Continental tropical forests are thought to be resistant to alien plant invasion due to a lack of disturbance, or low propagule pressure from introduced species. We assessed the importance of disturbance and edge effects by surveying areas of submontane and lowland forest of Amani Nature Reserve in the East Usambara mountains, Tanzania. These areas are in the vicinity of Amani Botanic Garden (ABG)-a propagule source for many alien plant species. We surveyed three edges in the vicinity of the ABG plantations, using plots interspersed along multiple 250 m transects. Survey plots were either in secondary or seminatural forest, representing a difference in past disturbance). Alien plant species richness and abundance declined with increasing distance from forest edges, indicating that edge effects were important. In addition, the effect of distance on richness and abundance of alien species as adults was much smaller in seminatural than secondary forest, emphasizing that invasion of seminatural forest is less likely to occur. Abundance and occurrence of individual species showed broadly similar declines with increasing distance from the forest edge, and lower abundance in seminatural compared to secondary forest. Alien species were dominant in 15 percent of plots surveyed. As 28 percent of the Amani nature reserve forest is within 250 m of an edge, the importance of disturbance and edges could make a potentially large proportion of the forest vulnerable to alien species invasion.
Bibliographical noteThis study was part of the Darwin Initiative project ‘Combating alien invasive plants threatening the East Usambara mountains in Tanzania’ (162/13/033) and the authors thank Defra and NERC for funding. The authors also thank the staff of the Amani Nature Reserve, especially the late Mr. Corodius Sawe and the Tropical Biology Association, especially Dr. Rosie Trevelyan, for logistical support. Thanks also to Dr. Ayub Oduor, for providing the abstract in Swahili. All necessary permissions and permits were granted by the Tanzanian Commission for Science and Technology (COSTECH), and the Tanzanian Forestry Research Institute (TAFORI).
- East Africa
- Invasive species
- Protected area
- Tropical forest