GUIDE TO THE NATURALIZED AND INVASIVE PLANTS OF EASTERN AFRICA

Abstract

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. Yet the management of invasive species in the region, and of invasive alien plant species in particular, has long been constrained by a number of factors. Common constraints have included weak policy and weak institutional mechanisms; lack of awareness and of access to critical information; inedaquate provision for prevention and control, and a general lack of capacity. 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. Without effective IAS management, many of the goals agreed to under these and other protocols, such as the 2030 Sustainable Development Goals, will remain elusive. 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). This selection is undoubtedly an underestimate, given that many parts of the region were not surveyed. At the very least, though, we are confident that all the most widespread and abundant invasive plant species present in the region are covered in the Guide. establishing dense stands. Many of the invasive species profiled in the Guide have been recorded as naturalized or invasive only in the East Usambaras, as escapes from the Amani Botanical Gardens. This area has been well studied, so there are good records of the presence of exotic species that have escaped cultivation, but which have not yet been recorded as being problematic elsewhere in the region, such as Maesopsis eminii Engl. (Rhamnaceae), Hura crepitans L. (Euphorbiaceae), Hevea brasiliensis (Willd. ex A.Juss.) Müll. Arg. (Euphorbiaceae), Arenga pinnata (Wurmb) Merr. (Arecaceae), Castilla elastica Cerv. (Moraceae), Cordia alliodora (Ruiz & Pav.) Oken (Boraginaceae), Piper aduncum L. (Piperaceae) and others. Some exotic species, such as neem and leucaena, seem to favour coastal habitats, and are particularly invasive there, along with species such as coral creeper (Antigonon leptopus Hook. & Arn.; Polygonaceae). Mesquite is also invasive along the northeastern coast of Kenya. Species such as devil weed seem to favour savannah habitats, while other species, such as famine weed, lantana, goatweed [Ageratum conyzoides (L.) L.; Asteraceae], and common thorn apple (Datura stramonium L.; Solanaceae), are adapted to survive and to proliferate across a wider range of habitats, wherever there is sufficient rainfall or soil moisture. Semi-aquatic species, such as giant sensitive plant (Mimosa pigra L.; Fabaceae), thrive on floodplains and around the edges of swamps and waterbodies. Other species, such as Brugmansia suaveolens (Humb. & Bonpl. ex Willd.) Bercht. & J. Presl (Solanaceae) are generally invasive only along streams and on riverbanks in highland areas, while spectacular cassia [Senna spectabilis F (DC.) H.S. Irwin & Barneby; Fabaceae] and yellow cestrum (Cestrum aurantiacum Lindl.; Solanaceae) are often problematic in forests and in woodlands. Many introduced vines or ‘climbers’, such as Madeira vine [Anredera cordifolia (Ten.) Steenis; Basselaceae], balloon vine (Cardiospermum grandiflorum Sw.; Sapindaceae) and pereskia (Pereskia aculeata Mill.; Cactaceae), which are only now starting to escape from gardens, may soon pose a significant threat to forest diversity. 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.