Abstract
There is a strong trend of declining populations in many species of both animals and plants. Dwindling numbers of species can eventually lead to their functional extinction. Functional, or ecological, extinction occurs when a species becomes too rare to fulfill its ecological, interactive role in the ecosystem, leading to true (numerical) extinction of other depending species. Recent theoretical work on food webs suggests that the frequency of functional extinction might be surprisingly high. However, little is known about the risk of functional species extinctions in networks with other types of interactions than trophic ones. Here, we explore the frequency of functional extinctions in model ecological networks having different proportions of antagonistic and mutualistic links. Furthermore, we investigate the topological relationship between functionally and numerically extinct species. We find that (1) the frequency of functional extinctions is higher in networks containing a mixture of antagonistic and mutualistic interactions than in networks with only one type of interaction, (2) increased mortality rate of species having both mutualistic and antagonistic links is more likely to lead to extinction of another species than to extinction of the species itself compared to species having only mutualistic or antagonistic links, and (3) trophic distance (shortest path) between functionally and numerically extinct species is, on average, longer than one, indicating the importance of indirect effects. These results generalize the findings of an earlier study on food webs, demonstrating the potential importance of functional extinction in a variety of ecological network types.
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Acknowledgments
We thank two anonymous reviewers for their valuable comments. This work was financed through a Faculty grant from Linköping University.
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Appendix
Appendix
Frequency of functional extinctions as a function of the strength of intraspecific competition and the proportion of mutualistic links. Boxplots represent the proportion of perturbations that leads to functional extinction in 100 replicate bipartite networks with different fractions of mutualistic links and strengths of intraspecific competition
The strength of intraspecific interactions relative to that of interspecific interactions in bipartite networks. Defined as the mean of the absolute values of all elements on the diagonal of the interaction matrix divided by the mean of the absolute values of all non-zero off-diagonal elements of the interaction matrix
Regression tree explaining the frequency of functional extinctions within networks. Predictor variables: α ii strength of intraspecific competition, pM proportion of mutualistic links in the network
Trophic distance (shortest path) between functionally extinct species and numerically extinct species in bipartite networks. Observed number of numerical extinctions at different distances from the functionally extinct species
Tendency of numerical extinction at different trophic distances from the functionally extinct species in bipartite networks. A value of 0 indicates that the observed number of numerical extinctions at a specific distance is the same as would be expected if all species were equally likely to go extinct; a value smaller than 0 or larger than 0 indicates that the observed number of numerical extinctions tends to be smaller or higher than the random expectation, respectively
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Sellman, S., Säterberg, T. & Ebenman, B. Pattern of functional extinctions in ecological networks with a variety of interaction types. Theor Ecol 9, 83–94 (2016). https://doi.org/10.1007/s12080-015-0275-7
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DOI: https://doi.org/10.1007/s12080-015-0275-7