s most biodiverse communities and buffer storm surges, protecting beaches and coasts. These characteristics make kelp forests a good area to monitor for scientists investigating ocean health during climate change.7 One of the biggest threats to kelp forests today are sea urchins. In a balanced ecosystem, sea urchins eat kelp for nutrients, but they can destroy kelp forests if their population is left unchecked.8 This is where sea otters come in. Otters feed on shellfish, including the sea urchin.6 By keeping the sea urchin population under control, the kelp forest ecosystem can thrive and continue to support a large variety of species[LJ1] . 

Changing ocean conditions have affected this ecosystem and highlighted the importance of sea otters as a keystone species. Warming ocean waters have slowed the growth of kelp forests and the numbers of sea urchin’s predators have declined, allowing the population of the purple sea urchin to increase more than 60-fold.8 This has led to a decrease in 90% of the kelp in the ocean off of Northern California.8 Global warming, coupled by the sea otters’ endangered status after human hunting practices, have left kelp forests in grave danger, a clear sign that ocean health is being affected by anthropogenic practices. 

A variety of other organisms also act as keystone species. The disappearance of the purple sea star (Pisaster ochraceus) halved the biodiversity in Tatoosh Island during a study investigating the impacts of its removal. Purple sea stars are predators of muscles and barnacles, and their disappearance allowed muscles to overrun the ecosystem, inching out many species by taking over their living spaces.3 The blue crab (Callinectes sapidu) is a keystone species in salt marshes, areas that serve as pollution filters and fish nurseries. They feed on the periwinkle snail, which in turn feeds on the marsh grasses present. Upon removal of the blue crab, the population of the snail increases, allowing them to overgraze on the grasses in the marsh. The overconsumption of the grass turns the marsh into a mudflat, a barren area of mud.9 When the crab is overfished, we would see previously healthy marshes turn to mudflats. Another example of overfished keystone species would be most species of shark. Their absence decreases the biodiversity of coral reefs, one of the most important habitats of the ocean.10 The monitoring of these keystone species could give us insight into the biodiversity of certain ocean communities and tell us how to regulate our fishing industry.   

Indicator species[LJ2]  are another major signal of ocean health. They reflect the state of an environment and its biodiversity and evidence of change13, which helps us to determine risks to the human population. Such risks can include water or air pollution.3 Oysters are indicator species, as well as other bivalves. As filter feeders, they can filter out the pollutants and sediments that get introduced into the ecosystem. Changes in their population or health can tell us more about what pollutants are present in our oceans. According to the Jellyfish Project, jellyfish are also an indicator of our ocean’s health. Their increasing populations tell us that their predators are overfished and the rising ocean temperature and acidity is affecting their proliferation.11

As we continue investigating the effects of climate change and ocean threats, it’s important to take action now. Establishing reserves to protect marine life and coral reefs, reducing pollution, consuming less fish, and promoting sustainable energy are some important action items we should be pursuing.5 It is also important to support the organizations that conduct research into ocean conservation. For example, The Marine Mammal Center in California, USA12 is a non-profit marine mammal hospital serving to promote ocean conservation through rescuing marine mammals, conducting research, and educating the public. The Ocean Conservation Trust and the National Oceanic and Atmospheric Administration (USA) are also committed to protecting the ocean.14,15 [LJ3] 

By interpreting and analysing changes in keystone species and their populations, we can tell a lot about our ocean. As anthropogenic actions increasingly affect ocean health, it’s important that we continuously monitor the impacts we are making. The ocean is vitally important to the world, including our own health and civilization. With global warming and overfishing threatening biodiversity everywhere, keystone and indicator species provide us with an alarm system to regulate our damaging actions. It’s all dependent on if we listen.


1.     OECD. The Ocean Economy in 2030 [Internet]. OECD; 2016. Available from:

2.     Elliott M, Cutts ND, Trono A. A typology of marine and estuarine hazards and risks as vectors of change: A review for vulnerable coasts and their management. Ocean & Coastal Management [Internet]. 2014 Jun [cited 2022 Oct 30];93:88–99. Available from:

3.     National Geographic. Role of Keystone Species in an Ecosystem | National Geographic Society [Internet]. 2022. Available from:

4.  NOAA. Why Should We Care about the ocean? [Internet]. National Ocean Service. NOAA; 2021. Available from:

5.  Fava M. The Treaths to the Ocean in 2022 and How to Prevent Them [Internet]. Ocean Literacy Portal. UNESCO: Intergovernmental Oceanographic Commission; 2022. Available from:

6.     National Geographic. Sea Otter | National Geographic [Internet]. National Geographic. National Geographic; 2010. Available from:

7.     NOAA. What is a kelp forest? [Internet]. National Ocean Service. NOAA; 2021. Available from:

8.     National Geographic Society. A Delicate Balance | National Geographic Society [Internet]. National Geographic. National Geographic; 2022 [cited 2022 Nov 2]. Available from:

9.     Nielsen M, Conneely B, Lipworth J, Bonetta L. Keystone Species Sampler: Blue Crab [Internet]. Exploring Keystone Species. HHMI Biointeractive; 2022. Available from:

10.     Nielsen M, Conneely B, Lipworth J, Bonetta L. Keystone Species Sampler: Shark [Internet]. Exploring Keystone Species. HHMI Biointeractive; 2022. Available from:

11.     The Jellyfish Project. Why The Jellyfish? [Internet]. The Jellyfish Project. [cited 2022 Nov 2]. Available from:

12.  The Marine Mammal Center. The Marine Mammal Center : Home [Internet]. The Marine Mammal Center. 2022. Available from:[LJ7] 

13. Lawton JH, Gaston KJ. Indicator Species. In: Encyclopedia of Biodiversity [Internet]. Elsevier Inc.; 2001. p. 437–50. Available from:

14. NOAA. About Our Agency | National Oceanic and Atmospheric Administration [Internet]. 2018. Available from:

15. Ocean Conservation Trust. About Us | Ocean Conservation Trust [Internet]. Ocean Conservation Trust. 2018. Available from:

add reference [LJ1]

What is an indicator species? Definition [LJ2]

add reference [LJ3]

@Lie, Asia Overall a very interesting article about an important topic! 🙂 It will make a great addition to the website. I learnt a lot reading it. [LJ4] [LJ4]

There are 2 sections which need referencing, please add references here. Afterwards, I can send it to be published. 🙂

I have made some other editing suggestions if you would like to consider them. In future, please reference journal articles rather than websites for the main content of your article.

 [LA5]thank you! For future reference, should I look for references of the organizations in journal articles to avoid citing their websites? (Ex. Marine mammal center or NOAA) [LA5]

Thank you for making your edits so quickly 🙂 [LJ6]

It’s fine to cite the websites of the organisations @Lie, Asia

in future, reference journal articles. Only reference websites in very specific circumstances (e.g. The Marine Mammal Center website) [LJ7]

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