Diversity, evolutionary specialization and geographic distribution of a mutualistic ant-plant complexMacarangaandCrematogasterin South East Asia

Abstract

The most conspicuous and species-rich ant-plant mutualism in the Malesian region is found in the important pioneer tree genusMacaranga, yet little is known about the identities or community ecology of the species involved. Our studies have revealed a far more complex system than previously thought. This paper presents the first extensive investigation in the whole distribution area of myrmecophyticMacaranga. All ant-inhabited species were restricted to the moister parts of SE Asia: Peninsular Malaysia, South and East Thailand, Sumatra and Borneo. We found a rather strict and similar altitudinal zonation of myrmecophyticMacarangaspecies in all regions. Here we focus on the majority of the 19Macarangaspecies obligatorily associated with ants of the genusCrematogaster. We identified a total of 2163 ant queens which belonged to at least eight (morpho)species of the small subgenusDecacremaas well as to one non-Decacrema(probably fromAtopogyne). The ant species were not randomly distributed among theMacarangaspecies but distinct patterns of associations emerged. Despite common sympatric distribution ofMacarangaspecies, 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 found. These colonization patterns usually but not always mirror existing taxonomic sections within the genusMacaranga. Possible mechanisms of specificity are discussed. The results are compared with other ant-plant mutualisms.