Diversity, evolutionary specialization and geographic distribution of a mutualistic ant-plant complex: Macaranga and Crematogaster in South East Asia

The most conspicuous and species-rich ant-plant mutualism in the Malesian r egion is found in the important pioneer tree genus Macaranga, yet little is known about the identities or community ecology of the species involved. O ur studies have revealed a far more complex system than previously thought. This paper presents the first extensive investigation in the whole distrib ution area of myrmecophytic Macaranga. All ant-inhabited species were restr icted to the moister parts of SE Asia: Peninsular Malaysia, South and East Thailand, Sumatra and Borneo. We found a rather strict and similar altitudi nal zonation of myrmecophytic Macaranga species in all regions. Here we foc us on the majority of the 19. Macaranga species obligatorily associated wit h ants of the genus Crematogaster. We identified a total of 2163 ant queens which belonged to at least eight (morpho)species of the small subgenus Dec acrema as well as to one non-Decacrema (probably from Atopogyne). The ant s pecies there not randomly distributed among the Macaranga species but disti nct patterns of associations emerged. Despite common sympatric distribution of Macaranga species, in most cases a surprisingly high specificity of ant colonization was maintained which was, however, often not species-specific but groups of certain plant species with identical ant partners could be f ound. These colonization patterns usually but not always mirror existing ta xonomic sections within the genus Macaranga. Possible mechanisms of specifi city are discussed. The results are compared with other ant-plant mutualism s. (C) 1999 The Linnean Society of London.