Why every island's wildlife ends up looking alike

Located to the east of Madagascar, the bountiful, volcanic French island of Réunion has sometimes been called a sister to Hawaiian volcanoes because of the similarity in their climate and geographical nature. Those familiar with its seemingly pristine vegetation may be surprised to find out that half of the plants there were introduced by humans, even though they arrived on the island in the 16th century. So how and why did some species arrive and not others?