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Description: The genus Ficus is made up of more than 750 species from pantropical and subtropical origins (Lansky and Paavilianen 2010). Plants in the genus are all woody, ranging from trees and shrubs to climbers (Neal 1965). The potentially giant banyan (F. microcarpa) is an evergreen, medium to very large Ficus tree with a dense, spreading crown; it can grow up to 25 m (~80 ft) tall. The variable leaves of F. microcarpa are alternately arranged on leaf stalks; they are coriaceous, oblong, broadly elliptic or obovate, usually 5-8 cm (2-3 in) long, 3-5 cm (1-2 in) wide, glabrous and have entire margins. The epithet or species name, microcarpa, of this banyan refers to the diminutive size of the compound fruit, technically known as a synconia which is “sessile, arising among or just below the leaves, depressed-globose, 6-10 mm in diameter, subtended by 3 broadly ovate, more or less persistent bracts” (Wagner et al., 1999). The synconia or compound fruits are spread by birds, and a species-specific wasp (Euprestina verticillata Waterst.) is necessary for pollination. Indeed, the syconium (or fig) and the reproductive systems of species in the genus Ficus are exceptional, as Starr et al. (2003) described:
“Each species of Ficus has an associated species of agaonid wasp (Hymenoptera: Chalcoidea: Agaonidae). Ficus species can only be pollinated by their associated agaonid wasps and in turn, the wasps can only lay eggs within their associated Ficus fruit. For successful pollination and reproduction of Ficus species to occur, its associated pollinator wasp must be present. Conversely, for successful reproduction of agaonid wasps to occur, their associated Ficus species must be present [see Janzen 1979].”
Small as F. microcarpa fruits may be, this tree sometimes serves as a large ornamental tree planted in parks and gardens. F. microcarpa is distinguished by its relatively short but often very thick trunk and the fruits that are paired at their base; it is also notable for numerous, often lengthy aerial roots about the trunk or hanging hair-like from its lower branches. In addition, like other botanical members of the family Moraceae, F. microcarpa is characterized by the milky juice or white latex exuding from cuts. Overall this variable-sized tree species differs in its habit, occurring as an epiphyte (“air plant”) or as a subscandent (slightly spreading) shrub when young, and in maturity sometimes spreading out as an evergreen tree with large branches and numerous aerial roots hanging from its trunk and branches; these aerial roots sometimes reach the soil to form pillar-like appendages. Krieger (1943:17) summed up the uniqueness of the big banyan fig species such as Ficus microcarpa and Ficus prolixa (see below):
“Of special interest on account of their method of germination and growth are the giant banyans (Ficus spp.) of the forest [which] usually begin their existence upon other trees, sending down aerial [adventious] roots which interlace and grow together, clasping the trunk of their host, and eventually strangling it. They then lead an independent existence, their great spreading limbs sending down more roots, which are like pendent threads at first, but soon thicken after gaining a foothold in the earth, and serve as columns to support the great dome of foliage overhead, as well as to supple it with nourishment and moisture.”
Distribution: Ficus microcarpa has been often misidentified as F. retusa or F. nitida (syn. F. benjamina). According to Stone (1970:251): “It is difficult to distinguish [F. microcarpa] easily from F. prolixa, but the presence of cystoliths [appearing as minuit dots] only on the lower surfaces of the leaves in F. prolixa allows a distinction.” The native range of F. microcarpa reportedly extends from Ceylon to India, southern China, the Ryukyu Islands, Australia and New Caledonia; it is also cultivated and naturalized elsewhere. In its native range, such as parts of southern Asia, F. microcarpa thrives in moist regions, where it is common up to 6,000 ft (1,829 m; e.g., see Neal 1965).As a widely cultivated tree throughout the world, Riffle (1998) noted that F. microcarpa is one of the most common street trees in warm climates around the world. Indeed, today it is grown as an ornamental tree in many tropical locations of the world and in some areas it has become a problematic invasive species, such as Hawai’i where this species has become a serious large pest species in disturbed areas, choking out many other plant species. In the case of Hawai’i, and other places where it has become troublesome, Starr et al. (2003:1) described the deleterious advance of this tree:
“The pollinator wasp has been introduced to a number of places where the tree is cultivated, including Hawai’i, allowing this species to spread beyond initial plantings. F. microcarpa is a notorious invader in Hawai’i, Florida, Bermuda, and from Central to South America. Tiny seeds within small sized fruit are ingested by many fruit eating animals, such as birds. Seeds are capable of germinating and growing almost anywhere they land, even in cracks in concrete or in the crotch of other trees. The small seedling begins to grow on its host, sending down aerial roots, and eventually strangling and replacing the host tree or structure.”
Motooka et al. (2003) summed up more problems with the large, invasive F. microcarpa fig tree:
“Besides shading with its broad canopy, it is a threat to host plants. Banyan roots are very destructive to infrastructures: pavement, home foundations, irrigation structures.”
Presence on Island or Island Group: In Micronesia, Fosberg and his colleagues (e.g., Fosberg et al. 1979), and others have reported on and/or collected specimens of Ficus microcarpa from islands in Palau, the Mariana Islands, Yap, Chuuk, Kosrae and the Marshall Islands. For example, Kitalong (2014:9) listed F. microcarpa as a native species in Palau, and as one of the dominant plants on the limestone areas with “limestone substrate throughout Palau” (e.g., the relatively inexcessible “rock” islands), especially where organic matter from the vegetation and faunal deposits form a thin layer in places over the coral rock; and in such habitats, epiphytic plants such as this fig species are well adapted to the naturally steep, porous, and extremely rugged terrain. Stone (1970:27) referred to Ficus microcarpa L.f. var. saffordii (Merrill) Corner, as well as Ficus prolixa var. subcordata Corner [see below], as varieties of native species of Guam (including those of the Marianas in general). Stone (1970:251-251) noted that similar “to the other banyan species in Guam, F. microcarpa is known as “nunu,” a local “name applied to various species of Ficus in Micronesia.” Manner (2013:241) listed Ficus microcarpa var. latifolia (Miq.) Corner as an occasional tree in the flora of Romonum Island, Chuuk Lagoon, Federated States of Micronesia, but did not indicate a Chuukese name or use of this banyan fig. As noted above, F. microcarpa is apparently not native to several of the other Micronesian Islands where it occurs today; and indeed, as also noted previously similar to several other tropical environments where this large fig tree species has been introduced, F. microcarpa can be invasive and thus problematic in parts of Micronesia for a number or reasons, including its potential to reach massive size and become an environmental and economic burden. However, Vande Velde (2003:10) pointed out that in Marshall Islands (all low coralline atolls or coral reef islands), F. microcarpa is rare and may remain so on these islands; e.g., on Majuro only two trees had been observed in the early part of this century on that atoll: “one in field by school, another in a private yard nearby; (although this species receives a somewhat high rating as an invasive, spread is unlikely in the Marshalls without the wasps and birds needed in typical dissemination).”