Abstract: This paper introduces the effects of species diversity on ecosystem functioning: the mechanisms and hypotheses, including the redundant species hypothesis, rivet hypothesis, idiosyncratic hypothesis, null hypothesis, compensatory/keystone species hypothesis, non-linear hypothesis, and monotonic/unimodal hypothesis. To date, no single hypothesis is accepted as truly reflecting the real relations, if they do exist, between species diversity and ecosystem function. According to our analysis, there is an intrinsic relationship among these hypotheses. They either include or complement each other and may all be considered as related to the redundant species hypothesis.