Human Emotional Responses to the Natural World
It is also claimed that biodiversity is valuable because the psychological makeup of human beings causes them to feel an intimate connection with the natural world which might be expressed variously in emotions such as love of, or respect for, nature. The idea that such emotional responses are a result of our evolved psychology was promoted by Wilson (1984) and Kellert and Wilson (1993). We note that the so-called Biophilia Hypothesis has received limited support in the literature (Simaika and Samways 2010 p. 903), but let us assume for the moment that we do share a common innate love of nature.
There are two important problems with grounding conservation in common emotional responses. Firstly, such responses are not always reliable guides to rational action. There is after all some fundamental fact about human beings that also causes them to see cigarettes as valuable. We don't think that this implies that we should 'conserve' cigarettes, because we don't think that this common emotional response is adaptive. Human beings feel positively disposed toward all sorts of things that are not actually good for us. But if we must then judge the adaptiveness of our feelings toward biodiversity, it seems that conservation justified thereby would not be a consequence of our feelings towards biodiversity, but rather of the utility of biodiversity to human populations (to which we turn shortly). Secondly, people clearly differ a great deal in the extent to which they feel positive emotions toward biodiversity (Einarsson 1993). If a general measure of biodiversity is to be inferred from emotional responses to biodiversity, then it seems that we will either have to discount the responses of outliers or average across a relatively large range of responses. Finally, this style of justification for conservation suffers from the same problems as conservation based on intrinsic value. Even if it were true that almost everyone attached the same equally strong positive emotion to the conservation of the biosphere, it is hard to see how we could turn universal love of nature into a practically applicable general measure of biodiversity. For these reasons, we think it implausible that common emotional responses to nature will justify general measure of biodiversity.
The benefits conferred by biodiversity on humanity (and indeed on other species) are themselves diverse (aesthetic, ecological, economic, epistemic etc.). Moreover, as Elliott Sober (1986) so eloquently points out, species differ a great deal in their apparent instrumental value. The great majority of species have small geographic ranges, do not perform unique ecological functions within their ecosystems and are not currently of important economic or psychological value to human populations. So Sober asks whether these facts justify the 'rational attrition' of species whose instrumental value is very small or unknown. This question about whether we should conserve 'unremarkable species' is closely related to the question of whether we should employ a general measure of biodiversity which would see us conserve species and ecosystems over and above those currently known to be of important instrumental value.
The strongest reason for conservation based on a general measure of biodiversity is that preferences or circumstances are likely to change so as to make valuable some proportion of the species in question. It is true that we have at times been overenthusiastic in our predictions about the possible future value of biodiversity such as the claims about the future value of bioprospecting in the Convention on Biodiversity (for more detail, see Maclaurin and Sterelny 2008, pp. 164–7). It is also true that a great deal of economic value resides in ecosystems that have low diversity, viz farms. That said, there has been huge growth in our appreciation for, and enjoyment of, natural variety through ecotourism, national parks, eco-sanctuaries etc. As noted in section “Measures we rule out”, there is also evidence that biodiversity is correlated with a wide range of ecosystem services. Furthermore, we should be careful not to base our predictions about future value on current categories. Just as ecotourism and bioprospecting are relatively recent ideas, we may in future discover new types of endeavour which place the value of extant species in a new light. In short, there is a prima facie reason for conservation based on a general measure of biodiversity, namely that we hedge our bets against an uncertain future. This idea was originally proposed by McNeely et al. (1990) as an instance of option value, but the use of option value in this context has been controversial. Option value is an idea imported from economics. It is essentially a willingness-to-pay measure—the additional amount a person would pay for some amenity over and above its current value in consumption to maintain the option of having that amenity available for the future (van Kooten and Bulte 2000, p. 295). Although one of us has previously expressed enthusiasm for the option value idea (Maclaurin and Sterelny 2008, section 8.4) we now think that the answer lies elsewhere.
The crucial problem with option value is that it ties the value of biodiversity to judgements about value made by ordinary people (consumers in the economist's terms). Clearly actual assessments of such option value will be difficult (Norton 1988). Even if we could assess such judgements, human beings are not good at reasoning about risk and they have limited biological knowledge. So it might be that people's actual assessments about the option value in natural systems would be very poor guides to the likely effects of conservation on future human communities or on future ecosystems. If we hedge our bets to maximise future outcomes then we should do so based on our best information about the probability of such outcomes rather than on the estimates that consumers might make about such outcomes.
In light of these issues, the value of biodiversity is better analysed as an instance of consequentialism, broadly applied. We should conserve biodiversity, not because people want to, but because doing so will on average lead to better outcomes for people and human communities of perhaps more broadly for moral patients (organisms capable of experiencing suffering).
However, even the consequentialist interpretation faces an important objection developed at length in chapter 6 of Maier (2012). It might be objected that our uncertainty about future states of the biosphere and future goals and preferences of people implies that conservation based on a general measure of biodiversity is as likely to produce net harm as it is net benefit (after all, the species we are conserving include many whose effects on human populations are currently unknown).
There are of course instances in which diversity works against us, as when we are threatened by a diversity of pathogens. That said, ours is an extremely successful species with an extremely broad niche. We have become adept at harnessing a great variety of features of the natural world to an astounding variety of ends. The number of species that pose a serious threat to humanity is a vanishingly small proportion of the total species count. Moreover, a great number of weeds and pests are not harmful in their native habitat, but only become harmful when that habitat is radically disturbed or when they are introduced by humans into other ecosystems (Baker 1974).
We therefore think it implausible that conserving unremarkable species will on average produce more harm than benefit. Put another way—were possible, at the press of a button, to destroy all those species and biological communities not known to be of special value to humanity, we think it would be irrational to do so. Humanity (and perhaps other sufficiently sentient species) would almost certainly be worse off. So where we cannot assess the likely payoff for conserving an individual unremarkable species, it is nonetheless rational to assume that that payoff will be positive. This does not of course tell us anything about how large such a payoff will be and we acknowledge that there is an interesting and difficult question about weighing the benefits of such conservation against the opportunity cost of forgoing alternative projects (e.g. if we used conservation funding to fight diseases or conservation land to grow more food for burgeoning populations in poor countries). However, we note that this problem of assessing opportunity costs is a global one, affecting all aspects of public policy and hence too large a topic to treat here. Our purpose is to determine how we should in general rank and assess biological systems as candidates for conservation. We leave it to others to determine how what proportion of total human effort ought to be spent on conservation.
-  This idea has been championed particularly by Dan Faith. For excellent discussions of the option value represented by biodiversity see Faith (1992, 1994, 2013)
-  Although not explicitly consequentialist and still somewhat confusingly called option value, the approach taken by Faith (2013, p. 72) is similar to the current proposal