Islands have long been of interest to ecologists. Their isolation from the mainland means that species tend to live undisturbed by invasion from non-native plants and animals. Over time, they evolve to adapt to their environment, creating an ecosystem unique to that island.The world’s islands range from those with significant human populations and economies such as New Zealand and Cuba, to tiny uninhabited patches of rock with just a few species trying to survive.
Island ecosystems are vulnerable, particularly if they are small and remote. Limited resources means that species are more likely to become extinct on an island than they are on the mainland. Therefore, islands are home to many of the world’s endangered species. An island is sometimes colonized by new species dispersing there, especially if it is not far from the mainland. There are many conservation projects underway around the world that support island ecologies by monitoring the health of species and protecting their habitats.
Historical Background and Scientific Foundations
An island is an area of land that is surrounded by water. Islands and their surrounding waters cover around one sixth of Earth’s surface. Today, over 100,000 islands are home to 500 million people. There are various types of islands. Old continental islands, including New Caledonia and New Zealand, were once part of a continent. Hawaii is an oceanic island and volcanic in nature. There are also coral atolls and the barrier islands that are found parallel to and close to mainland coast. Ecologists sometimes broaden the definition and call an island any area surrounded by some kind of barrier, such as a lake surrounded by forest.
Islands have played an important role in our understanding of ecology and evolution, with the most famous example being the exploration of the Galapagos Islands by English naturalist Charles Darwin (1809–1882). The Galapagos are an archipelago of 16 main islands lying around 600 mi (965 km) from Ecuador, to which they belong. Darwin’s visit was part of his famous voyage on the ship The Beagle, which started in 1831 and lasted for five years. The islands have a harsh landscape formed of volcanic rock and each has its unique population of species, including thrushes, finches, giant tortoises, turtles, and cacti. It was the finches that fascinated Darwin most. He counted 14 species (now known as Darwin’s finches) of these brown or black, rather insignificant looking, birds. What was striking was the variation in their beak size and shapes, and their songs. Darwin later realized that each finch species came from a different island and its beak had evolved to adapt to the food source available on its native island.
The Galapagos Islands have a similar climate but lie separated from one another by deep, fast water, and they are undisturbed by strong winds. These factors create a degree of isolation on each island that allows the local ecological communities to evolve undisturbed by the migration of plants and animals from elsewhere. Taking the finches as an example, the birds whose beaks were best adapted to the existing food source, such as seeds or berries, would live longer and produce more offspring. Therefore, the next generation would have more individuals with well-adapted beaks, and this characteristic would gradually evolve in line with the nature of the food supply, eventually leading to speciation, which is the separation of different species. This, in a nutshell, is what drives evolution and shows why an island is a good place to study such trends.
In 1967, the biologists Robert MacArthur and Edward Wilson put forward their theory of island bio-¥geography. This explains why islands tend to have a lower biodiversity than the mainland. Here biodiversity means the variety of species living in a particular habitat. Islands are limited in their resources so they can support fewer individuals of each species. Competition for resources may also cause some species to become extinct. The isolation of an island also means that the migration of species from elsewhere is limited. The ability of organisms to travel to new habitats is known as dispersal. Most birds can fly and therefore may travel very long distances. Seeds are borne on the wind and lighter ones will travel farther. Organisms with limited dispersal will tend to be endemic, or stay in one place, like turtles, lizards, and tortoises.
Overall, new species are not very likely to colonize an island, and its species will tend to be endemic. The smaller and more remote an island is, the more likely it is that the trend to extinction will outweigh the trend to colonization and the less biodiverse its ecology will be. Oceanic islands tend to share many plant and animal species with the mainland, while more remote islands tend to be populated with endemic species. MacArthur and Wilson provided evidence for their theory with their studies of reptiles and amphibians in the islands of the West Indies. Cuba, which is 100 times bigger than nearby Monserrat, has 10 times as many amphibians.
WORDS TO KNOW
BIODIVERSITY: Literally, “life diversity”: the wide range of plants and animals that exist within any given geographical region.
COLONIZATION: The process by which a species populates a new area.
EXTINCTION: The total disappearance of a species or the disappearance of a species from a given area.
SUSTAINABLE: Capable of being sustained or continued for an indefinite period without exhausting necessary resources or otherwise self-destructing: often applied to human activities such as farming, energy generation, or the maintenance of a society as a whole.
Impacts and Issues
Island species are naturally vulnerable to the threat of extinction. Islands contain more rare, endangered, or threatened species than mainland habitats. They act as ecological indicators, being the first to show the impact of threats like climate change and the introduction of invasive species. Other impacts include rising sea levels and pollution due to changing land use. It can be diffi-
cult to combat these challenges because of the isolation of islands and their relatively small human populations.
Work is going on all around the world on the conservation of islands, supported by organizations like the Nature Conservancy. The Parties to the Convention on Biodiversity launched the Global Island Partnership in 2005, with the aim of protecting and managing in a sustainable manner the world’s island ecosystems.
A good example of ongoing conservation involves Santa Catalina, one of the eight Channel Islands in the Pacific Ocean off the coast of Southern California, and the only one with a significant human population. The ecosystem of the Santa Catalina includes trees, shrubs, wildflowers, grasses, ferns, insects, birds, reptiles, mammals, amphibians, fungi, and lichens. It is home to at least 16 endemic species including the Island fox, Beechey’s squirrel, the California quail, a number of beetles, and six plant species.
One of the ecological threats on Santa Catalina involves the introduction of non-native species that may harm habitats and compete with endemic species. Santa Calatina has a number of these, such as rats, feral cats, goats, and starlings. For example, a small number of bison were introduced in 1924 as part of a filming project. Research by the Catalina Island Conservancy shows that the bison have had a number of adverse effects. They themselves are in poorer health than bison on the mainland, suggesting that Santa Catalina is a less-than-ideal home for them. Their foraging for food is begining to destroy some of the plants, including the endemic scrub oak, and they also act as an agent for the dispersal of non-native grasses, which could upset the native plant community still further.
A recent study of the bison’s impact on island ecology has suggested that their impact could be considerably reduced if they were confined to a smaller area and their numbers kept to no more than 150 to 200. Their removal from Santa Catalina could be even better for the health of the ecosystem, but there are other considerations. The animals are a considerable tourist attraction and residents regard them as part of island life. The Santa Catalina bison exemplify the ongoing challenge of sustainable management of island ecosystems.
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