Abstract: The study on the ecological security is an important issue in the study on the contemporary ecological economics and needs to pay a great attention to in the research domain of sustainable development. In this paper, it is considered that the ecological security is a status, which can provide an ecological guarantee for the security and sustainable development of the ecological economic system in a region, a country and even the world. Ecological security is characterized by foundation, entirety, regionalization, hierarchical difference, far-reaching influence, etc., to the ecological economic system. An index system of assessing the ecological security is designed from the status-pressure-response conditions, and it is verified and analyzed by a case study at the middle reaches of Heihe River watershed, Zhangye Prefecture, Gansu Province, which is the second longest continental river in northwest China. In the calculation, the weights of the indexes are determined by the variant coefficient method. The results show that the ecological security in Zhangye Prefecture was in a trend of continuous improvement during the 7 years from 1993 to 1999, and the security index was increased from 0.4421 in 1993 to 0.6084 in 1999. However, the assessment of the ecological security status in the prefecture shows that the pressure to the local ecological environment was continuously increased in the 7 years (the status assessing value was decreased from 0.1882 in 1993 to 0.1586 in 1999). The assessment of the pressure to the ecological security shows that the practical (or potential) influence of the economic activities to the ecological security is firstly increased then decreased. The fluctuation of the response index reveals that the efforts of the local residents to protecting the ecological environment are not steady. Therefore, it is concluded that the modern ecological security is not only decided by the carrying capacity of the nature itself, but more importantly depends on the construction and protection degrees of the ecological environment. In order to ensure the ecological security for the sustainable development, the natural resources must be utilized rationally and compensated properly at the same time.

Key words: ecological security, status-pressure-response, Heihe River watershed, Zhangye Prefecture