Recovery of ant species diversity on the Krakatau Islands, Indonesia
Graduate School of Science and Engineering, Kagoshima University, Japan
Krakatau, or Krakatoa in English, is a name collectively applied to the four small islands located in the Sunda Straits between Java and Sumatra, Indonesia. This island group, the Krakataus or Krakatau Islands, has attracted many field biologists, especially biogeographers, because it has provided the stage for an natural experiment in studying dynamic island biogeography. After the catastrophic eruptions in 1883 that are said to have terminated almost all biota on the Krakataus, the newly colonising fauna and flora were intensively studied from 1919 until 1934 by K. W. Dammerman and W. M. Docters van Leeuwen to reveal the recolonisation process of plants and animals. We have conducted myrmecological surveys on the Krakataus in 1982 and 2005-07 to monitor the ant colonization after Dammerman’s survey. All the collection records of ants obtained in the five survey periods since the catastrophic eruption in 1883 are presented for each of the four islands of the Krakatau group. In 2005-07 the actual species number was 99, and the colonisation curve was still rising without any indication of reaching an equilibrium species number. The equilibrium has not been reached yet probably because 1) the ants are generally passive dispersers so that the colonization process should be slower than in active dispersers like birds, 2) the pool of ant species on Java and Sumatra is so enormous that the depletion effect is not visible, or 3) the extinction curve actually declined with species buildup. The speed of colonization was different among groups of ants. Formicinae were earlier colonists, while Myrmicinae and Ponerinae tended to have arrived later at both genus and species level. Nesting biology of ants were also studied. Among the 49 species for which the nesting site was confirmed 74% were found from dead wood and twigs, while 14% from ground surface and in soil. A Leptogenys species adopting fission as colony mutiplication was found in 1982 and 2007. All this indicates that the immigration of many species was by rafting on the sea but that dispersal by air also should have occurred. Early colonisation of pioneer plants with extrafloral nectaries, i.e., Macaranga tanarius and Hibiscus tiliaceus, may have been important for the early ant colonizers to survive under poor vegetation. Some species preferring disturbed habitats like Tetraponera rufonigra, Polyrhachis dives etc., had been common on Rakata and Sertung in earlier periods (1909-1933), and on Anak Krakatau in 1982, but was not confirmed in 2005-07 on any of the four islands. Probably these species became extinct due to the reduction of disturbed area on Rakata and Sertung along with the development of forests. Most turnover events are considered to have occurred in association with vegetation succession. Sixteen tramp species have been so far recorded, of which only one (Pheidole megacepahla) has become extinct. On the whole Krakataus tramp species accounted for 15% (15/99) of all the species collected in 2005-07. The proportion of tramp species was relatively constant throughout the survey periods (0.14-0.18), and distinctly higher on Anak Krakatau (0.23 for 2005-07) than on Rakata (0.13). Strangely the famous tramp ant Solenopsis geminata has never been seen on the Krakataus. In 2005-07 only a few tamp species, e.g. Monomorium floricola and M. destructor, were relatively common, others including M. pharaonis being rather rare. The ant species diversity is constantly recovering. The actual number (99) during the fifth survey period was already larger than that of the Thousand Islands Archipelago (48) located off Jakarta. The latter island group has the tramp species rate higher than that of the Krakataus (0.21 vs. 0.15). This means that human habitation affects both the ant species diversity and the proportion of tramp species.
Keywords: Insect, Sunda Straits, colony and Thousand Islands Archipelago