This is an erudite and in some ways an important book, but it is not, as the back cover claims, ‘essential reading for philosophers of biology, environmental philosophers, conservation biologists, and indeed anyone interested in one of the most pressing issues of our time: the conservation of biodiversity’. For a number of philosophers of biology and conservation biologists, perhaps; others, however, will drown in often unexplained technical jargon, and sentences such as ‘Compare this allopatric case with sympatric speciation in plants due to polyploidy’ (36). This is a pity, for the nature and goals of biodiversity and its conservation really are among the pressing issues of our time, and this book often clears a path through much of the related theoretical undergrowth, fearlessly criticising the theories of all-comers, Dawkins, Gould and Lewontin among them.

Biological diversity, as the authors argue, has more than one dimension, and consists in more than just species richness. This kind of diversity needs to be distinguished from morphological or phenotypic disparity; for communities with equal species richness could be more diverse (and have a stronger prospect of survival too) if their species embodied more internal variety and (more importantly) occupied a greater range of morphological possibilities (morphospace). Species richness sometimes serves in practice as a surrogate for morphological disparity, and the two variables are not entirely independent of one another, but their capacity for co-variance can only be tested if, as the authors maintain, they can be conceived and measured separately.

This stance requires clarification of the concept of species. Species are “something like” natural kinds (40), unlike higher taxa such as genera and families, and are to be identified with ‘metapopulations in partial stasis’, although this identification holds good for only ‘some chunks of the tree of life’ (40). Earlier, no less than seven kinds of species concepts have been reviewed, but this evolutionary concept of species emerges as the preferred one. ‘Stasis’ seems to mean ‘stability’ (a large mutation from its ancient meaning and derivation), while a metapopulation seems to add to a population (a more-or-less contiguous cluster of individuals capable of inter-breeding) neighbouring populations with which the given population interacts. (‘Seems’ because no definition is offered where the term first appears on pages 30 and 37, and the meaning only becomes as much as partially clear at 118). Thus ‘the collection of independently evolving lineages in a region is a key component … of that region's biological diversity’ (40).

Morphological difference, nonetheless, is convincingly argued to differ from species diversity; and this matters, because ‘all else equal, the more disparate the biota, the greater its standing morphological-phenotypic variation, the more resilient it will be in face of change’ (84). Yet both these kinds of diversity are to be contrasted with plasticity; for ‘two populations can be phenotypically similar yet vary in their genetic resources or in the distribution of these resources’ (85), if, for example, one population is plentiful and the other is minimal. Plasticity could presumably be held to add to biodiversity if diachronic richness is included in that concept.

Besides, there is perhaps more still to biodiversity in the form of the causally salient properties that biological communities are here argued to carry. In this connection, the authors review the controversy about whether diversity causes stability (which would exemplify this conclusion if true). This once prevalent theory appeared to have been undermined by Robert May in the 1970s, but has received new support from the work of David Tilman in the 1990s; however, ‘no definitive case for the diversity-stability hypothesis has yet been made’ (123). Yet even without its acceptance, there are probably grounds to credit the causal properties of communities when ecological patterns are to be explained and predicted, or at least (so the authors more modestly remark) this a promising area for further research (131). (Reference is made just before this to ‘alpha biodiversity’ and ‘beta biodiversity’ (128), but these recognised technical indicators of biodiversity are not defined or expounded until the following chapter (135), nor sufficiently related even then to the earlier passage. Maybe this kind of discontinuity is among the perils of joint authorship.

The multiple dimensions of biodiversity make it difficult if not impossible to find either single measures or credible surrogates (chapter 7); hence reflection is called for on the goals of conservation biology, and thus on kinds of value. Here (chapter 8) the authors entirely dismiss intrinsic value because it severs the link between value and valuation; for they rightly discern that what is intrinsically valuable would remain so even if no one valued it (150). This is held to make intrinsic value so lacking in tractability and measurability that it is of no practical use. (Yet since ‘valuable’ means not ‘what is valued’ but ‘what there is reason to value’, and ‘this is intrinsically valuable’ means that there is reason to value it non-derivatively, the link between value and actual valuation is much more tenuous than their reasoning seems to assume; besides, few are deterred from holding beliefs about what has intrinsic value, such as happiness, health, or well-being, by difficulties about putting them into practice.) So they resort instead to a combination of demand value and what they term ‘option value’, the value that biological phenomena might turn out to have, and that we risk losing, if future changes render them valuable to humanity (whether instrumentally, aesthetically or in some other manner) in dimly foreseeable ways not predictable at present. This anthropocentric account is well elaborated through study of practices such as bio-prospecting; accordingly upholding option value is placed at the core of the rationale of conservation biology. These are, as the authors avow, light green conclusions rather than dark green ones.

