Figure 7.17

Degrees of barriers. Levels of resistance to dispersal vary depending on distance from the source of dispersing species and the severity of the barrier. Here, a continental mainland represents the source of species. The species on island 1 are filtered by the ocean: Some species from the mainland reach island 1, but not all. The distance to island 2 filters most dispersers. Reaching Island 3 directly from the mainland through sweepstakes dispersal is exceedingly unlikely. Using the islands as stepping-stones, however, increases the chances a species can reach the remote island.