Urbanisation and its implications for tropical urban ecosystems in Bangkok, Thailand

Southeast Asian cities have been growing rapidly since entering the 21st century, including Bangkok where their urban agglomeration increased from 6.4 million in 2000 to 10.5 million in 2020. The regional urban growth creates marked landscape transformations, most of which are within the region’s biodiversity hotspot. The consequent loss of vegetation cover tends to reduce biodiversity and ecosystem services, and thus well-being benefits. Most people live and work in urban areas, and there is growing interest in improving the quality of urban ecosystems. However, relatively limited urban ecological research has been conducted in rapidly urbanising tropical regions that can inform planning and management for biodiversity conservation and sustainable urban development. This thesis uses Bangkok as a case study of a rapidly urbanising tropical mega-city and investigates i) landscape transformation and vegetation dynamics arising from the recent urban development ii) biodiversity consequences of environmental change due to increasing urbanisation intensity, and iii) possible mitigation via nature-based solutions to improve the quality of the urban environment. The study region was defined as a 70 km × 80 km area (5,600 km2) centred on Metropolitan Bangkok and surrounding provinces. Using classification of high resolution aerial imagery, landcover maps were generated for two recent time points (i.e. ~2004 and ~2018) to assess landscape transformation and temporal changes in vegetation dynamics along an urbanisation gradient, and distinguishing impacts of urban expansion and densification. A subset of this region was delimited as 2 km × 2 km grid cells with ≥25% impervious surface cover. Random stratification was then used to select 150 1 km × 1 km sampling grid cells along the gradient of urbanisation intensity in the urban Bangkok region. Biodiversity surveys were conducted for trees, birds, and small arboreal mammals using 50 m fixed radius survey plots located at the centre of the randomly selected cells. Equivalent surveys were conducted in the largest patches of woodland or areas of trees within each cell. Comparing biodiversity patterns along the urbanisation gradient at these different types of locations enables us to assess how retaining or creating wooded patches can help mitigate urbanisation impacts on biodiversity and ecosystem services. The Bangkok region has undergone intense urban development, resulting in a considerable loss of vegetation cover. Urban expansion and densification shows similar effects on the loss of total vegetation at the grid cell resolution, but urban growth has primarily arisen through the expansion which thus has greater impacts on vegetation dynamics. Notably, however, infill densification has substantially reduced tree cover contrasting with increasing tree cover in the rest of the region including that undergoing urban expansion. 162 tree, 142 bird, 3 squirrels, and 1 tree-shrew species were recorded across all survey sites during March to July 2018, with literature based comparisons suggesting urbanisation has substantially reduced species richness of the focal taxa relative to nearby natural settings. There is interspecific-variation in population responses of squirrels and tree-shrews to urbanisation intensity, with Callosciurus finlaysonii becoming more abundant in highly urbanised locations whilst other species exhibited negative responses. Species richness and ecosystem services of tree assemblages appear to be maintained across the urbanisation gradient. Avian biodiversity declines linearly along the urbanisation gradient contrasting with the unimodal patterns typically observed in temperate regions, with the rate of the decline being reduced in the woodland locations. My results highlight the differences in biodiversity responses to urbanisation arising in tropical systems compared to the much more documented temperate regions. Results from woodland survey points also highlight the potential of woodland retention to enhance biodiversity and ecosystem service provision — even in highly urbanised locations. Although further urbanisation in Bangkok will adversely impact vegetation dynamics and biodiversity, policy interventions could mitigate some of these impacts by increasing urban tree cover to benefit biodiversity and ecosystem service provision.