Squirrel and tree‐shrew responses along an urbanisation gradient in a tropical mega‐city – reduced biodiversity, increased hybridisation of Callosciurus squirrels, and effects of habitat quality

Urbanisation is rapidly transforming terrestrial environments, especially in the tropics. Many squirrel species tolerate urbanisation, but studies are biased towards temperate regions. We quantify the distribution and abundance of squirrels and (ecologically similar) tree-shrews along an urbanisation gradient in a rapidly urbanising tropical mega-city (Bangkok, Thailand) located within the Indo-Burma biodiversity hotspot. We used repeated point counts in 150 1 km cells, selected using random stratification across the urbanisation gradient. We quantified species responses to (i) urbanisation intensity (measured using impervious surface cover), (ii) environmental conditions (woodland quantity and quality, human disturbance and predation pressure from free-ranging cats and dogs) and (iii) urbanisation impacts on hybridisation between congeneric Callosciurus squirrels. Three of the six species from the regional species pool were extremely rare or absent within our study region (Tamiops macclellandi, Callosciurus caniceps and Menetes berdmorei). Of the three more widespread species (Tupaia belangeri, Callosciurus finlaysonii and Callosciurus erythraeus) only C. finlaysonii had a higher abundance in more urban locations. The increasing intensity of urbanisation has thus markedly reduced squirrel diversity and abundance, contrasting with the perception from temperate regions that squirrels typically tolerate urbanisation. Urbanisation is thus likely to have reduced important ecological functions provided by squirrels, such as seed dispersal. Models of species responses to environmental conditions suggest that improving habitat quality by increasing tree cover and diversity at local and landscape scales and reducing human disturbance and numbers of feral dogs would partially mitigating adverse impacts of urbanisation on tropical squirrels and tree-shrews. Urban infrastructure (bridge construction across the Chao-Praya River) appears to have increased the permeability of a geographic barrier that previously separated C. finlaysonii and C. erythraeus distributions, increasing hybridisation rates. Our study enhances understanding of the ecological impacts of urbanisation in biodiverse tropical regions and the action required to mitigate these impacts

Avian species richness and tropical urbanization gradients: effects of woodland retention and human disturbance

Urbanization is a major driver of tropical biodiversity loss. In temperate regions avian species richness–urbanization intensity relationships typically exhibit unimodal patterns, with peak richness at intermediate urbanization levels. In tropical regions, the form of such relationships and the extent to which they are moderated by patches of seminatural habitat are unclear. We address these questions in Bangkok, Thailand (one of the largest and most rapidly expanding tropical mega-cities) and generate conservation recommendations for tropical biodiversity in urban locations. We use repeated point count surveys at a random location, and the largest available woodland patch, in 150 1 km × 1 km grid cells selected along the urbanization gradient. Woodland patches support higher species richness compared with randomized locations (except for non-natives), and avian species richness declines linearly with increasing urbanization. The contrast with unimodal patterns in temperate regions is probably driven by divergent patterns of habitat heterogeneity along tropical and temperate urbanization gradients. Moreover, we provide novel evidence that retaining patches of urban woodland moderates adverse impacts of urbanization on avian species richness. For most species groups, the benefits of woodland increase as urbanization intensifies, despite such woodland patches being very small (mean of 0.38 ha). Avian species richness in woodland patches is maximized, and community composition less similar to that in randomized locations, when woodland patches are larger and visited by fewer people. Assemblages of forest-dependent species (which provide additional ecological functions) have higher richness, and are less similar to those in randomized locations, in patches of woodland with higher tree species richness and biomass. Finally, species richness in randomized sites is greatest when they are closer to woodland patches, and such assemblages more closely resemble those of woodland sites. Our work highlights four strategies for tropical urban bird conservation: (1) conserving woodland patches across the urbanization gradient regardless of patch size, (2) improving the quality of existing woodland by increasing tree biomass and diversity, (3) creating additional woodland that is well distributed throughout the urban area to minimize effects of habitat isolation and (4) reducing human disturbance, especially in areas of the highest habitat quality, while ensuring that the benefits of connecting people to nature are realized in other locations