Allocating and mapping ecosystem service demands with spatial flow from built-up areas to natural spaces.

Co-urbanized areas around large cities in developing countries face the problem of spatial disconnection between supply and demand areas of ecosystem services (ES). To explore the reflection of human needs in the nonadjacent surrounding natural spaces and identify the response of the existing natural space system to the ES demand in terms of total amount and spatial distribution, a new method for ES demand mapping in co-urbanized areas was proposed. Based on the theory of the ES delivery chain, urban built-up areas are identified as service benefiting areas (SBAs) and the sources where demands are generated, natural spaces are regarded as service provision areas (SPAs) and the sinks and destinations where demands are satisfied, and ES spatial flow is considered as the delivery mechanism and ecological process that promotes the demand flow from sources to sinks. An indicator cluster composed of four multidimensional indicators, including flow quantity, flow boundary, flow direction and allocation mode along the distance, was used to characterize the spatial flow and represent the four key links in the technical path of allocating ES demand from built-up areas to natural spaces with spatial flow to intuitively reflect the spatial characteristics of human social demands projected in them. We quantified and mapped the distribution of three ES demands in built-up areas and surrounding natural spaces. In the former, the high-demand spaces are concentrated in the areas with high population density or high aging degree; while in the latter, the high-demand spaces are mainly adjacent to the built-up areas or the large-scale natural spaces. By controlling the flow quantity, expanding the flow area, increasing the flow directions and improving the ES supply capacity of SPAs within a given distance, the high ES demands in the above spaces can be effectively regulated.