How Isolation Affects Rates of Turnover of Species on Islands

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Abstract
Turnover occurs as species become extinct and/or colonize an island. MacArthur and Wilson predicted that turnover rates should be inversely related to island area and to island isolation. There is a weak inverse relation between turnover rates and island area among birds on islands in Gatun Lake, Panama. There is a much stronger relation between turnover rates and island isolation, but it is modal. Turnover rates first increase with increases in isolation and then decline with further increases in isolation. A model of stochastic population fluctuation predicts this modal relationship whenever immigration rates are large. This condition is met by the study system. The islands are all within four km of the mainland, and avian immigrants arrived frequently. When immigration rates are lower, the model predicts that turnover rates should always be inversely related to isolation.