How & Why Are Starfish Keystone Species? – (Let’s Know)

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How are starfish Keystone Species?

In a marine environment, starfish are considered to be a keystone species that through their participation in their respective ecological niche can keep a balance of other species populations in the ecological communities available there.

Starfish acts as Keystone Species as they due to their presence in the ecosystem are known to keep tidal pools of the respective marine environment in an ecological balance. And so, they do impactfully keep in check and prevent other species like mussels, barnacles, limpets, etc. from taking over the ecosystem.

Starfish do so by feeding on the mussels, barnacles, etc. and so they do act as natural population controllers in their respective marine ecosystem.

Organisms like mussels, barnacles, etc. feed on seaweeds like algae. They also feed on other organisms like bacteria, and other small, organic particles filtered from the water column.

Mussels’ larvae also act as parasites on the gills and fins of freshwater fishes.

So, naturally keeping a check on these small organisms’ populations is a must to keep a balance in the ecosystem.

And, Starfish do this very well as they are the natural predators of these small organisms like mussels, barnacles, etc.


By doing so, Starfish (Sea Star) helps ensure healthy populations of seaweeds, and thus directly or indirectly promote the sustainability and growth of other marine communities that feed on the seaweeds.

So resultedly, the population of the species like sea urchins, sea snails, limpets, and bivalves who feeds on the seaweeds also remain constant and balanced.

And also, as an indirect result of this, the population of other species like the Sheephead fish, large species of Snails, etc. that feed on sea urchins, limpets, etc. are also seen to be kept in a constant and balanced count.

So, you can very clearly see how without the presence of Starfish an ecosystem can show a drastic radical change by being unbalanced, or may also result to cease to exist altogether. 

Why are starfish Keystone Species?

As the definition of Keystone Species states that, “Keystone species are referred to those plant, animal, fungi, or bacterial species that perform a top-notch ecological niche in the ecosystem and through their own activities ensures the balanced sustainability and population growth of all other species of the ecological communities present there including themselves.

In simple words, you can say that Keystone species helps to add and glue together the various other species to a proper-balanced ecological habitat present in their ecosystem, where all the species have all the resources required for their own survival and growth.

Starfish are considered Keystone Species as they act as predators in the tidal pool marine ecosystem by controlling the distribution and population of large numbers of prey species like mussels, barnacles, etc.

This in turn helps to control the distribution and population of seaweeds there in that ecosystem, which also, directly and indirectly, controls the distribution and population of other species that feed on the seaweeds, and so on.

According to the context of conservation biology, it is the predator-prey relationship between the Starfish and the Seaweed feeders like mussels, etc. that make starfish to be considered as Keystone Species.

Starfish as the small predators that take part in the ecosystem by preying (consuming) upon the small omnivores species that feed on phytoplankton and zooplankton species.

This prevents such omnivores species from removing all of the seaweed species that are primary producers in the ocean.

And this why so Starfish are considered Keystone species as per the predator-prey relationship of that ecosystem.

Starfish as Keystone Species – (As Experimented by Ecologist ‘Robert Treat Paine’)

DO YOU KNOW: Pisaster ochraceus (Common name: Ochre starfish) is a species of starfish that was discovered to be a keystone species on the Makah Reservation in Clallam County, Washington, United States. These were the first animals to be identified as keystone species.

Pisaster ochraceus starfish was discovered in the year 1966 by Ecologist ‘Robert Treat Paine’ (1933–2016). The term Keystone Species was first coined by Robert Treat Paine in the year 1966.

Famous Ecologist named Robert Treat Paine was a zoology faculty at the University of Washington in Seattle.

Robert Paine started his experiment in 1941 with his few students from the University of Washington. He had spent around 25 years or so conducting such an experiment.

During his experiment and research studies, he had extensively found out that there was a species of Starfish which is now named Pisaster ochraceus (Common name: Ochre starfish) that preyed upon the other small predators like mussels, barnacles, snails, limpets, and more.

