Interspecific competition and frequency changes among ants in Solomon Islands coconut plantations.
The following is virtually the author's summary of this account of investigations on Guadalcanal (Solomon Is.) based on observations by the author and previous workers [cf. RAE A 44 465; 48 58]. Four species of ants, Oecophylla smaragdina (F.), Iridomyrmex cordatus F. Sm., Anoplolepis longipes[Anoplolepis gracilipes] (Jerd.) and Pheidole mega-cephala (F.), are involved in the control of the Coreid Amblypelta cocophaga China, which causes premature nut-fall of coconut [cf. 47 168; 48 59]. The ants compete to occupy coconut palms and are mutually exclusive. The competition between them is complex, continuous and one-sided, with relative numbers a major factor in the outcome of any interaction. In the competition for palms between established colonies, the order of decreasing competitive ability of the species is I. cordatus, Anoplolepis longipes and P. megacephala (equal), O. smaragdina, and other minor species. This ranking depends on the size of the individuals, the population structure, and the main nest site and food source of each species. In the ability of the species to colonize vacant palms, the order is reversed. Although l. cordatus is dominant over any other single ant species, it has a low rate of increase. This is thought to be due to the nature of its food and also to the effects of combined competition from several other species. The frequencies of the four species in certain plantations on Guadalcanal have been recorded, mostly in unpublished reports, at intervals of 2-3 years since 1948. No results have been reported since 1958, however, and the records are here brought up to date. P. megacephala, the dominant species before the Second World War, was still common, although declining, in 1948. It was succeeded by O. smaragdina and then from 1952 until the late 1960's A. longipes was frequent. An account of these changes is given and of the subsequent fluctuations in frequency of O. smaragdina, P. megacephala and minor species. Throughout the 20 years, populations of I. cordatus typically remained stable or increased slowly. It was not possible earlier to account for these frequency variations, and some suggestions are made. In O. smaragdina, there is evidence of a cycle of approximately eight years, observations on individual colonies indicating that this is their average life span. The probable effects of a war-time vegetation change on the diversity of ant faunas is discussed, and it is concluded that the post-war return to well maintained plantations affected ant frequencies and was responsible for a sequence of population fluctuations that is still continuing.