什么决定了物种的多样性?

What determines species diversity?

物种多样性是生物多样性在物种水平上的表现.物种是构成生态系统多样性的基本单元,同时又是遗传多样性的主要载体.物种还是人类社会持续发展所依赖的、需要重点保护的资源.因此,回答"什么决定了物种的多样性"兼具重要的理论和实际意义.首先,从漫长的生命演化历史来看,物种多样性总量受物种形成和物种灭绝两个相互对立的力量所支配,这种"生"与"死"的较量决定了物种多样性的发展及其波动规律.其次,在局部和较小的时间尺度上,生物生存的地理区域以及气候条件是决定物种多样性的关键因素.尽管已有许多不同的假说被提出,但由于地域因素和气候因素无法分割,迄今仍很难通过某种特定模型对物种多样性做出准确的评估和预测.再次,生物类群的生物学特性也会影响自身的物种形成或灭绝的速率,从而造成不同类群在多样化速率上的不同以及面对外部环境变化在适应能力上的差异.最后,需要强调的是,人为因素是影响物种多样性水平及其变化的不可忽视的重要因子.这一方面来自人类社会发展和人口膨胀对物种多样性产生的不利影响;另一方面在于人类对自然界生命认识的欠缺,包括至今都没有一个明确、可操作的物种概念,物种发现、描述和分类等基础性工作仍任重而道远.

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.