American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Does evolutionary history correlate with contemporary extinction risk by influencing range size dynamics?”

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Andrew J. Tanentzap, Javier Igea, Matthew G. Johnston, and Matthew J. Larcombe (March 2020)

Range dynamics can explain why evolutionary age and diversification rate predict contemporary extinction in plants

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Are older species more at risk of going extinct?

A modern-day giant, the kauri tree (<i>Agathis australis</i>), is <br/><br/>an ancient species that dominates warm-temperate forests like these in <br/><br/>northern New Zealand.  In their study, Tanentzap et al. show that old <br/><br/>conifers face a higher extinction risk because they have smaller <br/><br/>ranges.<br />(Credit: Andrew J. Tanentzap)
A modern-day giant, the kauri tree (Agathis australis), is an ancient species that dominates warm-temperate forests like these in northern New Zealand. In their study, Tanentzap et al. show that old conifers face a higher extinction risk because they have smaller ranges.
(Credit: Andrew J. Tanentzap)

In a new note in The American Naturalist, Tanentzap et al. provide one of the most in-depth analyses of why patterns of extinction risk vary across the Tree of Life. It would have been hard to miss this headline: “1 million species at risk of extinction” according to the 2019 Global Assessment of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. But why these species and not millions of others on Earth? Common reasons that species become extinct are habitat loss, overexploitation, and the introduction of novel predators.

In their paper, Tanentzap et al. present new evidence that the amount of time that a taxon has existed – also known as its age – may also help predict its risk of contemporary extinction. Age is important because, among other reasons, it influences the physical and ecological space that taxa can occupy. For example, older taxa that evolved longer ago may become maladapted as their environments change. These changes can limit the number of habitats that taxa occupy and increase their extinction risk by making their geographic range and population size small and fragmented. Detecting an association between taxon age and extinction may make it easier to determine the risks faced by species and inform future conservation. However, evidence for an association between taxon age and extinction risk has varied and few studies have considered why.

Tanentzap et al. test how and why taxon age influences extinction risk across the plant kingdom. Using data from nearly 9,000 species, they found groups of species faced greater extinction risk when they evolved species more quickly. As range size is often smaller in recently evolved species, it can explain this correlation. They then tried to model how range size influenced these patterns by focusing on two large, well-sampled groups. In conifers, older species had smaller potential range sizes, which correlated with higher extinction risk. In palms, age was neither directly nor indirectly correlated with extinction. Despite the different patterns, the general message is that range size can explain why taxon age correlates with extinction risk. As range size can vary in taxa of the same age, because of different geographic histories, it can explain why other studies have found both young and old taxa face greater extinction. Ultimately, these results further emphasize the importance of large ranges for biodiversity conservation.


Abstract

Extinction threatens many species, yet is predicted by few factors across the plant Tree of Life (ToL). Taxon age is one factor that may associate with extinction if occupancy of geographic and adaptive zones varies with time, but evidence for such an association has been equivocal. Age-dependent occupancy can also influence diversification rates and thus extinction risk where new taxa have small range and population sizes. To test how age, diversification, and range size were correlated with extinction, we analyzed 639 well-sampled genera representing 8,937 species from across the plant ToL. We found a greater proportion of species were threatened by contemporary extinction in younger and faster-diversifying genera. When we directly tested how range size mediated this pattern in two large, well-sampled groups, our results varied. In conifers, potential range size was smaller in older species and was correlated with higher extinction risk. Age on its own had no direct effect on extinction when accounting for its influence on range size. In palm species, age was neither directly nor indirectly correlated with extinction risk. Our results suggest range size dynamics may explain differing patterns of extinction risk across the ToL with consequences for biodiversity conservation.