Cut off from the continental land masses by stretches of ocean water, islands often have a range of plant and animal life that is uniquely their own. Some islands were once part of a larger land mass, while others are formed by underwater volcanoes rising from the ocean floor. As soon as an island forms, it begins to be colonized by plants and animals. Islands close to the main­land are colonized more quickly than remote ones.

Plants

Plant seeds can travel across the oceans. They rely on the wind or water to carry them to islands. Seeds can also be carried in the fur or feathers of animals, or even in their digestive systems. The light seeds of small plants can travel further on the winds than those of large trees, so there are sometimes few, if any, trees on the most remote islands. But these small plants may, over many years, evolve into larger ones.

Adapting to island life

Large predators are often absent from islands, which means that some island animals do not need to be fast-moving. It also means that some birds no longer need their escape mechanism of flight. Their wings become small and useless, since they spend all their time on the ground. This adaptation produces flightless birds like the takahe, a New Zealand species of rail—but they become easy prey when predators are later introduced by humans.

Taking advantage of abundant food supplies, some animals, including insects, can grow to huge sizes in island habitats. The giant weta, a cricket-like insect from New Zealand, is about four times as large as an ordinary cricket. Having no natural predators on its island, it has become flightless, but is now threatened by introduced predators, such as rats.

Galapagos species

There are three species of land iguana living on the Galapagos Islands, two of which are found on just one island apiece, including a pink variety that was discovered only in 1986. The iguanas were once abundant, but their numbers have since been reduced by hunting, and from being preyed on by rats, cats and dogs.

Giant tortoises

Giant tortoises are found in the Galapagos Islands and also in the Seychelles in the Indian Ocean. Because of the lack of predators, they can be large, slow-moving animals yet remain unthreatened. Some giant tortoises graze on low grasses, and have a broad shell that fits close to the neck. Others have domed shells with high, arched openings, allowing them to stretch their necks up to taller vegetation. The introduction by humans of grazing animals reduces the tortoises’ food supply and threatens their survival.