## 1 Introduction  
Net primary productivity (NPP) quantifies the total amount of net carbon fixed by plants through photosynthesis. NPP is the key component of the global carbon cycle [1], and provides a measurable and unified boundary for terrestrial ecosystems [2]. Now, it is identified as a key variable to indicating the change in ecosystem function and vegetation dynamics [3], and is widely used in the study of biodiversity [4] and environmental degradation, such as desertification [5] and grassland degradation [6; 7]. Numerous studies have found that human activities have become the major factor driving the changes in terrestrial NPP [7; 8; 9], and NPP has been used as an indicator to assess the impact of human activities on terrestrial ecosystems [10; 11; 12]. Most of the studies have used the difference between potential NPP and actual NPP to assess the impact of human activities on terrestrial ecosystems [13; 14; 15]. The potential NPP (PNPP) is the terrestrial NPP of potential vegetation, assuming that the growth conditions of vegetation are affected only by climate change without interference from human activities. In addition, the actual NPP (ANPP) is the terrestrial NPP of the actual situation of vegetation considering the combined effects of climate change and human activities. For instance, the difference between PNPP and ANPP was employed to estimate the NPP loss in the process of global land degradation caused by human activities [13], and also to assess the situation of grassland degradation [14; 15] and desertification [5]. Furthermore, the slope trends of ANPP and PNPP and their difference were used to determine the relative impacts of climate change and human activities on terrestrial dynamics, as well as to assess the progress of desertification [16] and vegetation degradation [17]. In addition, their wide applications in Qinghai-Tibet Plateau [5], Shiyang River Basin [18], Heihe Basin [19], and Northwest China [20] provided useful references for analyzing the adaptability of these methods in different regions. However, the above methods only considered the changes in terrestrial PNPP and ANPP, while the further impacts of human activities such as grazing and crop harvest on the terrestrial ANPP were ignored. Some researchers [21; 22; 23] have proposed that the terrestrial NPP loss from human harvests is also a part of the impact of human activities on the terrestrial ecosystems, and the total amount of human impact on the terrestrial NPP is the sum of the difference between the PNPP and ANPP as well as the impact of human harvests. For instance, grazing or moving for hay and silage can induce NPP loss on grassland, which is similar to harvesting for rice, wheat, and other grain crops and economic crops on cropland. It is necessary to establish a composite indicator by combining the difference between ANPP and PNPP and the impact of human harvests. Therefore, an indicator named the human activities index (HAI) was developed in this study to quantitatively assess the impact of human activities on terrestrial net primary productivity. Grasslands are one of the most important terrestrial ecosystems, covering approximately 25% of the global terrestrial surface and holding nearly 20% of the global carbon storage [24; 25]. It is sensitively fragile and susceptible to human activities [26]. Global warming and human activities, such as overgrazing and land use change, have induced widespread grassland degradation over recent decades [5; 7; 14]. Grassland degradation is related to the problems of declining productivity, biodiversity loss, and changing vegetation cover, and is usually monitored by these factors. Among them, the vegetation cover was found to have a significant linear correlation with the Normalized Difference Vegetation Index (NDVI) [27], and the grassland degradation index (GDI) based on vegetation cover was regarded as one of the widely used and effective factors to monitor the condition of grassland degradation [28; 29].