The reason for positive interspecific covariation was that these species used the resource similarly. 种对正协变的存在是由于这些物种对环境资源的利用具有相似性所致。
Water is essential for human being, economic activities, and important environmental resources as well. 水资源是人类生存和社会经济发展的重要物质基础，也是重要的环境资源，水危机已严重影响人们生活，损害人类健康，阻碍经济发展，危及人类自身生存与发展。
