Naturalized and invasive alien flora of Ghana

The paper provides an inventory of the naturalized vascular flora of Ghana, based on newly gathered information from the literature and consultations with local experts. We adopted a rather conservative approach by including only species for which strong evidence exists that they are alien in Ghana. We recorded 291 species of naturalized alien plants; for 237 of them there is information on their distribution in the 10 administrative regions of Ghana. Twenty-five species (i.e. 8.6% of the total number of naturalized species) are classified as invasive. There are 21 widely distributed species (7.2% of the total naturalized flora) that occur in at least eight regions, and five of them are distributed all over the country: Azadirachta indica, Echinochloa colona, Leucaena leucocephala, Senna occidentalis and S. siamea. Of these five, the first three are classified as invasive. The naturalized flora of Ghana includes 71 families, with Leguminosae (66 species), Compositae (22) and Poaceae (18) most represented. The majority of species have their native range in South and North America, contributing 161 and 127 species, respectively. Tropical Asia (98 species) and Africa (37 species) are less represented and only three species have their origin in Europe. The dominance of the Americas as a source region is even more pronounced for the subset of invasive species, with this continent accounting for 18 species out of 25 (72%). Annual and biennial/perennial herbs, shrubs and trees are evenly represented in the naturalized alien species pool. Among invasive species, biennial/perennial herbs are markedly over-represented compared to their contribution to the naturalized flora as a whole (52% vs 38%). The same is true for aquatic species (12% vs 2%). Species that occur as naturalized in more than 40 regions in Africa are also more widely distributed within Ghana. The numbers of naturalized species in the administrative regions of Ghana varied from 34 in Upper West to 173 in Greater Accra. These numbers increased with human population density and decreased with distance from the sea, but this pattern was driven by the city of Accra region. The only predictor that remained significant after removing this outlier was the amount of precipitation in the dry period, which was positively associated with the regional richness of naturalized species. This study could serve as a stimulus for other countries in hitherto understudied tropical regions to work towards comprehensive inventories, for which basic data on the alien flora often is still incomplete.