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.2022 Jul 27;289(1979):20220440.
doi: 10.1098/rspb.2022.0440. Epub 2022 Jul 27.

Does functional redundancy determine the ecological severity of a mass extinction event?

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Does functional redundancy determine the ecological severity of a mass extinction event?

Daniel G Dick et al. Proc Biol Sci..

Abstract

Many authors have noted the apparent 'decoupling' of the taxonomic and ecological severity of mass extinction events, with no widely accepted mechanistic explanation for this pattern having been offered. Here, we test between two key factors that potentially influence ecological severity: biosphere entropy (a measure of functional redundancy), and the degree of functional selectivity (in terms of deviation from a pattern of random extinction with respect to functional entities). While theoretical simulations suggest that the Shannon entropy of a given community prior to an extinction event determines the expected outcome following a perturbation of a given magnitude, actual variation in Shannon entropy between major extinction intervals is insufficient to explain the observed variation in ecological severity. Within this information-theoretic framework, we show that it is the degree of functional selectivity that is expected to primarily determine the ecological impact of a given perturbation when levels of functional redundancy are not substantially different.

Keywords: Ordovician; Permian; ecological severity; extinction severity; functional redundancy; mass extinctions.

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Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
Ten different possible communities consisting of 100 taxa arranged intok functional entities such that the distribution of taxa corresponds to a chosen entropy (plot labels indicate the entropy of the distribution). As can be seen in this figure, increasing the entropy of a distribution corresponds to an increase in the number of less redundant functional entities, ending with the maximum entropy distribution where there is no redundancy present.
Figure 2.
Figure 2.
Graphical illustration of the process used here to test for the relative importance of two factors influencing the ecological impact of an extinction event (functional redundancy and functional selectivity). This analysis assumes the following hypothesis is being tested: variation in functional redundancy between these two communities is sufficient to completely explain the variation in outcome. (a) Begin with two communities with variable levels of functional redundancy. (b) Test for a significant difference in functional redundancy between these communities (here we used the Kruskal–Wallis test for this comparison)—if the result is not significant, reject the hypothesis, as there is no significant difference in functional redundancy between these two communities. (c) If the difference in functional redundancy is statistically significant, test whether the expected outcome of a random extinction event is different when applied to these two distributions (here we used the Kolmogorov–Smirnov test for this comparison). (d) If the result is not significant, reject the hypothesis, as the significant difference in redundancy does not lead to a significant difference in the distribution of expected outcomes. (e) Next, test for a significant difference between the observed outcome (in terms of per cent change in Shannon entropy) and the distribution of expected outcomes (here we used thet-statistic for this comparison). If the difference is significant, reject the hypothesis, as the previously established significant difference in redundancy and expected outcome are not sufficient to account for the difference in the observed outcome. If the difference is not significant, fail to reject the hypothesis, as variation in functional redundancy and expected outcome are sufficient to explain the difference without invoking variation in selectivity.
Figure 3.
Figure 3.
Violin-and-box plots showing the distribution of changes in entropy following 100 000 random extinction events at three levels of severity, when applied to hypothetical communities with increasing initial Shannon entropy. Results indicate that increasing the degree of functional redundancy (as represented by lowering the Shannon entropy) increases the probability that the biosphere will show no structural change following a perturbation, regardless of magnitude. Plot labels indicate the initial entropy of the simulated community. (Online version in colour.)
Figure 4.
Figure 4.
(a) Time series showing the loss of genera and functional entities across the end-Ordovician extinction, following the onset of extensive glaciation (g) and subsequent melting (m). (b) Time series showing the loss of genera and functional entities across the end-Permian extinction, following the onset of flood volcanism (v). (c) Bar charts showing the distribution of genera within the 77 functional entities defined for this study, at four critical intervals. The Shannon entropy of the distribution at each interval is listed below (H). (Online version in colour.)
Figure 5.
Figure 5.
Violin-and-box plots showing the distribution of changes in entropy following 100 000 random extinction events at three levels of severity, when applied to the latest Ordovician and latest Permian samples used here. Dashed red lines indicate the actual per cent change in entropy that occurred following the event. Despite a statistically significant difference in the mean expected outcome, the near-complete overlap between the Ordovician–Silurian and Permian–Triassic distributions indicates the difference between the expected outcome of a random extinction applied to these two samples would have been quite small (even at equal magnitudes). Furthermore, results indicate that the selective pattern observed following the end-Ordovician was within the range of what would be expected following an extinction event that targeted taxa randomly with respect to functional role, while the degree of change observed following the end-Permian falls well outside the range of what would be expected to have occurred if taxa had been eliminated randomly, indicating a greater degree of functional selectivity. (Online version in colour.)
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