Putative grounds given for disregarding concepts of environmental ethics such as intrinsic value include the book being concerned with science as opposed to ethics; hence it is restricted to testable and tractable hypotheses, accessible to scientific methodology (149). But the book is also concerned with the future direction of conservation biology, and this is in part an ethical issue. Besides, in seeking to resolve what this direction should be, they inevitably embody their own account of intrinsic value, which lies somewhere in the realm of human interests, present, future and/or hypothetical. Thus the effect of their rejection of discussion of intrinsic value (including that of ecosystems and their components) is the more or less undefended adoption of an anthropocentric theory of value, issuing from the supposed implications of scientific methodology and of a distinction between science and ethics. But to travel this route is to sail perilously close to scientism.

In a book on biodiversity, this is not an unmitigated tragedy, as it eases the way to defensible conclusions such as that biological diversity, of whichever of the above kinds, is not intrinsically valuable, that neither ecosystems nor species are either, and that people need to be selective about which tracts of nature they seek to preserve, because we cannot preserve everything. Besides, global warming and the migration of species from traditional habitats is liable to change the scope of conservation biology and (given its standard aims) to require selective triage-like action to rescue key species that would otherwise be lost (177).

However, a theory of value that thus prioritises human interests is itself liable to underplay some of the grounds for conservation. As Donald Brown has remarked (in American Heat), biocentric theories of value (for which the flourishing of living creatures has intrinsic value) considerably strengthen the grounds for mitigating climate change, and the case for more stringent targets than those conventionally discussed. The urgency of all this may even suggest that the aims of conservation biology are first and foremost served by such mitigation of greenhouse-gas emissions, together, perhaps, with integrated policies of sustainable development, rather than by piecemeal acts of adaptation, however well theorised. (Biocentric approaches to value, incidentally, can be held without adopting the view of Arne Naess and of Paul Taylor that intrinsic value belongs to all creatures in equal measure.)

There again, even within the more traditional sphere of conservation biology, a rejection of the intrinsic value of the well-being of creatures themselves could issue in neglect that could be unjustified. Recently, living creatures were found two miles beneath the Antarctic ice-sheet. Being previously undiscovered, they lacked demand-value, and being so distantly related to ecosystems inhabited by human beings they quite probably lack option value too. Yet imaginably human actions could subvert their ecosystem, and arguably actions that do so should be avoided. But anthropocentric theories are hard-pressed to uphold this judgement.

To what degree a biocentric theory of intrinsic value requires changes to the implications of the implicit theory of value of this book it would be hard to specify within the confines of a review. But the probability that it would make a significant difference may be recognised if we consider the sphere of animal welfare legislation. If this were entirely premised on anthropocentric considerations, the outcome would be very different from that implicit in an ethic that recognises animal suffering as intrinsically bad – and also, arguably, from the legislation that we actually have. Thus, even granted that resources are finite, and that we cannot preserve everything, conservation policies (some of which require omissions and abstentions rather than the deployment of resources) might acquire a tangibly different content if value other than demand-value and option-value were to be recognised.

To return from ethics to the theory of morphospace, the authors ask a question long since tackled, as it happens, by a philosopher. ‘No six-limbed vertebrates have ever evolved from four-limbed ancestors. Is this evidence of the developmental impossibility of centaurs?’ (104). If we set aside the possibility that centaurs might be four-limbed rather than six-limbed, and focus simply on whether creatures that are half-human and half-equine would be viable, the view that they could arise as products of natural selection, apparently favoured by the Greek philosopher Empedocles, has to encounter the objection that the equine half would attain adulthood at three years of age, while the human half would still be in infancy; by the time the human half had attained maturity, its alter ego, the horse, would be dying of old age. Centaurs would thus be dysfunctional, which probably amounts to ‘developmentally impossible’. This was the objection of that early adherent of natural selection, Lucretius (De Rerum Natura, V, 878-924).