While many introduced species are highly regarded because of the benefits they can provide, these same species have in some cases become serious threats to forests and the forest sector. Such conflict species are a considerable problem from a management perspective requiring a clear and unbiased analysis of the costs and benefits of their use.
Some examples of conflict species include the following.
• Pinus and Eucalyptus species are the most important introduced species used in commercial forestry enterprises worldwide and most particularly in the tropics and subtropics (Richardson, 1998; Richardson and Higgins, 1998; FAO, 2000b). Many introduced commercial species such as rubber (Hevea brasiliensis), coconut (Cocos nucifera) and oil palm (Elaeis guineensi) are becoming important sources of wood and fibre (FAO, 2000b). Several of these alien forest trees have spread beyond the areas in which they were planted with devastating impacts. The main impacts are considered to be caused by reduced structural diversity, increased biomass, disruption of existing vegetation dynamics and altered nutrient cycling (Richardson, 1998; Richardson and Higgins, 1998).
• Many species of Australian Acacia have been introduced into the Cape Floristic Region of South Africa for timber, fuelwood and building materials (A. mearnsii); for tannins which are used by leather industries (A. saligna, A. mearnsii); and for sand stabilization (A. cyclops, A. saligna) (McNeely, 1999; de Wit, Crookes and van Wilgen, 2001; Matthews and Brand, 2004). Such species have radically altered habitats for wildlife resulting in major changes in the distribution of species, particularly birds, and nutrient cycling regimes in nutrient poor ecosystems due to their ability to fix atmospheric nitrogen (van Wilgen et al., 2001). They have also decreased water supplies for nearby communities and increased fire hazards (McNeely et al., 2001; van Wilgen et al., 2001; Petit et al., 2004).
• Leucaena leucocephala has been widely introduced as a source of timber, fuelwood, fodder and shade and is also used to restore degraded lands, improve soils and stabilize sands. Leucaena is a fast-growing, nitrogen-fixing tree that is tolerant of arid conditions and saline soils and as such is highly regarded in arid regions in Asia and Africa (Matthews, 2004; Matthews and Brand, 2004). In areas where it has been introduced however, this species tends to form dense impenetrable thickets and readily invades forest margins, roadsides, wastelands, riparian areas and agricultural lands (McNeely, 1999). Also, the toxicity of its seeds and foliage decreases its value as a source of fodder (McNeely, 1999).
• Prosopis juliflora, introduced 70 years ago in the Thar Desert of India, has dense green vegetation which is very useful in controlling soil erosion, reducing the aridity of the area, and providing a source of fuelwood as well as fodder and shelter for both wild and domesticated animals (McNeely, 1999). Such benefits however are being overshadowed by the negative impacts of this species. P. juliflora displaces native flora resulting in reduced biodiversity and reduced diversity of products available to rural communities (McNeely, 1999). Its dense impenetrable thickets also render invaded lands useless for agricultural purposes (Richardson, 1998).
• Salt cedar (Tamarix spp.), introduced from central Asia to the southwestern United States nearly 200 years ago to control erosion along river banks, now forms dense thickets on more than 400 000 ha of riparian habitat severely impacting hydrological systems (McNeely et al., 2001).
• Australian brush-tailed possums (Trichosurus vulpecula), introduced into New Zealand for a successful fur industry, have caused considerable damage to native forests by changing forest composition and structure through the defoliation and eradication of preferred food plants (McNeely, 1999).
• The flatworm Platydemus manokwari has been introduced into many areas where it successfully controls populations of another alien invasive species, the giant African snail, Achatina fulica (McNeely, 1999). Although successful as a biological control agent, P. manokwari is now considered a significant threat to native gastropod species, including rare and endemic species, in the areas where it was introduced.
It is vital to ensure that such species serve the purposes for which they were introduced and do not escape to cause negative consequences on native ecosystems.