This starfish is the apex predator in the rocky intertidal ecosystems of the Makah Reservation in Clallam County, Washington, United States which is located in the Pacific Northwestern region.

He and his students instead of just observing the habitat of the Pisaster ochraceus starfish patiently conducted the experiment by altogether changing the habitat and removing the starfish from that place.

He had done so by selecting a shoreline at Mukkaw Bay, which was about 8 meters in length by 2 meters in width. This area was kept free of that starfish for conducting the experiment.

There he had demonstrated that at least on this rocky shoreline from where the starfish were removed impactfully resulted in an increase of dramatic changes in the temperate intertidal ecosystem as the apex predator was no more present there.

These dramatic changes were caused due to the increased population of mussels, barnacles, snails, limpets, etc. which destroyed the majority of the seaweeds present there.

And so as a result, the population of many other species that used to feed on the seaweeds was also consequently decreased.

This largely disturbed the ecosystem and the balanced food chains that were present there as there was no one to keep a check on the increased population of mussels, barnacles, snails, limpets, etc.

He finally concluded that the top predator Pisaster ochraceus controlled its prey and affected most of the other species in the community.

He had also stated that a single predator could control the abundance, diversity, and distribution of other organisms sharing its ecosystem.

This was in short how the famous Ecologist ‘Robert Treat Paine’ (1933–2016) conducted his experiment Pisaster ochraceus (Common name: Ochre starfish), and as a result coined the term “Keystone Species’ for the first time ever.

Paine highly gave light to the concept of keystone species to mainly explain the relationship between Pisaster ochraceus, and Mytilus californianus, a species of mussel.

Thus, his findings led to a major effect on conservation and ecology. As a result, many ecologists from around the globe started discovering and terming many other species as Keystone species of their respective ecosystems.

Sea Otters, for example, are now understood to have key roles in controlling the distribution, abundance, and diversity of many species in ocean ecosystems of kelp forest habitat. Sea Otters feed on organisms like Sea Urchins that destroy the kelp forest habitat.

What role do starfish play in the ecosystem? What makes them so important?

The roles that starfish play in the ecosystem by maintaining the structure and integrity of the community make them the keystone species there.

However, you must also note that a starfish that is considered as a keystone species in one particular place, may not always be considered as a keystone species of another place.

The consideration of keystone species of one particular place’s ecosystem depends on other participatory species’ ecological roles as well either directly or indirectly.

Here, I have listed some of the straight-forward points to indicate the roles that starfish play in the ecosystem:

1. Starfish act as keystone species in most of the Tidal Pool marine ecosystems. They act as keystone species by preying on the animals that have very little to no other natural predators. This helps maintain balance in the ecosystem and its relationships.

2. In some ecosystems, you will find that starfish feed on some marine animals in the ocean as their nutritious prey. This keeps a balance check in the population of starfish. In such cases, the starfish may not be always considered as a keystone species if other top-notch keystone species are there in the ecosystem.

3. They maintain the predator-prey relationship in most of the ecosystems like in the majority of the Tidal pool locations. This also balances the ocean food chain and protects a lot of species from getting endangered over time.

4. They balance the food chain of that particular marine ecosystem by keeping a check on the various species of the ecological communities. This protects the ecosystem from having a low amount of prey or a high amount of predators, thus maintaining the balance which altogether supports life and thus evolution over a greater period of time.

5. The presence of starfish prevents from occurring various radical changes in the ecosystem like new invasive species populating the habitat, or leading to the over-growth of prey population than predators, or causing the over-growth of predator population than prey, or other various negative consequences.

6. Sea stars are also known to feed on the various sea urchins. They are known to envelop themselves around the sea urchins and then they feed by everting their stomach against the sea urchin to dissolve away the flesh and shell and take it inside. By consuming the sea urchins they do also act as keystone species, and as a result they do prevent the destruction of kelp forests caused by the sea urchin population.

