What is Biodiversity? Maclaurin, J. and K.Sterelny . 2008 . The University of Chicago Press , Chicago , IL . 224 pp. $$60.00 (hardcover) . ISBN 0-226-50080-5 . $24.00 (paperback) . ISBN 0-226-50081-2 .
Customarily, biodiversity is defined as “variation at all levels of the biological hierarchy,” especially concerning genetic, species, and ecosystem variation. However, it is nearly impossible to measure and compare this notion across landscapes. Maclaurin and Sterelny provide a roadmap and a set of proposals for rethinking the definition of biodiversity is, how it can be measured, and how it is valuable.
To begin, the authors survey different taxonomic philosophies including pheneticism, evolutionary taxonomy, and phylogenetic systematics. Each philosophy attempts to solve the “units-and-differences problem” (i.e., one must identify units and a measure of similarity that is, both explanatory and predictive). Pheneticism runs into two problems—organisms have too many characters and there are too many measures of overall similarity. Evolutionary taxonomy is premised on the assumption that phylogeny and phenotypic similarity covary, which is generally false. Phylogenetic systematics provides a natural taxonomy; it looks at a special sort of similarity, shared derived characteristics, and requires taxa to be monophyletic. Maclaurin and Sterelny distinguish between diversity as cause and effect. Given the plurality of mechanisms operating within biological systems and the patterns so produced, there are many different ways of resolving the units-and-differences problem.
Maclaurin and Sterelny then turn to species concepts. Generally, measuring biodiversity starts with counts of species, and there has been serious disagreement over the nature of species in evolutionary biology. If there is so much disagreement, should species richness form the core of our notion of biodiversity? Clearly, there is no single species concept that does service to the diversity of organisms and interests of biologists. For example, the bio-
logical species concept does not apply to asexual taxa, the ecological concept requires the individuation of species’ niches, and the phylogenetic species concept increases the number of species classified immensely. Maclaurin and Sterelny resist this pessimism, and suggest that species are a “geographic and ecological mosaic,” and they generally have a characteristic life cycle: population to metapopulation to isolated population to incipient species. However, they grant that this account need not fit plants and certainly not microorganisms.
Following the work of Stephen Jay Gould, Maclaurin and Sterelny consider the important distinction between diversity and disparity. There are species-rich clades; however, these species may be very similar morphologically, especially if they are a result of a recent adaptive radiation. If this is so, then species richness need not covary with phenotypic diversity. Gould challenged what he termed the “cone of increasing diversity” by noting that, this view conflates diversity and disparity, and that the radiation that occurred at the Cambrian explosion increased very quickly the number of body plans, which decreased ever since. The authors challenge Gould's view in several ways (e.g., the Cambrian explosion was not so rapid, body plans are not “fixed” but are modular). They also argue there is no general, objective “morphospace” by which disparity can be measured and the objections concerning overall similarity apply to general morphospaces as well. Maclaurin and Sterelny do accept that one can define reasonable morphospaces that apply to how organisms have actually and could have evolved (specifically through work done by Thompson, Raup, and Niklas). However, the most sensible way to define such mophospaces is clade relative. A fully general morphospace would require articulating and measuring every character trait of interest, which is theoretically and empirically intractable. Likewise, they argue that evolutionary developmental biology teaches one that clades are developmental mosaics and mosaic structure “corresponds to the real dimensions of morphospace for that clade.”
Maclaurin and Sterelny then examine whether ecological communities represent a separate array of diversity that must augment species richness, or whether one can successfully represent ecological variation in an “individualistic” manner. That is, do ecological communities have emergent properties, internal regulation, and objective boundaries? Their results here are mixed because diversity-stability hypotheses, for example, that suggest communities have emergent properties are contentious at best. However, given the importance of niche construction and ecosystem engineering, coupled with other empirical evidence, the authors argue cautiously that communities are organized systems.
They turn to considering the problems in measuring biodiversity through “biodiversity surrogates.” Specifically, once species richness is measured, how might one augment this information with measures of phylogenetic distance and genetic diversity. Maclaurin and Sterelny examine work done by Daniel Faith, which, though extremely suggestive, is difficult to make tractable. Likewise, given the very indirect relationship between genome size and morphological variation, genetic variation is problematic to add to species richness as well. Often one is stuck with measuring species richness (along with evenness) and using higher taxonomic information as a surrogate.
Likewise, they examine a variety of rationales for preserving biodiversity. Maclaurin and Sterelny find claims of the intrinsic value of biodiversity wanting because they are too impractical to use. Likewise, the “demand value” that biodiversity provides in terms of ecosystem services or aesthetic preferences will catch some but not most of the diversity on the planet (e.g., the snail darter surely would be unimportant). Their solution is “option value theory” that states that the option value of a species is determined by how it would satisfy preferences in the future even if it does not now. They write, “Prudence requires us to treat communities and ecosystems as organized systems with crucial components whose continued operation cannot be taken for granted in the face of disturbance.” In summary, Maclaurin and Sterelny offer a multidimensional account of biodiversity. First, it is centered on species richness, given that notions of species are relatively precise and moderately easy to measure. Second, one can supplement species richness with an array of information concerning phylogeny, development, and ecology to enrich these depending on the interests at hand.
There is much to agree and disagree with in Maclaurin and Sterelny's work. I want to raise one objection. The authors concede a form of species pluralism—there is no single correct species concept. Moreover, they also agree that species arise from different mechanisms. However, once one concedes this point, then different species (e.g., animal, plant, microorganism) are not comparable—they need not represent the same taxonomic level in the Linnaean hierarchy. If this is so, then the species category is not a natural kind and cannot form the comparable basis for their account of biodiversity (Ereshefsky, M. 2006. Species and the Linnaean hierarchy. Pages 285–306 in R. Wilson, editor. Species: new interdisciplinary essays. MIT Press, Cambridge, Massachusetts.). Overall, however, What is Biodiversity? is an exceptionally interesting and useful work. Conservation biologists, ecologists, philosophers, and many others stand much to gain by carefully examining their views.