草原在生物多样性保护和生态系统服务供应等方面具有重要作用。放牧是内蒙古典型草原的主要土地利用方式，对草原生态系统服务及其关系有重要影响，过度放牧会影响草地的生态系统服务供应和草地可持续发展。本文以内蒙古典型草原为研究区，在对照（CK）、轻牧（LG）、中牧（MG）和重牧（HG） 4种放牧强度下，对支持服务，土壤矿质氮（SMN）；供给服务，牧草采食量（HI）、牧草质量（粗蛋白含量CP）、药用价值物种（MS）；调节服务，气候调节（土壤有机碳储量SOCS）、土壤保持（地下生物量BB）、土壤侵蚀防治（植被盖度CV）和土壤水分调节（土壤含水量SWC）8个关键的生态系统服务指标和物种丰富度（SR）、Shannon wiener 指数（SI）、Pielou均匀度指数（EVen）、功能均匀度（FVen）和功能离散度（FDiv）5个生物多样性指标进行量化，用均方根误差法量化生态系统服务对的权衡关系，分析生物多样性和生态系统服务间关系对放牧的响应，利用方差分解分析和结构方程模型探究放牧强度和生物多样性等多因子对生态系统服务的综合影响。主要结论如下：
（1）生物多样性与不同生态系统服务对放牧强度的响应不同。放牧显著影响物种多样性指标和功能离散度，而对功能均匀度没有显著性影响。物种多样性随放牧强度增加呈现增加的趋势。物种丰富度在重牧下最高。随着放牧强度的增强，支持服务没有明显的变化，供给服务增加，多数调节服务减少，说明不同种生态系统服务之间存在此消彼长的权衡关系。随着放牧强度的增加，总生态系统服务下降。
（2）放牧强度会影响生物多样性与生态系统服务之间的关系。在对照样地，有42.5%的弱权衡关系，42.5%的中度权衡关系和15%的强权衡关系，而在中牧下两者之间强权衡消失，弱权衡比例最大，为65%，说明适度放牧会削弱生物多样性与生态系统服务之间的权衡程度，而重牧会加剧两者之间的权衡程度。放牧强度会影响生态系统服务对之间的关系。放牧会或多或少的降低生态系统服务对的权衡程度，中牧的降低程度高于重牧。在内蒙古典型草原区，为实现生态系统服务利益最大化和生态系统服务对权衡最小化，最适的放牧强度为轻牧或中牧。
（3）对大多数生态系统服务而言，生物多样性是最重要的影响因子，其次是放牧强度，最后是生态系统特征（土壤容重和凋落物生物量）。物种多样性与功能多样性对不同的生态系统服务的重要性有所不同，物种多样性对大多数生态系统服务的直接效应高于功能多样性。对于土壤矿质氮、药用物种数和地下生物量三种生态系统服务，放牧通过对生物多样性或生态系统特征对其的间接效应大于放牧的直接效应。对于多数生态系统服务，放牧会通过调节生物多样性而间接影响生态系统服务。这证实了生物多样性在调节生态系统服务对放牧强度的响应方面具有重要作用。
本文探究了放牧强度对生态系统服务及其关系的影响，并研究了放牧强度对生态系统服务产生的直接效应与通过生物多样性而产生的间接效应，有助于更深入的了解草地管理对生态系统服务的影响，研究结果为内蒙古典型草原可持续发展的草地放牧管理政策提供决策依据。

Grassland plays an important role on biodiversity conservation and ecosystem services supply. Grazing is the main land use pattern of typical steppe in Inner Mongolia, which plays an important role in grassland ecosystem services and their relationships. Overgrazing will affect grassland ecosystem services delivery and the sustainable development. In this study, the typical steppe in Inner Mongolia was used as the study area. This paper quantified eight ecosystem services indicators, including soil mineralization nitrogen (SMN), herbage intake (HI), herbage quality (herbage crude protein content, CP), medicinal species (MS), climate regulation (soil organic carbon storage, SOCS), soil conservation (belowground biomass, BB), soil erosion prevention (plant cover, CV), soil water regulation (soil water content, SWC) and five biodiversity indicators, including species richness (SR), Shannon Wiener index (SI), Pielou evenness index (EVen), functional evenness (FEve) and functional divergence (FDiv) under four grazing intensities (GIs), such as check control (CK), light grazing (LG), moderate grazing (MG) and heavy grazing (HG). The root mean square error (RMSE) was used to quantify the level of trade-offs for ES pairs. This paper analyzed the responses of biodiversity, ecosystem services and their relationships to grazing. Variance partitioning analysis and structural equation modeling were used to illustrate the comprehensive effects of multiple factors such as grazing intensity and biodiversity on ESs. The main conclusions are as follows:
（1）Biodiversity and ecosystem services had different responses to grazing intensity. Grazing significantly affected species diversity index and functional divergence, but had no significant effect on functional evenness. With increasing GIs, species diversity index increased. Species richness was highest under heavy grazing. With the increase of grazing intensity, there was no significant change in supporting services, while provisioning services increased and most regulating services decreased, indicating trade-offs between different ecosystem services. The total ecosystem service (TES) decreased with with increasing GIs.
（2）Grazing intensity affects the relationship between biodiversity and ecosystem services. 42.5% of the relationships between biodiversity and ESs were weak trade-offs, 42.5% are moderate trade-offs and 15% are strong trade-offs under CK. However, under MG, the strong trade-offs disappeared, and the proportion of weak trade-offs was the largest (65%), shows MG could weaken the trade-offs between biodiversity and ecosystem services, HG enhances the degree of trade-offs between biodiversity and ecosystem services. Grazing intensity can affect the relationship between ecosystem service pairs. These results indicated that grazing also reduced the trade-offs between ES pairs to a greater or lesser extent, and the degree of reduction was higher in MG than in HG. In order to maximize the benefit of ESs and minimize the trade-offs between ES pairs, the optimal grazing intensity should be LG or MG for typical steppe of Inner Mongolia.
（3）Biodiversity is the most important influencing factor for most ecosystem services, followed by grazing intensity and finally by ecosystem characteristics (soil bulk density and litter biomass). Species diversity and functional diversity have different importance to different ESs, and the direct effect of species diversity on most ESs is greater than that of functional diversity. For soil mineralization nitrogen, medicinal species and belowground biomass, the indirect effect of GIs on them through biodiversity or ecosystem properties was greater than the direct effect of GIs. For most ecosystem services, grazing indirectly affects ecosystem services via biodiversity. This result confirms the importance of biodiversity in the impact of grazing intensity on ecosystem services.
Our study explored the effects of GIs on ESs and their relationships, as well as the direct and of GI on ecosystem services and indirect effects through biodiversity, which will help us to better understand the impact of grassland management on ecosystem services. The results of this study provide a basis for the management policy of grassland grazing for the sustainable development of typical steppe in Inner Mongolia.