Macaranga

The following is based on the Maschwitz et al. 2004.

The most prominent ant-plant system of perhumid South-East Asia consists of the pioneer tree genus Macaranga and its manifold associations with ants. The genus Macaranga (Euphorbiaceae) comprises species which are not ant-inhabited and/or facultatively ant-associated as well as obligate ant-plants (review in Fiala 1996, Fiala et al. 1999). While food bodies and/or extrafloral nectar produced by non-myrmecophytic species attract various opportunistic ants for occasional visits, obligate myrmecophytes also offer nesting space for their specific ant partners. In obligate myrmecophytes, the ants also harvest honeydew produced by specific scale insect partners maintained inside the internodes (Heckroth et al. 1998). In exchange for the provision of food and nesting sites, the ant partners effectively protect their host plants against herbivore damage, competition by climbers (e.g., Fiala et al. 1989, Heil et al. 2001) and fungal infection (Heil et al. 1999). For further details on ecology and biology of ant-associated Macaranga we refer to the extensive work of our group in the last 15 years (for overview see, e.g., Fiala & Maschwitz 1990, 1992; Fiala 1996, Fiala et al. 1999, Blattner et al. 2001, Federle et al. 2001, Feldhaar et al. 2003a, and references therein).

Macaranga comprises about 280 species with a range stretching from West Africa throughout the Oriental region eastward to the Fiji Islands (Whitmore 1973). It is the only plant genus in the palaeotropics exhibiting such a substantial radiation of myrmecophytes (about 29 known myrmecophytic species in the Malayan Archipelago, see recent revision of Davies 2001). Most myrmecophytic species are found in two sections (Pachystemon and Pruinosae) each of which, although closely related, represents separate evolutionary acquisitions of myrmecophytism (Blattner et al. 2001, Davies et al. 2001). The majority of partner ants belong to the myrmicine genus Crematogaster (subgenus Decacrema). Additionally, two Macaranga species (M. puncticulata and M. lamellata) were found to be specifically inhabited by species of the genus Camponotus, subfamily Formicinae (Maschwitz et al. 1996; Federle et al. 1998a, b; Fiala et al. 1999).

The center of diversity of this group of three-partner associations (plants, ants and scale insects) is Borneo, spreading westward to Sumatra and northward to the Malay Peninsula (Fiala et al. 1999), with Borneo hosting many more endemic host plant species (see revision of Davies 2001) and more ant lineages as well (Fiala et al. 1999, Feldhaar et al. 2003b). So far no information exists on the age and geographic origin of the Macaranga-ant system. The first (nonmyrmecophytic) Macaranga species – thought to be bushlike pioneers of open places – was hypothesized to originate in the Oligocene or early Miocene (38 to 15 mya) when the climate was predominantly seasonal and dry (Slik & van Welzen 2001). Rainforest, the habitat of all myrmecophytic Macaranga species, colonized the region in the mid-Miocene when the climate changed to perhumid (Morley 2000) and also when the greatest northward extension of rainforest occurred, so we assume a development of the first close Macaranga-ant associations not before this period. For any interpretation of the evolutionary development of the Macaranga-ant associations, information on their biogeography is required. The section Pachystemon, which comprises most myrmecophytic species, is restricted to the moist tropics within the floristic boundaries of West Malesia (sensu van Steenis 1950) and does not extend into the more seasonal monsoon forest regions (e.g., Whitmore 1975, Fiala et al. 1999, Davies 2001). We do not know what limits the distribution of the myrmecophytic associations. Climatic factors certainly play an important role since low temperature and increased dry periods do not allow continuous food production of the plants for their partner ants. Inhabited myrmecophytic trees in the Malay Archipelago do not occur above 1250–1400 m (Fiala et al. 1999), although ants not associated with myrmecophytes were still found foraging on vegetation up to 2300 m in the Mt. Kinabalu area, Sabah (Kern 1996). Since ants and plants as well as associated coccids are usually not able to survive without each other (e.g., Fiala 1996, Heckroth et al. 1998), and ants were never found outside their host plants, it is difficult to tell whether Decacrema ants alone could survive in other climatic zones. The interdependency of all partners is so strong that obviously their distribution range is always correlated.

Related Pages

 * Ants and Plants
 * Myrmecophytes