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Oceanic Islands

Oceanic Islands

What do the islands of Honshu, Iceland, Mauritius, Tonga, and Hawaii have in common? They vary drastically in size (and human population), and are found in different corners of the globe. But one trait that they do share is their formation process. All five are oceanic islands.

Earth is home to thousands of oceanic islands. In fact, the continent of Oceania establishes the Pacific Ocean as the defining characteristic (and namesake) of the continent.


Oceanic Islands: Definition

An island is a body of land surrounded by water. There are countless islands around the world, varying in size, climate, and biodiversity.

Islands that are less than one acre in size are known as islets.

Greenland is the world's biggest island, covering 836000 square miles.

There are six types of island, classified according to their formation.

  • Continental islands are formed by breaking continents or rising sea levels. Examples include Greenland and the British Isles.

  • Tidal islands are a kind of continental island that is only separated from the mainland during high tide. Examples include Mont Saint-Michel in France and Mersea Island in Essex.

  • Barrier islands are narrow islands lying parallel to the coastline, separated by a lagoon. They are formed by sediment deposits or coral reefs. An example is Padre Island, Texas.

  • Coral islands (atolls) are reefs that grow in a ring around an oceanic island. An example is the Bahamas.

  • Artificial islands are made by humans. A famous example is the Palm Islands of Dubai.

  • Oceanic islands (also known as volcanic islands) are formed by the eruptions of underwater volcanoes.

The Formation of Oceanic Islands

Oceanic islands are formed by the eruptions of underwater volcanoes. Gradually, layers of lava build up and eventually break through the water's surface to form an island.

A seamount is a volcano that's yet to break through the surface of the water.

Volcanoes can be found at plate boundaries or hotspots.

Plate Boundaries

The Earth's crust is split into tectonic plates, which can be oceanic or continental. The plates interact with each other at tectonic plate boundaries, where earthquakes and volcanic eruptions are common.

Some volcanoes form in subduction zones, where a dense oceanic plate sinks below a less dense continental plate.

The island of Honshu was formed in a subduction zone.

Other volcanoes form in constructive zones, where two tectonic plates are moving apart from each other. Magma rises to fill the gap between the plates, resulting in volcanoes.

Iceland was formed in a constructive zone.

Hotspots

Magma bubbles up from hotspots, forming volcanoes far from plate boundaries.

A hotspot is a break in the Earth's crust outside a tectonic plate boundary.

The Hawaiian archipelago was formed by a hotspot over millions of years.

The Climate of Oceanic Islands

Like continents, the climate of oceanic islands depends on their position on Earth. For example, Iceland is situated just south of the Arctic Circle. It experiences a cold and cloudy climate, with some seasonal temperature variation. That differs considerably from equatorial islands like the Galápagos, which experience a tropical climate with little seasonal variation.

The climate of oceanic islands differs from continents. Islands typically experience less temperature variation (i.e. slightly milder climate), increased humidity, and increased wind.

Precipitation on islands is often determined by relief – windward slopes of mountainous islands receive the most rain. Ocean currents also play an important role. Cold currents promote stable air stratification and decrease precipitation. In contrast, warm currents intensify air convection and increase precipitation.

The Gulf Stream originates in the Gulf of Mexico and brings warm water towards northwest Europe. As a result, the British Isles experience milder winters and more precipitation than other regions at a similar latitude.

Names and Examples of Oceanic Islands

There are thousands of oceanic islands around the globe, but let's focus on a few examples.

IslandLocationSize (square miles)Climate ClassificationEndemic Species of Note
MauritiusIndian Ocean790Tropical Monsoon and Tropical SavannaDodo (extinct bird)
TongaPacific Ocean288Tropical RainforestMany endemic species, including the Tongan whistler and black foxface
SurtseySouthern coast of Iceland0.5Subpolar OceanicNone so far – but the island was only formed in the 1960s!

Unique Features of Oceanic Islands

Oceanic islands possess unique features due to their isolation. These islands are typically far away from continents – especially oceanic islands that were formed by hotspots. Isolation is a key factor influencing island biology. Species richness is negatively associated with island isolation – the more remote an island is, the fewer species that live there.

Limited Faunal Diversity

Islands exhibit different community compositions to their continental counterparts. Firstly, oceanic islands lack natural populations of amphibians. Their skin is sensitive to salt water, preventing them from travelling across the ocean. Secondly, islands support fewer mammal species; sometimes just bats.

There are no records of any terrestrial mammal naturally occurring on an island over 300 miles from its nearest continent.

