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High frequency of functional extinctions in ecological networks

Abstract

Intensified exploitation of natural populations and habitats has led to increased mortality rates and decreased abundances of many species1,2. There is a growing concern that this might cause critical abundance thresholds of species to be crossed1,3,4,5, with extinction cascades and state shifts in ecosystems as a consequence4,6,7. When increased mortality rate and decreased abundance of a given species lead to extinction of other species, this species can be characterized as functionally extinct even though it still exists. Although such functional extinctions have been observed in some ecosystems3,4,8, their frequency is largely unknown. Here we use a new modelling approach to explore the frequency and pattern of functional extinctions in ecological networks. Specifically, we analytically derive critical abundance thresholds of species by increasing their mortality rates until an extinction occurs in the network. Applying this approach on natural and theoretical food webs, we show that the species most likely to go extinct first is not the one whose mortality rate is increased but instead another species. Indeed, up to 80% of all first extinctions are of another species, suggesting that a species’ ecological functionality is often lost before its own existence is threatened. Furthermore, we find that large-bodied species at the top of the food chains can only be exposed to small increases in mortality rate and small decreases in abundance before going functionally extinct compared to small-bodied species lower in the food chains. These results illustrate the potential importance of functional extinctions in ecological networks and lend strong support to arguments advocating a more community-oriented approach in conservation biology, with target levels for populations based on ecological functionality rather than on mere persistence8,9,10,11.

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Figure 1: Increased mortality rate of a species most often leads to extinction of another species.
Figure 2: Functional extinction is more likely than numerical extinction in species with large biomass.
Figure 3: Species at the top of food webs can only lose few individuals and sustain small extra mortality rates before going functionally extinct.

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Acknowledgements

We thank N. Virgo, A. Clark, R. Law, O. Petchey, P. Münger, D. Gilljam, A. Curtsdotter and U. Wennergren for comments and discussion. D. Gilljam and A. Curtsdotter also provided computer code for part of the analysis. This project was supported by a Faculty grant from Linköping University to B.E.

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Authors

Contributions

T.S. and B.E. designed the project, T.S., B.E. and S.S. performed research, T.S. analysed the data, T.S. wrote the Supplementary Information, T.S. and B.E. wrote the manuscript with contributions from S.S.

Corresponding author

Correspondence to Bo Ebenman.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Figures 1-9, Supplementary Methods, Supplementary Tables 1-8 and Supplementary References. (PDF 1862 kb)

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Säterberg, T., Sellman, S. & Ebenman, B. High frequency of functional extinctions in ecological networks. Nature 499, 468–470 (2013). https://doi.org/10.1038/nature12277

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