Measures We Rule Out
A general measure of biodiversity must be capable of guiding large-scale and longterm conservation effort. We think this rules out two types of biodiversity measures: biodiversity surrogates and measures based on ecosystem services. Both are, of course, important tools in conservation, but for the reasons set out below, they cannot underpin a general measure of biodiversity.
Surrogates of Biodiversity
As noted above, most of the growth in biodiversity metrics has been in the development of new surrogates for biodiversity, i.e. measures of features whose presence is correlated with high biodiversity. If biodiversity measurement is to succeed as a large-scale goal of conservation, then we must be able to assess the success of biodiversity surrogates and we can only do that if we understand what it is that these metrics are surrogates for. Sarkar et al. (2006) argue that “general biodiversity is too diffuse a term to be precisely defined”. The best we can do is to agree to “some convention or consensus about what constitutes the relevant features of biodiversity in a given context”. We think this 'nothing but surrogates' view of biodiversity measurement, in effect, risks giving up on the idea of biodiversity as an overarching goal for conservation. Crucially this convention-based view on how we should characterise biodiversity appears not to rest on underlying principles for the assessment of the conventions underpinning such a consensus on biodiversity measurement.
On our view, a general measure of biodiversity must be definable (or at least capable of clear characterisation) and it must be a feature of biological systems that we can practically assess across clades and ecosystems. This is essential if such a measure is to assist us in forging large-scale conservation policy. Moreover, it must not itself be a surrogate for some further more basic characteristic of living systems that can also be measured across clades and ecosystems.
The idea of ecosystem services as a foundation for a general measure of biodiversity is fraught with difficulty. This is partly because the whole idea of ecosystem services is at best very open ended. The Millennium Ecosystem Assessment report (2005) defines ecosystem services as “benefits people obtain from ecosystems”. Despite gallant attempts to assess the global value of ecosystem services in dollar terms (e.g. Costanza et al. 1997), many of the psychological and social benefits are difficult to measure even at small scales and, as a group, the benefits people obtain from ecosystems seem incommensurate with one another (Boyd and Banzhaf 2007). Moreover, while ecosystem services are usually interpreted as inventories of current benefits to humanity, conservation is inherently forward-looking and it is even more difficult to accurately assess the benefits that species and ecosystems will provide to our descendants. Indeed, even if we could agree on a reliable set of measures and agree on a way to aggregate them, many environmental ethicists and many members of the public would balk at the idea that only human interests need be taken into account in conservation decision-making (see for example Stone 1972). So although ecosystem services are an important driver of conservation effort, we think this tool is too limited to form a plausible basis for a general measure of biodiversity.
The idea of biodiversity should capture the diverse features of life not the diverse interests of people. While we grant to Reyes et al. (2012) that there is 'functional overlap' between these two features of biological systems we agree with Faith (2012) that ecosystem services and biodiversity are distinct. It is in the interests of humanity to preserve biodiversity, but this fact does not warrant defining biodiversity in terms of current human needs and interests. Moreover, there is practical utility in keeping these ideas separate. Differentiating between ecosystem services and biodiversity has allowed research into whether these features co-vary and what biodiversity targets yield ecosystem services (Benayas et al. 2009; Mace et al. 2012; Worm et al. 2006). In certain cases we may want to prioritize the maintenance or reinstatement of ecosystem services. Differentiating the services from the diversity serves to distinguish such conservation that focuses squarely on the economic and social needs of human populations.