THE FORM OF THE CURVES - A DIRECT EVALUATION OF MACARTHUR AND WILSONSCLASSIC THEORY

1. We calculate the yearly numbers of bird species immigrating to - an d becoming extinct on - 13 small islands of the British Isles, using a long and relatively complete data record. 2. We estimate the size of the colonist pool for each island using four methods. 3. We assume tha t immigrations and extinctions are distributed binomially, and use a m aximum likelihood method to fit concave immigration and extinction fun ctions to the data, utilizing all four species pool estimates. 4. Exti nction rates increase significantly and consistently with increasing n umbers of breeding species on each island. For nine of the 13 islands the extinction functions are significantly concave. 5. Immigration rat es decrease consistently with increasing numbers of breeding species o n each island. Seven islands have significantly concave immigration fu nctions. 6. Immigration rates and extinction rates decline consistentl y, but not significantly, with island distance and island size, respec tively. The number of breeding species does not always reflect the num ber of species likely to have reached an island. Moreover, some specie s may choose not to breed when their chance of extinction is high. The se factors; plus the modest range of island areas and distances in our database, reduce our chances of finding the theoretically predicted e ffects of area and distance on extinction and immigration rates.