THE DISTANCE EFFECT
Species richness decreases as a community is more geographically isolated, partly due to the distance effect. Island communities are less diverse than mainland communities with similar environments, because many species experience difficulties in reaching and colonizing islands that are far away. If a species undergoes local extinction, then recolonization of isolated islands or mountaintops by other members of that species is less likely. Isolated areas, which tend to be smaller, also may contain fewer potential ecological niches. This is why the percentage of bird species (with respect to New Guinea, at 100%) on islands in the South Pacific declines as the distance from New Guinea increases.