How well ancient ecological communities bounce back after a mass extinction doesn’t depend on the number of different species in that system, according to new work published in Science this week. Using fossil data and modeling, researchers found that the structure of communities leading up to an extinction event was the best predictor of how stable they were through the traumatic upheaval: Food webs remained stable if they were dominated by large reptiles and mammal relatives, and if they lacked smaller ones.
"There is no real precedent for what's happening to our planet at the moment," Peter Roopnarine from the California Academy of Sciences says in a statement. "We can't look into recent history and find this particular cocktail of accelerated climate change, habitat destruction, and global extinction. We can, however, explore instances of extreme crises in the fossil record – looking far back in time to reconstruct what happened, and how ecosystems responded."
He and Kenneth Angielczyk from the Field Museum of Natural History reconstructed terrestrial communities of the Karoo Basin in South Africa during the Permian-Triassic extinction, also known as the Great Dying, some 250 million years ago. Based on fossil records, the duo developed a mathematical model to analyze the stability of various guilds (groups of different species utilizing the same resource) shortly before, during, and just after the event. They created 100 alternate food webs for each community by making alternations like changing the roles each animal played or reorganizing the links between prey and their predators.
"We saw that, after disturbing a pre-extinction community and all of its alternate models, the real community always emerged as the most stable," Roopnarine says. "Since we held species richness constant, we know that each species' ecological roles – the jobs in the food web – are the key factors influencing big-picture stability. It's amazing that some of these ecosystems may have remained relatively stable despite huge biodiversity loss."
Increase in local stability was thanks to functional organization, and not the change in biodiversity richness. This elevated stability was also partly due to the extinction of small-bodied amniotes, all vertebrates except fish and amphibians. During the early phase of the extinction event, reptiles and ancient mammal relatives were the most at-risk. “It was a bad time to be a rat,” Roopnarine adds. “We think they can survive anything now, but during the Permian and Triassic, their ancient cousins played an unlucky role in the larger community.” Those ancient food webs remained stable if they were dominated by large amniotes and lacked smaller ones – but not the other way around. The smaller species were individually successful, but collectively, they couldn’t support stable communities.
Detail of part of a model food web for the Karoo paleocommunity prior to onset of the mass extinction. The nodes (spheres) represent individual species, and the arrows between them are predator-prey links showing the direction of energy flow. Peter Roopnarine