Species diversity is the number of different species in a given community or region and represents one of major forms of biological diversity at the species level. As a group of individuals that can interbreed with one another, species is the basic component of genetic diversity and ecosystem diversity as well as the most practical unit for measuring biodiversity. Particularly, species forms the biological wealth or resources on which our most important industries relied, including agriculture, forestry, fisheries, medicals, and tourism. In another words, species on this planet comprise an interwoven linkage with the atmosphere, climate, soil, water, and many other essential for the existence of life. Therefore, to answer what determines species diversity is of great importance both theoretically and practically. Here I outline a few of points that may be relevant to the topic and try to highlight the potential consequences and causes of species diversity. First, historically, species diversity results from two contrasting evolutionary forces, i.e., speciation and extinction. The former is a process on which a new species arises and the latter is one where an extant species expires. On the one hand, the evolutionary process generates numerous new species consistently during the immensely long time since the origin of life. Fossil record indicates, on the other hand, that hundreds of millions of species have existed since life appears on Earth about 35 billion years ago and 99% of them have become extinct. It is the balance between the two opposing processes that determines the species diversity and its dynamics. Second, a number of different factors such as climate, geography and topography might shape the species diversity at the local scale. Within a particular kind of environment, for example, landscapes such as deep valleys, high mountains and wide rivers are all potential environment to harbor high level of species diversity than uniform terrain. Many hypotheses to explain these factors have been proposed and yet to be investigated further. Third, biological features of organisms may also contribute to the species diversity, including genetic diversity within species, the dispersal ability and adaptability of species, the species interaction within the community etc. which all may have different impacts on the speciation and extinction of the organisms. Forth, human activities including population expansion, urbanization, agriculture during recent decades have imposed severe impacts on species diversity, resulting in habitat destruction, hunting, alien species invasion, etc. Importantly, extinctions caused by human activities are currently occurring in the time frame of centuries or even decades, rather than millions of years in early history. The last but not the least is the fact that we still do not know how many species are there on our planet. The reasons that we cannot get an accurate count of the species number include under exploration of species diversity on particularly many tropical and subtropical areas, insufficient research in taxonomy including inconsistent definitions of species, different lines of evidence in taxonomic study and practices as well as the research scope and depth of different taxa or in different regions across the world. Therefore, further work on inventory of species, extensive scientific research using multiple approaches, biological conservation and managements as well as sustainable utilization of biological resources are yet undertaken to answer this big question. © 2017, Science Press. All right reserved.
