Exotic Species Invasion and Biodiversity in Bangladesh Forest Ecosystems
Mohammad Belal Uddin
- Both, biological invasion by exotic plant species and biodiversity including spatial patterns and drivers are two major issues in tropical forest ecosystems. This dissertation deals with these two issues in a tropical forest ecosystem in Bangladesh. Considering the first issue, it comprises two manuscripts: a systematic review and a field survey in Bangladesh forest ecosystem. The review was done based on a formalized literature search in order to summarize the approaches that were hitherto applied as well as to mark gaps in tropical invasion research. A considerable number of primary research papers focused on invasion by plants in tropical forests were reviewed. The results identified ample gaps of research. Adressing these gaps may generate promising future research to understand and mitigate this great challenge in different types of tropical forests. Then a case study was conducted to examine the invasiveness and invasibility characteristics in a forest ecosystem of Bangladesh. This study seeks to find out the characteristics of exotic species and relationships between native species richness, environmental variables, disturbances and exotic plant invasion in this ecosystem. Boosted Regression Trees and Detrended Correspondence Analysis are used to determine these relationships. Most exotics are trees followed by shrubs and herbs. Fabaceae and Asteraceae contribute a large proportion of exotic species. Most of them originated from other tropical areas. Native species richness was found to be the best predictor for the number and percentage of exotic species in the study area. However, a unimodal relationship was found. Multiple other factors also influence the success of exotic species. The number and the percentage of exotic species are positively correlated with frequency of disturbances and with soil attributes (phosphorus and bulk density) but negatively correlated with topography (elevation) and conservation patterns (protection). Considering the biodiversity issue, it encompases another two manuscripts based on a case study conducting a systematic field work in the same forest ecosystem of Bangladesh. They are the first spatially explicit analysis of drivers and patterns of biodiversity in this terrestrial ecosystem based on multivariate approaches, similarity analysis and variation partitioning. One manuscript assesses the relationships between landscape and habitat characteristics, conservation patterns, and plant diversity in this tropical forest ecosystem. This study analyses the effects of soils, topographic conditions, disturbances and nature protection on plant species richness and species composition. The results reveal that biodiversity patterns in the study area are positively correlated with protection and elevation. These patterns are, however, negatively correlated with disturbances. The other manuscript focuses on the stand characteristics and spatial patterns of biodiversity as they are rarely studied in the tropics in general and in Bangladesh in particular. Data on tree species are used as they are the most conspicuous element of these ecosystems. Tree species composition was recorded in a systematic plot design and diameter was measured at breast height for each individual tree. Distance-decay approach was applied to analyze the spatial pattern of biodiversity for the whole study area and two subsamples from Satchari National Park and Satchari Reserve Forest. Analyses showed that biomass increased significantly with protection status. Plots in the Reserve Forest were associated with higher species turnover than in the National Park. This dissertation identifyies, for the first time in a systematic approach, the major drivers for invasion and biodiversity pattern in a forested area in Bangladesh. In conclusion, both, biological invasion by exotic plant species as well as biodiversity are strongly related to the disturbance regime and nature protection.