The maintenance of the ant mosaic in Ghana cocoa farms.
Abstract
The factors that maintain the mosaic distribution pattern of dominant ants in cacao plantations in Ghana were investigated in a series of observations and in surveys and a field experiment in which dominant ants were selectively mechanically removed from plots of cacao.The heterogeneity of the environment was one factor contributing to the mosaic or patchy ant distribution. For instance, different species of ants occurred in particular densities of cacao canopy. Although Macromischoides aculeatus (Mayr) was associated with dense cacao canopy, it spread into trees with a thinner when adjacent dominants were removed. This suggests that its distribution was also limited by competition from other ants. The interspecific ant mosaic is maintained by a combination of competition and habitat requirements. Interspecific competition may be for food or for nesting and foraging sites or it may take the form of aggressive competition between adjacent colonies or mature colonies and founding queens.Aggressive behaviour was particularly intense between inter-specific blocks of the mosaic. M. aculeatus was able to reduce competition with Crematogaster depressa (Latr.) within its territory by spacing out its foraging time, and C. castanea F.Sm. could co-exist with Oecophylla longinoda (Latr.) by adopting a similar colony odour. O. longinoda may aid species segregation by having queens which select their habitats when dispersing. New colony establishment was rare in mature cacao since changes in the ant mosaic were usually compensated for by lateral spread of existing colonies. The role of dispersing queens is probably more important in developing cacao farms. Climate and weather influenced the structure of the mosaic by directly influencing the features of the habitat, the availability of certain types of food or by physically weakening colonies of certain species. It would be feasible to alter the structure of the ant mosaic by artificially interfering with the competitive balance between species or by altering the nature of the habitat so as to favour the spread of certain species.