Abstract
Extinctions stemming from environmental change often trigger trophic cascades and coextinctions. Bottom–up cascades occur when changes in the primary producers in a network elicit flow‐on effects to higher trophic levels. However, it remains unclear what determines a species' vulnerability to bottom–up cascades and whether such cascades were a large contributor to the megafauna extinctions that swept across several continents in the Late Pleistocene. The pathways to megafauna extinctions are particularly unclear for Sahul (landmass comprising Australia and New Guinea), where extinctions happened earlier than on other continents. We investigated the potential role of bottom–up trophic cascades in the megafauna extinctions in Late Pleistocene Sahul by first developing synthetic networks that varied in topology to identify how network position (trophic level, diet breadth, basal connections) influences vulnerability to bottom–up cascades. We then constructed pre‐extinction (~80 ka) network models of the ecological community of Naracoorte, south‐eastern Sahul, to assess whether the observed megafauna extinctions could be explained by bottom–up cascades. Synthetic networks showed that node vulnerability to bottom–up cascades decreased with increasing trophic level, diet breadth and basal connections. Extinct species in the Naracoorte community were more vulnerable overall to these cascades than were species that survived. The position of extinct species in the network – tending to be of low trophic level and therefore having relatively narrow diet breadths and fewer connections to plants – made them vulnerable. However, these species also tended to have few or no predators, a network‐position attribute that suggests they might have been particularly vulnerable to new predators. Together, these results suggest that trophic cascades and naivety to predators could have contributed to the megafauna extinction event in Sahul.
Highlights
Of all the extinctions that have ever occurred on Earth, many – potentially most – have been coextinctions (Dunn et al 2009)
We investigated the potential role of bottom–up trophic cascades in the megafauna extinctions in Late Pleistocene Sahul by first developing synthetic networks that varied in topology to identify how network position influences vulnerability to bottom–up cascades
Our analyses demonstrate that a species’ vulnerability to bottom–up coextinction cascades varies depending on its trophic level, diet breadth and number of basal connections
Summary
Of all the extinctions that have ever occurred on Earth, many – potentially most – have been coextinctions (Dunn et al 2009). Changes in the primary-producer component of a community can trigger bottom–up cascades and profoundly alter ecological communities (Kagata and Ohgushi 2006). It is unclear which species are most vulnerable to bottom–up cascades. Extinction-risk assessments by the International Union for Conservation of Nature (IUCN) and recorded recent extinctions suggest that herbivorous terrestrial vertebrates are vulnerable to extinction (Atwood et al 2020), a pattern that might partly be explained by the sensitivity of lower trophic levels to bottom–up cascades. The uncertainty regarding how vulnerability to bottom–up cascades varies with species traits (such as trophic level) has limited our ability to assess the importance of bottom–up cascades in past extinction events and to predict how such cascades might unfold in the future
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