Islands lack large mammalian predators for two reasons:

  • Insufficient range (the island is not large enough to support a viable population)

  • Difficulties reaching the island

Due to an absence of predators, flightless birds have evolved on many islands. They are seldom found elsewhere.

Oceanic Islands weka flightless bird endemism StudySmarterFig. 1 - The Weka is a species of flightless bird endemic to New Zealand. Unsplash

The Island Effect is an evolutionary rule stating that members of a species get larger or smaller depending on the resources available in their environment.

Small animals experience island gigantism, evolving a larger body size over time. Possible causes are reduced predation risk and vulnerability to scarcity.

The Galápagos giant tortoise can weigh up to 400 kilograms. That's approximately seven times larger than their closest living relative!

In contrast, large animals experience insular dwarfism, evolving a smaller body size over time. Possible causes are limited resources and competition.

The pygmy three-toed sloth is endemic to a small island in the Caribbean. It's much smaller than the other members of the genus.

Increased Floral Diversity

Although animal diversity is reduced, oceanic islands experience high rates of plant diversity.

The island of New Guinea is home to more than 13,000 plant species, of which two-thirds are endemic.

The genetic diversity of island plants is low, while morphological diversity is high. Why?

Island plants often have limited dispersal and pollination abilities, leading to asexual reproduction. This has resulted in low genetic diversity. It's been theorised that low genetic diversity has enabled rapid speciation.

Endemism

Endemic species are common on islands because they are geographically and ecologically isolated from other species.

Endemism refers to a species that is found in a single defined geological location.

The Island Speciation Process

  • Individuals of a population travel to an oceanic island, where they become geographically isolated from other members of the population.

  • Conditions on the island will be different from the continent.

  • In these new conditions, certain alleles will become more advantageous than others. These alleles will begin to increase in frequency over time.

  • Eventually, the individuals living on the island will have changed so much that they can no longer breed with the original continental population. A new species has been born!

Why are Endemic Species Important?

Endemic species are only found in certain regions, which puts them at increased risk of extinction. Furthermore, endemic species often have unique characteristics that directly impact the survival of other species in the ecosystem.

Threats to Oceanic Islands

Sadly, these unique locations are under threat from human activities. In the past 500 years, human colonisation of islands has caused the extinction of at least 800 species.

Overexploitation

Globally, overexploitation is a major driver of biodiversity loss. Oceanic islands have not escaped this problem. The high rates of endemism means that overexploitation is a serious threat that has the potential to drive species to extinction.

Overexploitation is the act of harvesting species faster than they can recover.

Introduced Species

Introduced species are found outside their natural range due to human activities. Many introduced species disrupt the natural ecosystem and threaten the endemic species of islands.

Kiwis are flightless birds native to New Zealand – an island with no native terrestrial mammals. However, humans introduced a range of terrestrial mammals that threaten the kiwi. Not all of them are predators; many compete for their food. The kiwi population has fallen from 5 million to just 50,000.

Habitat Destruction

Habitat loss threatens island biodiversity – and endemic species in particular. Their already-limited range puts them at a high risk of extinction.

Sea Level Rise

Burning fossil fuels results in greenhouse gas emissions, which affect the global climate. Higher temperatures cause ice to melt, thus increasing sea levels.

Many oceanic islands are low-lying, putting them at risk from sea level rise.

The Maldives is an archipelago of over 1000 islands. The country is renowned for its beautiful beaches and its low elevation – most of the islands are just 1 metre above sea level. Scientists have predicted that rising sea levels will have inundated the nation by the end of the twenty-first century.

Oceanic Islands maldives sea level rise StudySmarterFig. 2 - Rising sea levels will affect the beaches of the Maldives, affecting tourism and income. Unsplash

Conserving Oceanic Islands

Oceanic islands are hotspots for endemic species, who face a high rate of extinction, so it's important to protect them from threat.

Eradication of Introduced Species

As explained earlier, introduced species disrupt the natural ecosystem – and present an extra danger to flightless birds, who are adapted to predator-free conditions.

Eradication refers to the complete and permanent removal of wild populations of a species.

Eradication strategies are most successful when carried out as soon as possible, giving the species minimal time to establish and grow in population. To minimise the eradication time, rapid detection systems need to be put in place. This often involves public education campaigns.

Control of Development and Visitors

Sustainable development is difficult on islands; resources and land space are limited. Urbanisation is considered inevitable for islands near the mainland, or close to hubs of economic activity.

65 million people live on Small Island Developing States (SIDS). These regions face unique challenges related to their geography, including high import costs, small population sizes, and high vulnerability to climate change.

