Why do islands give rise to such unusual creatures?
1. Land of opportunity
Islands make up a sixth of all land on Earth. Some are formed from lava expelled by volcanoes under the sea, such as the Hawaiian Islands. Others, like Madagascar, are created when landmasses break apart over hundreds of millions of years.
Isolated from the mainland, a new island often provides a unique habitat. With its own set of conditions, life can evolve differently. Some creatures develop the most unusual traits to survive – and thrive – in their new home. In many cases, separated from their mainland ancestors, they eventually transform into new species altogether.
2. Making the journey
The water that surrounds islands makes them difficult to reach, but there are four main ways that animals and plants can discover remote areas.
Wind plays a major role in transporting spores from ferns and mosses. Because they are so light, they can be carried for thousands of miles. Some plants have seeds that are shaped specially for this purpose – including daisies and dandelions, relatives of which are found throughout the Galapagos Islands. Small insects and even tiny snails can be blown to new shores. Some spiders even make ‘balloons’ from their own silk to hitch a ride through the air.
Water surrounding new or remote islands provides a barrier for most species – but not marine animals, such as turtles and crabs. Unimpeded by the sea, these creatures are among the first to arrive on land. They may be drawn to the area in search of nesting grounds, or simply drift there on the ocean currents. Land animals can reach islands by floating on ‘natural rafts’, such as large logs or carpets of vegetation, or on debris discarded by humans. Some seeds can survive months or even years at sea too. Once afloat, they are at the mercy of ocean currents, which can propel them towards remote islands.
Migratory bird species can fly hundreds or even thousands of miles every year. Sea birds are efficient long distance fliers, so their journeys may simply take them to new islands along the way. But long migrations can prove more difficult for smaller birds, so they may settle on a new island if wind directions are against them. Seeds might hitch a ride too – attached to birds' feathers, feet, or in their guts. Other aerial creatures, such as bats and some insects, have also colonised islands.
Humans can introduce non-native species to islands.They may be brought in deliberately. For example, rabbits were introduced to New Zealand in the early 19th Century as a source of food and sport. They have since colonised the entire South Island, proving a particular nuisance to farmers and costing millions of dollars to control. However, some species are introduced accidentally. Mice and rats are notorious for hitching rides on boats. Marine animals can also make the trip in ballast water, which is used to stabilise ships but is loaded and discharged at different locations.
3. Evolution runs riot
When an animal or plant arrives at an island for the first time, the new environment will usually be different from the one it’s used to. As a result, a species will gradually adapt to survive or die out.
Over time, its appearance, behaviour and the way its body works might change – to become more successful at evading a predator, or more efficient at exploiting a new food source. For example, it might develop longer limbs, or lose those it no longer needs. Eventually, it may alter so much that it can no longer breed with its original species: it’s evolved into an entirely new species.
An island, especially a remote one, may be colonised by relatively few species. This allows the members of one species to exploit numerous different lifestyles, or ‘niches’ – a phenomenon called adaptive radiation. As the individual groups adapt to their different niches, they may evolve into distinct species. This is how one ancestor can eventually lead to the evolution of many new species – often looking and behaving quite different from each other.
4. Unusual adaptations
These three island creatures have evolved some extraordinary traits.
Blown to Hawaii on the wind, the ancestor of this moth (below left) fed on plants. But the abundance of fruit flies on the islands offered it an unprecedented opportunity to swap vegetarianism for a more nutritious protein-based diet. Rather than munch on shoots and leaves, the caterpillar lay in wait to ambush insects that happened to pass by. As a result of adaptive radiation, the islands have 18 species of carnivorous ambushing caterpillars. Their behaviour is unique to Hawaii. When the moth arrived, no other bugs that could grasp with their forelegs, such as praying mantises, had colonised the islands. So an unlikely candidate like a caterpillar occupied the carnivore niche. What’s more, the caterpillars do not switch to a vegetarian diet, even if they are starving. When they get hungry, they eat one of their own kind rather than a plant.
When the ancestors of lemurs arrived on Madagascar, the island they found was rich with different habitats and lacked competitors. Adapting to these new conditions triggered the evolution of at least 100 species – the most bizarre arguably being the aye-aye (below middle). Aye-ayes are primates, but they live like woodpeckers. As woodpeckers don’t inhabit Madagascar, the aye-aye has evolved to take advantage of this niche. Large incisors (for gnawing at bark) and special, elongated fingers allow them to acquire food inaccessible to other lemurs – such as grubs in trees.
True masters of disguise, leaf-tailed geckos (below right) present one of most stunning examples of adaptive radiation in Madagascar. Sharing the tree-living niche with some of their predators, such as snakes, they’ve evolved an extraordinary specialisation to survive. To blend into their surroundings, the reptiles mimic dead leaves or bark. There are at least 15 species – each evolving to become ‘invisible’ on different backgrounds in their habitats.