THE RELATIONSHIP BETWEEN SPECIES DIVERSITY AND ECOSYSTEM FUNCTIONING
We have an intuitive and experiential imperative for conservation and by now an accumulated experience in how to restore degraded systems; is there a great deal more that ecology can contribute to conservation? I will start by considering one of the most global questions: the relationship between species diversity and ecosystem functioning. This question was raised in the 1970s and given prominence in the 'rivet popper' analogy of Ehrlich and Ehrlich (1981). It was explicit in several of the imperatives of the Ecological Society of America's Sustainable Biosphere Initiative (Lubchenco et al., 1991) and has spawned a great deal of debate in the 1990s, with large programmes of research (e.g. Tilman, 1996;Lawton et al., 1998). Covich (1996) treated the question in the context of stream ecosystems. The question is central to conservation: do we need to conserve biodiversity in order to maintain the 'ecological services' of ecosystems? Should we apply our conservation efforts in preserving species or preserving the correct functioning of ecosystems?
It is surprising that within the discipline that has evolved as 'conservation biology' this issue does not receive much attention and research. For instance, in the 1997 International Meeting of the Society for Conservation Biology at Macquarie University, Sydney, Australia, there were no presentations on this theme. On the other hand, there appears to be a fervour of activity within the ecological community on the topic, with two scientific meetings in The Netherlands in 1998 (Wardle, 1999), at least one book (Schulze and Mooney, 1994) and many papers (e.g. Tilman, 1996;Naeem, 1998). A general trend in the results of research in this area is that species diversity in itself is not as important in ecosystem functioning as is the particular composition of species and functional-group composition. Many ecologists and perhaps the diverse assemblage that could be called the 'conservation community' are reacting to this evidence with dismay and sometimes antagonism because it seems to diminish the importance of conserving species. Two general points counter this reaction: (i) many if not most studies show the importance of at least some form of diversity, whether it be in terms of functional groups (Tilman, 1996), redundant elements (Walker, 1995;Naeem, 1998) or other aspects, and it is impossible to separate the conservation of general biodiversity from conservation of this specifically important diversity, and (ii) there are many other important scientific, utilitarian and ethical reasons for conserving species.