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.