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.

Macaranga griffithiana
Macaranga griffithiana in Peninsula Malaysia is a very common lowland ant-plant found mainly in open disturbed habitats with moist to swampy soils, i.e. on plains which can be flooded during wet times, on banks of streams, etc.. Roadside drains are frequently occupied by M. griffithiana. It was usually associated with Crematogaster (Decacrema) msp. 1 and with a specific coccid partner (Coccus caviramicolus) although a few other scale species also occurred. In south-east Thailand 160 km south-east of Bangkok, about 80 km from the Cambodian border, we found M. griffithiana, associated, however, with a different ant species: Colobopsis markli from the subfamiliy Formicinae.

Plant characters. M. griffithiana grew up to 15 m within forested habitats but was most common as a bushy treelet or broadly ramified shrub at open sites and forest edges. Thin young stem parts and branches, as well the undersides of the rather weakly trilobed leaves, are waxy and whitish in color. The stem becomes hollow by pith degeneration thus forming intermodal domatia. Seedlings can begin to develop domatia at a height of less than 0.5 m; below that height stems often still have a rather small diameter (about 5 mm only).

The paired stipules are rather short lived and persist only in the two or three most apical leaves of a shoot. Food bodies are found beneath the stipules and (as seen on saplings kept in the greenhouse) in large numbers on both sides of young leaves, preferably close to the veins.

Several characters of the association partners are characteristic of close myrmecophytic relationships:

– maintenance of the full set of ant-plant characters in M. griffithiana, i.e., formation of thin-walled, fragile domatia, and a high rate of food body production,

– the high specificity of associations at several separated subpopulations in open and in forested habitats, monopolization of trees by colonizing ants although other arboreal ant species were present,

– host plant localization, and domatium opening behavior (chewing of entrance holes into living plant tissue) of the colony-founding queen,

– frequent food body consumption by the partner ant,

– aggressiveness and probable pruning behavior (this was also reported from C. (Colobopsis) colonizing M. puncticulata, Federle et al. 1998a),

– presence of a single coccid partner species. This homopteran tending as well as food body harvesting can make the ants independent of food searching away from the plant.

All these characters taken together speak for a close association of all three partners. It obviously represents another example of the independent origin of a myrmecophytic association within the diverse genus Macaranga. The two other associations of Macaranga plants with Colobopsis species differ in many aspects to the one reported in this study. The Macaranga species involved are not closely related, M. griffithiana (West Malaysia, Thailand) and Macaranga lamellata (Borneo) belong to different clades within the section Pachystemon, and Macaranga puncticulata (West Malaysia) is even more distant, forming an own clade (Blattner et al. 2001, Davies 2001). On the ant side as well, initial phylogenetic analyses based on mitochondrial DNA sequence data of all Camponotus species involved revealed that C. markli is not closely related to either of the other Macaranga-colonizing Colobopsis species (J. Gadau et al., unpubl. results), i.e., has obviously evolved independently as a plant-ant.

Related Pages

 * Ants and Plants
 * Myrmecophytes