Why is the flora and fauna of many islands unique?

Since islands are completely surrounded by water, they are totally isolated from other land masses, their vegetation, and wildlife.

 Plants and animals from a nearby mainland can reach an Island in different ways. Seeds can be carried to an island by the wind. Sometimes, seed capsules like those of the coconut, are washed ashore by the waves.

 These seeds take root on the island, and grow into plants. People who come to inhabit the island also bring saplings and plant them. The new plants often adapt to the different conditions found on the island, and become one of a kind species, not found anywhere else in the world.

Similarly, many islands have animals that have adapted themselves to become a new species that is unique to that particular island. Lemurs, Tasmanian devils, and Galapagos penguins are some examples of such animals. Tuatara is a very rare creature that is found only in New Zealand.