ANTS ON SWOLLEN THORN ACACIAS - SPECIES COEXISTENCE IN A SIMPLE SYSTEM

On the black cotton soils of the Laikipia ecosystem in Kenya, two swol len-thorn acacia species support nine ant species, four of which are a pparently obligate plant-ants. Among the ants, there are five species of Crematogaster, two species of Camponotus, and one each of Tetrapone ra and Lepisota. Acacia drepanolobium is host to four ant species that are both common and mutually exclusive. These four ant species, and a n additional non-exclusive ant species, tend to occur on trees of diff erent sizes, implying a succession of ant occupants. Nonetheless, all four exclusive species occur in substantial proportions on trees of in termediate size. There is direct evidence that an early successional a nt species (Tetraponera penzigi) is actively evicted by two late succe ssional ant species in the genus Crematogaster. There was also some ev idence of height differentiation among ant species resident on A. seya l. Different acacia-ant species had different direct effects on A. dre panolobium. Extrafloral nectaries were eaten and destroyed only on tre es inhabited by Tetraponera. Axillary shoots were eaten only on trees inhabited by C. nigriceps (potentially another early successional ant) . This was associated with more new terminal shoots and healthier leav es than other trees, but also the virtual elimination of flowering and fruiting. Different resident acacia;ant species also had characterist ic relationships with other insects. Among the four mutually exclusive ant species, only Crematogaster sjostedti was associated with two spe cies of Camponotus, at least one of which (C. rufoglaucus) appears to be a foraging non-resident. A. drepanolobium trees occupied by C. sjos tedti were also far more heavily infested with leaf galls than were tr ees occupied by other ant species. A. drepanolobium trees occupied by C. mimosae and C. sjostedti uniquely had tended adult scale insects. T his diversity of ant inhabitants, and their strikingly different relat ionships with their hosts and other insect species, are examples of co existing diversity on an apparently uniform resource.