Introduction

On August 27, 1883 Krakatau, an island about the size of Manhattan located between Sumatra and Java, underwent a series of volcanic eruptions releasing as much energy as 100 megatons of TNT (Wilson 1992). Magma, ash, and rock flew 5 km into the air and fell back into the sea, creating a tsunami 40 m in height, washing away villages in Java and Sumatra, killing 40,000 people. Waves were still a meter high when they came ashore in Sri Lanka. A total of over 18 cubic kilometers of rock and ash was thrown into the air with dust and sulfuric acid aerosol reaching 50 km into the stratosphere, where their effects were seen as brilliant sunsets for several years thereafter. All of this airborne material produced "darkness at noon" in areas near the former Krakatau.

Only the southern end of Krakatau remained. This island, which became known as Rakata, was covered by pumice 40 m thick. The pumice had been heated to between 300 and 850 °C, and all living things had been destroyed; Rakata was a sterile island. Yet living things soon began colonizing this lifeless rock. Nine months after the explosion a visitor found a small spider spinning its web. In the fall of 1884, a year after the eruption, biologists found a few shoots of grass. By 1886 there were 15 species of grasses and shrubs; by 1897 there were 49 species; and in 1928 300 species of plants were found. In 1919 there were patches of forest; by 1929 most of the island was forested, forcing the grasses into small pockets (Wilson 1992).

What Wilson described, as outlined in the preceding paragraphs, sounds like a typical successional sequence, proceeding from a community of colonizing species to a mature or "climax" forest. What we want to emphasize, as we consider the topic of metapopula-tions, is the two processes at work on Rakata. Obviously one of those processes is colonization. New species continually arrive on the island from the nearby mainlands. The other process is local extinction. Many species that arrived on this island, and were recorded early in the twentieth century, are no longer present. Again, this may not be surprising to students of succession. So-called climax species are supposed to outcompete and eliminate earlier successional species. But is that what happened? At least one animal species that we would associate with the more mature community, the reticulated python (Python reticulatus), was present as early as 1933, but was gone by the 1980s. The bird community is perhaps more to the point. In 1908, 13 species of birds were recorded on Krakatau; by 1920 there were 31 species; and in 1933, 30 species were found. Wilson (1992) believed that an "equilibrium" number for Krakatau was approximately 30 species of birds, and that number had been reached by about 36 years. More important, however, is that the actual composition of the bird community has not remained stable. During the interval between the 1920 and 1933 surveys, five species of birds went extinct on Krakatau, to be replaced by four new species (MacArthur and Wilson 1967). For example, the sooty-headed bulbul (Pycnonotus aurigaster) and the long-tailed shrike (Lanius schach) had become extinct on Rakata between 1920 and 1933.

The history of Krakatau illustrates two major points: (i) local populations are continuously subject to the twin processes of colonization and extinction; and (ii) communities are continuously changing. Even when the number of species in the community is static, the composition of the community is not.

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