Impacts of human appropriation of net primary production on ecosystem regulating services in Tibet

Humans are transforming the ecology of Earth and have reshaped the form and processes of ecosystems across the terrestrial biosphere. Net primary production (NPP) is a key ecosystem process, and humans interfere with this process by appropriating a fraction of NPP. This fraction is defined as human appropriation of net primary production (HANPP). HANPP could potentially be an important indicator of ecological sustainability. We provide a framework that elucidates the relationship between HANPP and ecosystem regulating services by revealing hypothetical trends of regulating services impacted by HANPP. We performed an empirical study in the Tibet Autonomous Region of China (hereafter, Tibet) by assessing the impacts of HANPP on an important regulating service, the wind erosion prevention (WEP) service. We estimated HANPP in percent of potential NPP (hereafter HANPP%) and the WEP service in Tibet from 1989 to 2015, and interpolated wind speed, annual precipitation, and mean annual temperature as climatic factors. The results showed that, at the county level, HANPP% became the dominant factor in south-central Tibet after the year 1998. Temporal change and space-for-time substitution results suggested a HANPP turning point of approximately 27% of the potential NPP with regard to the WEP service. On the basis of these results, we suggested a hypothesis that HANPP had two thresholds with regard to its impact on regulating services, the sustainability threshold and the upper limit threshold. Before HANPP reaches the sustainability threshold, ecosystems are considered to be sustainable; while after HANPP reaches the upper limit threshold, ecosystems will be at risk of collapse. The findings of this study provide an insight into how human activities affect ecosystems, and also provide an operational framework that enables the use of HANPP as a sustainability indicator.