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Both in Ethiopia and in the countries of East Africa, the continuing proliferation and spread of invasive alien species (IAS) is now recognized as a serious problem, which needs to be addressed. While this situation has improved dramatically over the past 10 years, further progress has been hampered by the absence, hitherto, of a comprehensive IAS database for the region. Countries in the region have repeatedly expressed the need for such a database, as a tool to assist in the identification of naturalized and invasive alien plant species, and in understanding their impacts, both existing and potential, while also providing pointers on what can be done to manage such species. This information is seen as essential, not only in enabling countries to develop effective IAS management strategies, but also in helping them to meet their obligations under various international agreements and treaties, including Article 8 (h) of the Convention on Biological Diversity (CBD) and Target 9 of the 2020 Aichi Biodiversity Targets. In providing such a database, this Guide is intended to give the countries of eastern Africa the information they require, in order to be able to develop effective strategies for combating the growing menace posed by invasive alien plants. It is further hoped that this Guide will foster increased regional collaboration, in responding to the challenges of managing shared invasive plant species. The Guide is based on the findings of extensive roadside surveys, carried out throughout the region, and on a review of the literature pertaining to naturalization and/or invasiveness among alien plants in eastern Africa. By this means, scores of exotic plant species were found to have escaped from cultivation, and to have established populations in the 'wild', to the detriment of natural resources and the millions of people in the region who depend on these resources. Included in the Guide are descriptions of roughly 200 exotic plant species which are either invasive already or which are deemed to have the potential to become invasive in the region. The profiled species include aquatic invasive plants or waterweeds (seven species); vines, creepers or climbers (20 species); terrestrial herbs, shrubs, and succulents (more than 30 species of each), and trees (more than 60 species). Also profiled in this Guide are many exotic plant species which, although their current distribution in the region may still be relatively localized, nevertheless have the potential to become considerably more widespread and problematic. The wide range of habitats and climatic conditions found within Ethiopia and across East Africa make the region as a whole particularly prone to invasions by a host of introduced plant species. Such invasions are being facilitated by increased land degradation, especially through overgrazing and deforestation, and also by climate change.

The impetus for the development of this Field Guide came about as a result of pleas from the community around the village of Doldol, Laikipia County, to initiate a control programme for Australian prickly pear [Opuntia stricta (Haw.) Haw.; Fabaceae], an invasive plant which was having a dramatic impact on livelihoods. However, a number of other exotic plants, which were less widespread, but had the potential of becoming invasive, were not seen as a potential problem. In order to avoid a similar situation from arising in the future, the community expressed a need for a Field Guide, which would include descriptions of naturalized and invasive species already present in, and those that were most likely to invade Laikipia County and, information on how best to manage them. An additional impetus was to contribute to the four main objectives of the National Strategy and Action Plan for the Management of Invasive Species in Kenya's Protected Areas. The Field Guide contributes in some or other way to all of these objectives which are to (i) Enhance awareness of invasive species to relevant actors; (ii) Prevent new invasions, manage established invasions and rehabilitate degraded habitats; (iii) Enhance research, monitoring and information management on invasive species; and (iv) Enhance capacity, resource mobilization and coordination. Extensive surveys revealed the presence of a number of introduced plant species which had escaped cultivation and established populations in the 'wild' to the detriment of natural resources and the people that depend on them. Introduced succulents, especially those in the genus Opuntia (Cactaceae), were found to be the most widespread and abundant invasive species in the semi-arid regions in the north and east of Laikipia County. Other succulents, those in the genus Bryophyllum (Crassulaceae), were also found to have escaped cultivation and were locally abundant. In the higher rainfall areas to the west and southwest, introduced trees such as black wattle (Acacia mearnsii De Wild.; Fabaceae) and Australian blackwood (Acacia melanoxylon R. Br.; Fabaceae) and the shrubs/climbers, Mauritius thorn [Caesalpinia decapetala (Roth) Alston; Fabaceae] and yellow cestrum (Cestrum aurantiacum Lindl.; Solanaceae), were invasive. Introduced plants, which have the potential to become problematic in Laikipia, unless eradicated or controlled, have also been included in the Guide. This includes species such as famine weed (Parthenium hysterophorus L.; Asteraceae) and 'mathenge' [Prosopis juliflora (Sw.) DC.; Fabaceae], which are already abundant in areas adjoining the County.

Despite the significant impacts of alien plant species (IAS), there has not been a concerted effort to tackle the problem across the region. This can mainly be ascribed to a lack of policy, little awareness and limited capacity at a national and regional level. The UN Environment-Global Environment Facility project, 'Removing Barriers to Invasive Species Management in Production and Protection Forests in SE Asia', which was active in Cambodia, Indonesia, the Philippines and Vietnam, identified these barriers and produced this Guide which will go a long way to creating awareness about invasive plants, their impacts and how best to manage them. This Guide will serve as an invaluable aid in the identification, mapping, monitoring, and management of IAS that are already present in ASEAN member states, or which may become problematic in the future, due to increased trade and travel, economic development and climate change. It is hoped that this Guide would trigger similar efforts in other countries in Southeast Asia as the region moves toward socio-economic integration.