Abstract: This paper discussed the scientific concept of species and the biological nature of species as a natural group. It is high diversity of species that makes it impossible to produce general , applicable and theoretically significant species concepts for all the organisms existing or once existed on the earth. Species communities of composition diversity should have characters which correspond with different biological aspects of individual species , i. e. species diversity should acturally be the biological diversity of species. There are many different methods to measure the degree of species diversity , but the total number of species should be the most explicit and direct one. A lot of studies in the last decade tried to value how many species exist in the world. The origin of species diversity is through speciation , and its loss through species extinction. The model and mechanism of speciation and extinction and the possible causes were briefly discussed. The author strongly argues that there exists a close relationship between biogeographic analysis and species diversity researches. Pattern identification and component analysis on fauna and flora should be an important part of species diversity researches on a relatively large scale.