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

Oceanic islands are islands that rise to the ocean surface from its floors. An island is a body of land surrounded by water. There are countless islands around the world, varying in size, climate, and biodiversity.

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

• Continental islands are formed by breaking continents or rising sea levels. In other words, they are islands formed by the same continental land near them, which has been surrounded by water, forming the islands. 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.

Volcanoes can be found at plate boundaries or hotspots.

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.

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.

Hotspots

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

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.

Names and Examples of Oceanic Islands

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

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.

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.

Fig. 1 - The Weka is a species of flightless bird endemic to New Zealand. Unsplash

Increased Floral Diversity

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

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.

The Island Speciation Process

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.

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.

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.

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 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.

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.

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