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Invasive Australian acacias on western Indian Ocean islands: a historical and ecological perspective

2007; Wiley; Volume: 46; Issue: 4 Linguagem: Inglês

10.1111/j.1365-2028.2007.00892.x

1365-2028

Christian A. Kull, Jacques Tassin, G. Rambeloarisoa, Jean-Michel Sarrailh,

Ecology and Vegetation Dynamics Studies

Trees and shrubs of the genus Acacia from Australia have been widely introduced in tropical and subtropical regions (Midgley & Turnbull, 2003; Brockwell et al., 2005). Having evolved in hard ecological conditions, many can colonize poor soils and have thus been heavily utilized in nonconventional forestry; a number of them are now considered problematic invasives (Cronk & Fuller, 1995; Richardson, 1998). Australian acacias were introduced to western Indian Ocean islands beginning in the nineteenth century; some species have become highly invasive (Kueffer et al., 2004; Tassin et al., 2006). This paper reviews the history of these introductions and their ecological consequences in this geographical area (Madagascar; Seychelles; Comoro Islands; Mascarene Islands; Fig. 1). We consider invasive an introduced species that extends beyond its previous range (Williamson, 1996). Map of western Indian Ocean locations (drawn by K. Rasmanis) At least 43 Australian acacia species were introduced to western Indian Ocean islands, the majority (39) to Madagascar, especially during the 1950s (27 species). Five species can currently be seen as invasive in this region, seven others have been registered as naturalized (sometimes invasive) in other countries (Table 1). The two most invasive species to date are bipinnate cool-climate wattles from south-eastern Australia. The first, Acacia dealbata Link (silver wattle) was introduced in 1841 to Réunion (Trouette, 1898), and probably contemporaneously to Mauritius (Polhill, 1990). It was widely planted in Réunion to control erosion and is locally naturalized. It was introduced around 1900 to Madagascar for afforestation, railway fuel, and roadside shade (François, 1925). Mid-century, it was widely sown, sometimes unsuccessfully from airplanes; by the late 1960s, it covered more than 30,000 ha (Roche, 1956; Chauvet, 1968). It is now widespread in cooler zones of the central highlands (over 1200 m) where we estimated varying densities of presence on circa 300,000 ha. Most growth is spontaneous from seeds and re-sprouting roots. Given seed persistence, multiple dispersal mechanisms (water, wind, ants), and the plant's behaviour in comparable environments (e.g. southern Africa), it has a high potential of becoming ecologically problematic and has been noted as invasive (Binggeli, 2003). However, most policymakers do not currently consider it a threat because of its utility (e.g. woodfuel, construction, fertilizer) and because it reforests open, degraded zones (Kull, Tassin & Rangan, 2007). The second, Acacia mearnsii De Wild. (black wattle), was introduced to Réunion in the late 1870s (Trouette, 1898), and widely planted in Madagascar in the 1920s (François, 1925). Seeking to emulate successful tannin production in Africa, the French established plantations around Moramanga and east of Fianarantsoa (Kull et al., 2007). The tree has naturalized around now-abandoned plantations and is known to pioneer forest clearings. It has been found in Ranomafana National Park, where it may become problematic (Vahinala Project, 2007). In Réunion, the species was promoted during the early 1950s to restore fertility in Pelargonium fallows but became invasive following the collapse of the market for this cash crop in 1960s. Bird diversity remains very poor in naturally regenerated A. mearnsii stands (Tassin & Balent, 2004). The phyllodinous cool-climate Acacia melanoxylon R. Br. (blackwood) is less widespread than the bipinnate wattles, yet also invasive in places. In Madagascar, it was one of hundreds of exotic trees introduced in the 1950s (Chauvet, 1968; FOFIFA, 1990). It has naturalized around a defunct arboretum at 2000 m in the Ankaratra mountains (C. Kull, personal observation). In Réunion, it is invasive and threatens some native populations of its close relative Acacia heterophylla (Lam.) Willd. The origins of A. heterophylla in Réunion, as a polyploid of A. melanoxylon, are not clear (Coulaud, Brown & Siljak-Yakovlev, 1995). Introduction through petrels, which, as burrowing seabirds, can disperse seeds (Mulder & Keall, 2001), is a possibility. Phyllodinous acacias from tropical Australia and adjacent islands are more recent introductions but present high potential for future spread. Their ability to grow on poor soils makes them highly competitive. These species –Acacia aulacocarpa A. Cunn. ex Benth., Acacia auriculiformis A. Cunn. ex Benth., Acacia crassicarpa A. Cunn. ex Benth., Acacia holosericea A. Cunn. ex G. Don, Acacia leptocarpa A. Cunn. ex Benth., Acacia mangium Willd. and Acacia trachycarpa E. Pritz.– were widely tested and disseminated beginning in the 1980s for reforestation and agroforestry (Chaix & Ramamonjisoa, 2001; Midgley & Turnbull, 2003). Acacia auriculiformis and A. mangium, are noted as invasive in the Comoros, where they invade padza badlands (Kueffer et al., 2004). These species, together with A. crassicarpa, grow well in low to mid elevation humid and subhumid zones on Madagascar, and have demonstrated invasive behaviour (G. Chaix, personal communication). In Réunion, foresters destroyed invasive patches of several of these trees in 2006 in the vicinity of old forestry trials. Some other Australian species are not yet invasive but may become problematic, such as Acacia polystachya A. Cunn. ex Benth. in Seychelles (Kueffer et al., 2004). No Australian Acacia species is recorded to be invasive in Mauritius and Rodrigues (Kueffer et al., 2004). Both bipinnate and phyllodinous groups of Acacia contain invasive species. Early detection and rapid control are needed to prevent new invasions. Most attention should be given to species with a history of heavy planting in numerous localities (particularly A. dealbata, A. mearnsii, but also the tropical phylodinous acacias) as invasion is more likely with longer presence and high propagule pressure (Krivanek, Pysek & Jarosik, 2006), and to additional species known to be invasive elsewhere [Table 1; e.g. Acacia longifolia (Andrews) Willd., Acacia pycnantha Benth., Acacia saligna (Labill.) H. L. Wendl.]. A main concern is that the ecological impacts of several acacia invasions, particularly on Madagascar, are currently under-evaluated, as opposed to documentation of their utility to farmers and for soil rehabilitation (Kull et al., 2007). There is a need (i) for further research on the ecological impacts of these introduced species (Binggeli, 2003); (ii) to develop and implement legislation to control the introduction of other species; and (iii) to develop early detection procedures near critical biodiversity conservation sites. J. Triolo and G. Chaix shared valuable information on the status of acacias in Réunion and Madagascar respectively.