7. They do also help in cleaning the ecosystem by consuming the scattered pieces of rubbish or remains of other small organic seaweeds and organisms that are at the ocean floor and nearby rocks. This also helps in creating new room for other organisms to survive and grow.

8. They due to their unique body physiology of Water Vascular System are able to filter the ocean water from debris. Thus, by making the water stay clean and clear they do promote the flourishment of new life and thus can be stated beneficial to the ocean ecosystem.

9. They as keystone species also indirectly take part in maintaining the balance of dissolved oxygen in the marine ecosystem. As they keep a direct check on the phytoplankton population as a part of their ecological niche, also ensures that enough oxygen is produced and the oxgen concentration remain balanced in the ecosystem for other species to respire.

10. They offer critical insights into the way ecosystem works, and so they have sparkled the ecologists to keep a search for other keystone species all over the world.

What will happen if starfish get extinct or are removed from an ecosystem?

By being keystone species starfish keeps a balance check of the various species in the ecosystem including itself. This keeps a balance in the ecosystem’s various food chains, food webs, and energy flow in trophic levels.

And so, if the starfish get extinct or are removed from an ecosystem then it will eventually cause a very drastic radical change that will alter the balance in the ecosystem.

In some of the ecosystems like that of the Mukkaw Bay ecosystem where Pisaster ochraceus (Common name: Ochre starfish) is the apex predator, there will be total decrease in the population count of the various other species if that starfish gets removed.

Like that in a wide variety of marine ecosystems, starfish act as prominent keystone species by maintaining a proper predator-prey ecological relationship.

So, if anything happens due to natural calamities or due to human intervention causing the removal of starfish from the environment, then this will eventually result in their prey population count increasing in number leading to the driving out of other species from that environment.

They help to prevent the negative consequences happening in an ecosystem. These negative consequences can be like over-growth of prey population than predators, or over-growth of predator population than prey, or disruption in the species relationships in the ecological communities.

So, in very simple words, due to the absence of starfish, ocean food chain will surely become unbalance and also causing many other species to become endangered due to the low amount of prey or high amount of predators.

As we know that, all of the keystone species are prominent and unique of their kind in the ecosystem due to their participation in their respective ecological niches which is almost incomparable to any other species’ ecological niche.

So, this also means that if the keystone species like starfish disappear or become extinct from their ecosystem, then it’s pretty much clear that no other species of that ecological community can take the role and match its ecological niche the same as the starfish did.

Let’s Know: What type of Keystone Species is a starfish?

We all should know that amongst the 2,000 species of starfish known so far, most of them are active predators that do feed on almost anything they come across their way including mussels, clams, oysters, barnacles, snails, limpets, etc.

So, those predatory starfish can be considered under the ‘Predator’ type of Keystone Species as they impactfully and largely take part in keeping a direct check on the population count of mussels, clams, oysters, barnacles, snails, limpets, etc. that come their way.

This also indirectly shapes the ecosystem by creating, modifying, or maintaining the surrounding natural landscape around them, as they help the seaweed population to remain balanced.

This balanced seaweed population can in turn maintain the population count of other species that depend on the seaweeds for their sustainibility.

So, you can also state them to be considered under the ‘Ecosystem Engineer’ type of Keystone Species as they do also perform a very little natural task to modify their marine habitats through their own biology or by physically changing biotic and abiotic factors in the environment indirectly.

However, you should always note that not all starfish are apex predators in some of the ecosystems as there may be another apex predator like sharks, etc. that do feed on the starfish.

In such a case, the starfish may not be a keystone species but act as Indicator species that will indicate very sensitive environmental changes in its ecosystem, or can act as flagship species by symbolizing an environmental habitat.

Here, according to the experiment performed by Ecologist ‘Robert Treat Paine’ showed that Pisaster ochraceus (Common name: Ochre starfish) is an apex predator type of keystone species on the Makah Reservation in Clallam County, Washington, United States.

But, for other places it may not always be the same predator type of keystone species. In the same way, different locations have different organisms acting as keystone species.

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