İstanbul, with a population of over 13 million, is Turkey's most populated metropolitan area and the economic powerhouse of the country. The city is located in a region that has rich biodiversity due partly to its unique location at the crossroads of two continental landmasses and two large water bodies. The geographical characteristics and the city's topography allow for diverse micro-climatic zones to exist in a relatively small area, 5,461 km 2 . Moreover, due to millennia of human settlement including sixteen centuries as an imperial capital, many exotic species have found their way to the region. This assessment provides an overview of the main challenges and opportunities that İstanbul is faced with in regards to biodiversity conservation and support for the ecosystem services upon which the city depends, while simultaneously managing population growth and economic development. The assessment will also highlight the Ömerli Watershed, which supplies most of the city's drinking water. The watershed's freshwater provisioning capacity has been degrading due to urbanization in its catchment area, while the demand for water in the city overall has been increasing. An ecological-asset evaluation of the watershed has been carried out to develop an ecosystem services-based spatial decision-making framework. The evaluation is part of the urban biosphere reserve initiative that may be a solution to prevent further decrease of the watershed's biodiversity and degradation of its ecosystem services. Keywords Illegal settlements (gecekondus) • Urban watersheds • Ecosystem services • Freshwater • İstanbul

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