The Concept of Island Or Insular Biogeography

Islands are one of the Earth’s most interesting features. Islands experience great diversity in their size, age, inhabitants, and origins. Islands either be oceanic when it is a product of volcanism, such as Hawaii, or it could have been connected to a part of a continent, therefore making it a continental island like Madagascar. Continental islands comprise of continental shelf fragments as it used to be connected to a continent either directly or indirectly by means of a land bridge. Narrow, shallow waters separate continental islands from continents. Separation from continents can occur as a result of postglacial sea-level rise. A rising sea-level leads to the isolation of species already occurring on the island from those on the mainland and as the island originated from a continent, the biological and geographical features would be similar.

The island of Madagascar is a continental island that is located in the Indian Ocean and is separated from Africa by the Mozambican channel. With an area of 581 500 km2, Madagascar is one of the largest islands in the world. After Gondwanaland broke up two rifting events occurred. During the first event 180 mya, India and Madagascar separated from Africa. The second event separated India from Madagascar 88 mya. The organisms located on this island range from ancient Gondwanan lineages to species that invaded after the separation from continents, and have undergone speciation.

Grub, 2003, noted important features regarding the fauna and flora of Madagascar. The high degree of endemism is a result of the historic isolation from Africa for about 125 million years. The wet vegetation areas on Madagascar have a lot of woody plant species. During the Pleistocene, Madagascar experienced ‘species-pumping’ in contrast to other places where mass extinctions were occurring. The species-richness and abundance of palms, tree-ferns, and bamboos were due to the wet climate experienced in the past as well as the present. There is an area of dry evergreen forest where trees evolved fire-resistant bark as an adaptation to frequent ground fires. Another thing to take note of was the thicket vegetation and succulents instead of grasses in the driest areas probably as the result of having a finite chance for rain throughout the year.

Oceanic islands are the result of volcanic activity in the ocean. Submarine volcanism can either occur from hotspots and mid-oceanic ridges or at subduction zones. This type of island has never been connected to another landmass and therefore all the species present came from somewhere else and might have undergone speciation. These islands do not exist for long and although some exist for millions of years, they will erode and subside back into the ocean eventually.

The Hawaiian Archipelago is found inside the tropics of the central Pacific and is located 4000 km from the closest continental land- which is the west coast of North America. These islands have never been connected to continental land as their geological history ensures continuous isolation. The islands of Hawaii consist of various volcanoes that developed from the ocean in a sequence- the oldest are located at the northwest side, whereas the youngest island lie in the southeast. The Hawaiian Islands have been isolated throughout history and therefore experienced a restriction on the number and diversity of organisms to establish themselves on the land. The restriction was caused by the isolation as only the ones with long-distance dispersal mechanisms were successful- due to this, the Hawaiian Islands lack faunal and floral groups that are key components for tropical communities. Examples of these groups include terrestrial reptiles and amphibians.

Generally, fauna and flora go through a process in order to reach islands. Floating vegetation, seeds and animals (on top of vegetation) can reach an island and then colonize it if it was able to withstand the saltwater and sun. Groups such as mammals and amphibians would not be able to survive over long distances due to their energy needs (mammals) and biology- amphibians have porous skin which would lead to them being poisoned by the seawater. Organisms like reptiles that have a leathery skin to prevent the loss of moisture would be able to successfully migrate over the ocean. An alternative way to reach an island includes getting picked up by the wind and be dispersed wherever the wind blows. In order to be dispersed via the wind, the organism needs to be light and not dense such as insects or spiders. Spiders can produce their web and then float it in the air in order to be carried away by the wind.

After the organisms have reached an island they tend to evolve and adapt according to their new habitat. Some lose features that they no longer need, such as the wings that flew them to the island while other animals and plants can colonize an area rapidly as there aren’t many predators on a new island. Due to the area being populated by a limited number of species that are all in need of resources, they evolve and adapt to fill different niches- if this is not done extinction could occur. Islands are inhabited by various species of plants and animals that either had to find their way to it or evolve while it was there.

A theory that aims to model the diversity and richness of an island’s species is the Equilibrium Theory of Island Biogeography by MacArthur and Wilson. This theory takes into account the size of the island as well as the distance from the mainland and is based on the process of immigration and extinction. This theory states that all species have an equal chance of making their way to an island as well as going extinct after arriving. The Earth is constantly changing- making new things and calling back others. Islands are the perfect example of the power of nature and the ability to create and establish life by overcoming challenges. By observing how island systems originate and function a lot can be learned.