Island Biogeography Theory Equilibrium theory of island biogeography = explains how species come to be distributed among oceanic islands Also applies to “habitat islands” – patches of one habitat type isolated within a “sea” of others Explains how the number of species on an island results from an equilibrium between immigration and extirpation Predicts an island’s species richness based on the island’s size and distance from the mainland

Species richness results from island size and distance Fewer species colonize an island far from the mainland Large islands have higher immigration rates Large islands have lower extinction rates

The species-area curve Large islands contain more species than small islands They are easier to find and have lower extinction rates They possess more habitats

Small “islands” of forest rapidly lose species Forests are fragmented by roads and logging Small forest fragments lose diversity fastest Starting with large species Fragmentation is one of the prime threats to biodiversity

General principles of island colonization: 1) The closer the island to another land mass, the higher the probability of colonization.  2) The older the island, the more likely it will be colonized.  3) The larger the island, the more species are likely to be established.  4) Geographic isolation reduces gene flow between populations.  5) Over time, colonial populations become genetically divergent from their parent population due to natural selection, mutation, and/or genetic drift.