However, some islands manage to support human populations and maintain biodiversity. The Galápagos Islands are a good example. In 1959, the islands adopted the strategy of land zoning. The Galápagos National Park covers 97% of the archipelago's land area, with human populations only found on four of the thirteen major islands. The National Park is maintained by rangers, who mitigate against damage and eradicate invasive species.

The islands require entrance fees and immigration control cards for visitation, to keep track of tourist numbers.


I hope that this article has clarified oceanic islands for you. Remember that they're formed by underwater volcanic eruptions, and their isolation has resulted in unusual animal biodiversity and many endemic species. Sadly, these islands are under threat from human activities.

Oceanic Islands - Key takeaways

  • Oceanic islands are formed by the eruption of underwater volcanoes. They can form at plate boundaries or volcanic hotspots.
  • The climate of oceanic islands is typically milder, windier, and more humid than continental regions. Ocean currents and island relief impact precipitation.
  • Oceanic islands are isolated, which impacts species richness and animal diversity. Terrestrial mammals (especially predators) are rare on oceanic islands – enabling the evolution of flightless birds. Endemic species are common on oceanic islands.
  • Oceanic islands are under threat from overexploitation, introduced species, habitat destruction, and sea level rise.
  • Conservation strategies for oceanic islands include eradication of introduced species, and control of development and tourism.

1. Charles Darwin, On the Origin of Species, 1859

2. CIA World Factbook, Tonga, 2022

3. David Elliott, This island's dazzling flora makes it the most plant-diverse on the planet, World Economic Forum, 2020

4. Discovering Galapagos, A Sustainable Galapgos 1/5: Population Growth and Land Zoning, 2022

5. Iceland Review, Surtsey Island 50 Percent Original Size, 2013

6. Janhvi Johorey, Why are Kiwis Endangered? - Facts and Figures, AnimalWised, 2017

7. Jeannie Evers, Island, National Geographic, 2022

8. José María Fernández-Palacios, Scientists’ warning – The outstanding biodiversity of islands is in peril, Global Ecology and Conservation, 2021

9. Matt White, 2002: Largest Tortoise, Guinness World Records, 2015

10. Permanent Court of Arbitration, Memorial of the Republic of Mauritius, 2012

11. Tristan McConnell, The Maldives is being swallowed by the sea. Can it adapt?, National Geographic, 2022

12. United Nations, About Small Island Developing States, Office of the High Representative for the Least Developed Countries, Landlocked Developing Countries and Small Island Developing States, 2022

Frequently Asked Questions about Oceanic Islands

Oceanic islands are formed by the eruption of underwater volcanoes.

The islands of the Hawaiian archipelago are oceanic, formed over a volcanic hotspot.

An oceanic island is an island that was formed by underwater volcanic eruptions.

The British Isles are not oceanic islands; they are continental islands.

There are thousands of oceanic islands in the world.

Final Oceanic Islands Quiz

Question

How are oceanic islands formed?

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Answer

Oceanic islands are formed by the eruptions of underwater volcanoes. 

Show question

Question

What is a seamount?

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Answer

A seamount is an underwater volcano that is yet to break through the surface.

Show question

Question

Define a hotspot.

Show answer

Answer

A hotspot is a break in the Earth's crust outside a tectonic plate boundary.

Show question

Question

Islands experience milder climates than nearby continental regions.

Show answer

Answer

True

Show question

Question

Warm ocean currents lead to reduced precipitation.

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Answer

False

Show question

Question

Define eradication.

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Answer

Eradication refers to the complete and permanent removal of wild populations of a species.

Show question

Question

Species richness is positively associated with island isolation.

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Answer

False

Show question

Question

Why do oceanic islands lack large mammalian predators?

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Answer

Insufficient range

Show question

Question

What unusual birds are mostly found on islands?

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Answer

Flightless birds are mostly found on islands.

Show question

Question

Define endemism.

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Answer

Endemism refers to a species that is found in a single defined geological location.

Show question

Question

Define overexploitation.

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Answer

Overexploitation is the act of harvesting species faster than they can recover.

Show question

Question

What are introduced species?

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Answer

Introduced species are species that are found outside their natural environment due to human activities.

Show question

Question

Why are many oceanic islands threatened by sea level rise?

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Answer

Many oceanic islands are low-lying.

Show question

Question

Are there natural amphibian populations on oceanic islands?

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Answer

There are no natural amphibian populations on oceanic islands.

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Question

Why are endemic species important?

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Answer

Endemic species at an increased risk of extinction. They also have unique characteristics that impact the survival of other species in the ecosystem. 

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