Thor Heyerdahl hypothesized Polynesians arrived by boats from lands to the east, and mounted the Kon Tiki series of voyages in the 1940s to prove it. Islands that are more isolated from the mainland will have a much lower biodiversity than islands near the mainland. The intervening sea between islands act as a barrier to dispersal. Think of a stepping-stone pattern of islands, with the farthest island having fewer species. Regarding the successful establishment upon arriving, the area of an island is most important as an island with greater area will have more hospitable habitats for arriving organisms in which to thrive.
Island life is also more hazardous .
Refugia , isolated areas protected from natural catastrophes, are scarce on an island, making survival difficult. Other disturbances include tsunami, wildfires, earthquakes, and landslides. By contrast, in the source region, chance extinction of a species is offset by more rapid immigration of a new species due to ease of travel. Therefore, the source region will always have a richer biodiversity , even though both may have similar habitats. For example, on mainland Panama in Central America, a very small two ha plot of tropical rainforest contained 56 bird species and 58 shrub species.