Ecology and conservation biology are two distinct fields with different aims. Unfortunately this distinction is not always appreciated, and the confusion makes it difficult to determine the scientific basis for some concepts used in ecology, conservation biology, and management. The widespread use of the term, biodiversity, in ecology and conservation biology has led many people to assume that there is broad scientific support for this concept. There are problems, however, with the definition of biodiversity, hypotheses about the consequences of changes in biodiversity, and the evidence that addresses these hypotheses. I attempt to clarify the definition of biodiversity and suggest that it is best to refer specifically to the element(s) of biodiversity that are under consideration (e.g., species diversity). I describe some of the hypotheses about the relationship between species diversity, specifically species richness, and ecosystem processes. I then critically review the studies by Naeem et al. 1994, 1995 and Tilman et al. 1996; the first experimental studies to examine these hypotheses. Scientists must acknowledge that they are in the early stages of this research, and it is inappropriate at present to conclude that there is scientific support for the commonly-held assumption that diversity per se is essential for ecosystems to function optimally.