T. R. New

Colonisation of the Krakatau islands by invertebrates

Arrival of invertebrates on the Krakatau islands, and the development of invertebrate assemblages there since 1883, are reviewed. Many data are imprecise, and difficulties of interpretation are outlined. A few well-known groups, such as butterflies, are used to suggest that much colonisation so far has been by relatively generalist species,and that the rate of addition of new species may become slower as the less vagile specialists characteristic of later successional vegetation may find natural colonisation harder. Studies of the assemblages developing in association with particular plant species are likely to be helpful in assessing future trends of invertebrate colonisation.

Anak Krakatau — a colonization model within a colonization model?

AbstractThe ppaer re-examines three suggestions previously made concerning the colonization of the Krakatau islands since the extirpating 1883 eruption that involve the more recently emergent volcanic island Anak Krakatau, which itself suffered a devastating eruption in 1952. The suggestions re-addressed in the light of recent comments by other workers are: (1)Anak Krakatau offers, in general terms, an analogy of the early successional processes that occurred on the three older islands in the first decades after the 1883 extirpating eruption.(2)Anak Krakatau (with the Sertung spit) may have acted as an ecological refuge for open country species whose habitat on the three older islands declined as a result of vegetation succession.(3)Mainland open country species that did not colonize the archipelago when the appropriate vegetational successional stage occurred on the three older islands may now do so through a reopened early-successional window — Anak Krakatau (and the Sertung spit). Mann-Whitney tests carried out on species naturally colonizing the Krakatau group and those colonizing Anak Krakatau since its self-devastating eruption of 1952 showed that in the case of spermatophytes, pteridophytes and resident land birds, the biota available on the older islands has contributed to Anak Krakataus developing biota with a bias towards species that arrived on the archipelago early. There are indications also for reptiles, frugivorous birds, bats and figs that successful colonists of Anak Krakatau from the older islands tend to have been early colonists to the archipelago also. For butterflies, however, Mann-Whitney does not indicate strong correspondence between early arrival on the archipelago and occurrence on Anak Krakatau. The butterfly fauna is ecologically strongly disharmonic, heavily biased towards species of coastal and near-coastal habitats and the coastal flora of the archipelago has changed little since 1897, Anak Krakataus flora comprising almost entirely such species. On both the archipelago and Anak Krakatau, over similar periods since their sterile beginnings, the proportion of marine-dispersed plant colonists has steadily declined and the animal-dispersed fraction increased. Thus in two aspects of colonization — order of species arrival and changes in dispersal mode spectra of plants, the colonization of Anak Krakatau from the older islands has been analogous to that of the older islands from the mainland in the first decades after 1883.Differences between the two models are: the presence of close sources, the early availability of animal dispersers of plants in the case of Anak Krakatau, and continuing volcanism. The second of these may explain the significant proportion of animal-dispersed taxa in the early plant colonists of Anak Krakatau after 1930 but not of the archipelago after 1883. Volcanism has affected Anak Krakataus biotic development sporadically since its birth and eruptions in 1972/3 probably constrained increase in species numbers and seriously set back plant succession. In contrast, the archipelagos biotic development has been affected only at a later stage in the process, and with different effects on different islands.Evidence is advanced that Anak Krakatau (with the Sertung spit) may have acted as an ecological refuge for some 18 open country animal species of which the preferred habitat on the older islands was gradually extirpated as a result of successional processes, and as a respening ealry-successional window for as many as 12 earl-successional mainland animal colonists, for which the window of opportunity on the older islands had become closed by succession to mixed forest.