Biographical note of the first author: Jin-Feng WANG, a professor who works in the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, B eij ing 100101, China.Correspondence should be addressed: Jin-Feng WANG, State key Laboratory of Resources and Environmental Information System, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China, Tel: 86-10-64888965, Fax: 86-10-64889630, E-mail: wangjf@Lreis.ac.cn; Xiao-Ying ZHENG, Institute of Population Research, Peking University/WHO Collaborating Center of Reproductive Health and Population Science, Beijing 100871, China, Tel: 86-10-62759185, Fax: 86-10-62751976, E-mail: xzheng@pku.edu.cn