Colonization of the Krakatau Islands by Land Birds, and the Approach to an Equilibrium Number of Species

The colonization of the Krakataus by resident land birds is re-examined in view of two additional good data points, 1951 and 1984-86, which were not available to MacArthur & Wilson (1967) in their seminal work on island biogeography. The bird data for all years were carefully assessed with respect to the explicit criteria needed for the calculation of rates of extinction, immigration and turnover; these parameters are defined and calculated. Calculated from data from the two most recent surveys, the equilibrium number of species (S) for Rakata, the largest island, is from 48 to 56 and for the archipelago as a whole from 44 to 58; the ranges reflect alternative assessments of the records. By fitting a colonization equation to data from all surveys, S for the archipelago is estimated as 36 species, and by a procedure attempting to account for cryptoturnover, 38 species. These estimates compare with 30 (MacArthur & Wilson 1967) and 40-45 (Mayr 1965). Thirty-seven species were present during 1984-86. The establishment of the secondary forest and closure of the canopy had the greatest effect on colonization parameters of resident land birds since 1883. Immigration and turnover peaked in the period 1908-21 and fell sharply over the next decade, when, in contrast, extinctions reached a peak. The species that have colonized the Krakataus are mostly wide-ranging species that are found in both Java and Sumatra and have wide ecological tolerances. The earlier colonists were open country, generalist species, but since forest formation there was an increase in the proportion that are true forest species and more specialist feeders; more recent colonists have been aerial predators and insect feeders. Analysis of the colonization data suggests that the first resident land birds colonized the Krakataus one or two decades after the 1883 eruption. The newly emergent island, Anak Krakatau, has provided early seral habitats for species that have been lost from the other islands of the archipelago because of ecological succession and consequently the island provides an ecological refuge, postponing the extinction of birds that depend on these open habitats. Anak Krakatau's emergence and continued existence will reduce species turnover and delay the achievement of equilibrium on the archipelago.