Invasive species: impacts on forests and forestry
Survey of the effects of Prosopis introduction in Sahelian Africa
In the 1960s and 70s, FAO and other international organizations helped countries in dry-zone sub-Saharan Africa and the Near East get access to new sources of tree and shrub species, which were introduced as part of national programmes on soil conservation, desertification control, fuelwood, fodder and food production, and overall environmental rehabilitation. At FAO, several units dealing with food security, agriculture and forestry were involved in the promotion of Acacia and Prosopis species. A number of programmes and projects on information gathering and exchange, access to and provision of germplasm, testing and introduction, fuelwood production and protection forestry were carried out in the Sahel, where repeated droughts coupled with food and fuelwood shortages contributed to severe environmental degradation and increased desertification.
After the introduction and testing phase, international attention to Prosopis decreased significantly, possibly on the assumption that the initial objectives of introduction projects had been achieved and that the resource was now available in target countries. More recently, with hindsight, as the use of Prosopis is expanding, concerns have been raised about negative impacts of Prosopis extension. Invasiveness is being reported in a number of sites throughout the Sahel. At the same time, there are a few success stories documenting environmental consolidation, and increased use of the tree by local populations.
Prosopis has naturalized in several areas of the Sahel and is now a constituent of both 'natural' and cultivated ecosystems; its total eradication, advocated by some environmental groups, is in many areas almost certainly technically and economically impossible. More to the point of this proposal, it is not an adequate analysis to simply state that Prosopis is an alien invader, and therefore must be eradicated. The case of Prosopis presents a complex equation involving a variety of benefits and costs, and opportunities and threats, across a range of social and economic landscapes.
From a biologist's perspective, the species, in its new environment, offers unique opportunities to study the integration and adaptation of an exotic woody plant over an extended period of time (in some countries, introductions have been documented since 1930, although most seed was imported in the 1960s and 70s). The tree is present in a wide range of agro-ecosystems throughout the Sahel, where it is expanding, naturally or through plantations. As a legume, it is likely to impact wildlife and domesticated animals, pollinators, soil fertility, water use, and the dynamics of the native vegetation and other introduced plants.
Although a multipurpose tree or shrub, providing a full range of products and services to local communities, Prosopis has in some cases negatively impacted traditional land uses, in particular through the decrease of pasture lands (in the drier areas) and irrigated agricultural areas (e.g. around Lake Chad in Niger). The species appears to naturally colonize degraded dry-zone areas, and in some ways fills an ecological niche previously occupied by native Acacias and other woody plants, which prior to Prosopis introductions were already seriously reduced by over-exploitation, therefore contributing to a partial restoration of a critical guild of producers within a fragile ecosystem.
From a socio-economic perspective, consistent reports point to different uses of the tree (and its different provenances) by different rural groups. This may be partly due to the fact that Prosopis species and varieties may have different genetic characteristics; but cultural experience and local knowledge also vary among human groups. These diverse uses influence livelihoods, especially among the poorest people. Gender issues are also an important aspect, since children, women and men use the tree in different ways.
From a biosecurity perspective, the case of Prosopis illustrate the empirical way that several plant introductions have been carried out with lack of concern for or awareness of invasiveness risks and issues in the agro-forestry sector. The situation is significantly different in fisheries, where impact studies on introduced fish species were started more than 20 years ago, and methodologies were developed to document risks and inform potential users. The case of Prosopis can provide insights related to future introductions of species and genotypes into new environments, as these too will likely involve a complex outcome of hazards and benefits.
With the financial support of the FAO-Netherlands Partnership Programme on Agro-Biodiversity, an interdisciplinary programme was initiated in 2003 to review and document the effects of introduced Prosopis spp. in Sahelian countries, using an ecosystem approach (including human activities). The programme will provide sectoral viewpoints and analyze the positive and negative effects of Prosopis on the environment and people's lives. A case study on perspective on uses and problems of Prosopis is being completed.