Abstract
Estimating whether the Earth’s biota is in the middle of a crisis relies heavily on comparisons between present and past data about biodiversity or biodiversity surrogates. Although the past is a crucial source of information to assess the severity of the current biodiversity crisis, substantive conceptual and methodological questions remain about how paleodiversity and biodiversity are to be properly compared. I argue that to justify claims of a current biodiversity crisis is harder than it appears. More precisely, I claim that paleodiversity measurements are incommensurable with contemporary measurements, given the different ways that biodiversity is conceptualized and operationalized. I conclude by proposing three possible ways of overcoming this incommensurability problem.
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Notes
Suppose you sample a plot p twice, obtaining: a) 100 individuals belonging to 28 species at time t1; b) 200 individuals belonging to 36 species an time t2. According to S, the second dataset is more diverse. Nevertheless, this conclusion might be an artifact of simply having sampled more individuals.
Suppose you have two datasets of 100 individuals, each made up of three species A, B and C, but represented respectively by 30 As , 30 Bs, 40 Cs, and 90 As, 10 Bs, 10 Cs. The diversity value resulting from applying Menhinick’s index to the first and second dataset will show two equally diverse communities (dataset one: \(D_{Mn} =3/10\); dataset 2: \(D_{Mn} =3/10\)).
If an evenly distributed community of one million species were to be annihilated except for 100 species, the \(D_{Gini}\) will drop from 0.999999 to 0.99 (1% drop)—not representing the real damage sustained by the community (Daly 2018, p.11). For a fuller philosophical review, see Crupi (2019) and Justus (2011).
This statement does not distinguish between various purposes of biodiversity measure, which might validate any of the indices above for a specific purpose. Assessing whether single indices can be validated for specific purposes is beyond the scope of this paper.
For example, Pierre Taberlet and colleagues (2012), using large-scale multispecies vegetation data, demonstrated that species richness does not co-vary with genetic diversity, another important aspect of biodiversity that should be accounted for in a biodiversity operationalization. Similarly, ecologist Helmut Hillebrand and collegues argue that measurements of species richness are not informative enough of biodiversity fluctuations and crisis as they “changes in species richness and its underlying components do not necessarily correlate with species compositional turnover” (Hillebrand et al. 2018, p.171), which constitute a critical metric in ecological studies.
Faith’s PD index is only one of the many measures of biodiversity conceived as phylogenetic distance. These measurements are made possible because of the increasing availability of molecular and genetic data (see Vellend 2011)
Bokulich also argues that paleocurves are better thought of not as representations of taxa fluctuations obtained from “pure data”. Rather, they should be conceived of as “data models”, namely as the outcome of filtering and improving inevitably incomplete data using post-collection techniques that have been shown to be effective. This improved data displays higher degree of fidelity to the phenomenon of interest and is more useful to paleontologists than raw taxic diversity estimates.
The old trope in paleontology to assert that the fossil record is not an exhaustive or reliable source of evidence is still alive. This is unfortunate because eminent studies oppose this view and demonstrate that the fossil record can adequately serve specific purposes. For example, Gould and Eldredge most famously defended the completeness of the fossil record and used it to overturn traditional views on the mode and tempo of macroevolutionary patterns (Gould and Eldredge 1972). Furthermore, sophisticated analytical techniques have been developed to deal with incompleteness (Kidwell and Holland 2002), such as reconstructing phylogenetic trees for specific lineages (Woolley et al. 2022).
It should be made clear here that talking about an ongoing biodiversity crisis and talking about entering a mass extinction are two separate issues. A mass extinction event is usually characterized by high rates of species extinction unbalanced by species origination or by a significant increase over the background extinction rate. Bocchi et al. (in press) provides a meticulous philosophical analysis of the concept of mass extinction and the evidence needed to support the claim that we are entering an extinction event comparable to those of the past). A biodiversity crisis is a more complex and multifaceted issue to assess than a mass extinction event. As I have already hinted at and as I will make explicit in Section 4, the former requires a type of evidence that is not reducible to evidence for an escalation of species extinction.
In Sepkoski’s historical reconstruction, E.O Wilson championed the past-present analogy to enhance his scientific agenda. Wilson and other environmentalists (Barbara Ward, René Dubos,Paul and Anne Erlich, David Ehrenfeld, Norman Myers) strongly advocated for the analogy regardless of their being aware of the speculative character of their estimates, as well as the protestations of paleontologists.
Species loss is a definitional feature of a mass extinction, which are catastrophic events measured on a global scale during which an meaningful portion of extant species went extinct. For a biodiversity crisis to be occurring, this condition does not necessarily need to be met (Racki 2021).
Some studies can quite accurately represent species fluctuations, but are generally rare and focused only on restricted numbers of taxa at lower spatial scales. See Fan et al., (2020)
Even if excellent work has been done in modelling how higher taxa track the diversity trends of lower taxa since the paleobiology revolution in the 1980s Grantham (2009), the strength of some of these methods in describing species patterns from genus or family patters are successful only when an accurate identification of species, genus and family has been carried out (de Oliveira Jr et al. 2020, p.8). This excludes the possibility of applying some of these method to the fossil record, where higher taxa and not species are normally identified
I am grateful to Thomas Hegna for a useful email exchange about genera extinction.
A great deal has been written about what Kuhn and Feyerabend meant by “incommensurability”, as well as about Kuhn’s reformulation of the concept from The Structure of Scientific Revolutions (1970) to The Road since Structure (2000). A conceptual distinction, introduced by Sankey and Hoyningen-Huene (2001) is that between semantic (or taxonomic) and methodological incommenesurability. Two theories are said “semantically incommensurable” when the theoretical terms they employ cannot be co-opted in other theories, because their definition is embedded within the specific context within which that concept was developed (see Sankey and Hoyningen-Huene 2001, p. ix). The semantic aspect of incommensurability is the most commonly mentioned in philosophy. Two theories are said “methodologically incommensurable” if they abide by different standards when it comes to adjudicating which, of two competing theories, best fits the data (see Sankey and Hoyningen-Huene 2001, p. xiii).
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Acknowledgements
I am grateful to Alisa Bokulich, Aja Watkins and the Phi-Geo research group, as well as the audience at the 2021 EPSA conference in Turin. I am especially grateful to Emily Parke, Lydia Patton, and two anonymous reviewers for useful feedback on a previous version of this manuscript.
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Bocchi, F. Biodiversity vs. paleodiversity measurements: the incommensurability problem. Euro Jnl Phil Sci 12, 64 (2022). https://doi.org/10.1007/s13194-022-00494-6
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DOI: https://doi.org/10.1007/s13194-022-00494-6