Species down south bounced back fastest

A mass extinction 66 million years ago wiped out three-quarters of all species – including the non-avian dinosaurs – but after the dust settled, southern hemisphere biodiversity may have bounced back faster than the northern.

Michael Donovan from Pennsylvania State University in the US and colleagues analysed insect nibbles on 3,646 fossilised plant leaves in Patagonia, Argentina, from the late Cretaceous to early Palaeogene – the period spanning the extinction – to estimate the variety of insects living at the time.

Their results, published in Nature Ecology & Evolution, revealed that insect herbivore diversity and terrestrial food webs in Patagonia recovered at twice the rate of North America after the Cretaceous-Palaeogene mass extinction event.

While extinction and recovery studies in the terrestrial realm have been undertaken extensively in western interior North America, it’s not been the case in the southern hemisphere, but new collections of plant fossils from Patagonia have allowed research to expand into those regions.

Palaeobiologists suspected the southern hemisphere back then was a biodiversity refuge for plants and insects.

This was based on, for instance, theories that being further from the Chicxulub impact site – the site of the collision thought to have caused the disaster – would mean fewer species succumbing to extinction.

To check this, Donovan and his crew examined the tiny etchings made by insect larvae as they gnaw on the surface of a leaf – called leaf mining.

Each larvae species has a certain leaf mining “signature”, and from these, the researchers could determine whether the markings were made by the same or closely related insect species both before and after the Cretaceous-Palaeogene extinction.

If they found the same leaf mining marks either side of the extinction event, that particular species was part of the 25% worldwide that managed to survive.

But no species fit the bill. From the Cretaceous to Palaeogene, the variety of bite marks in both North America and Patagonia decreased dramatically – insects and plants in the south were not spared the mass extinction of the north.

Leaf miner insects’ recovery patterns, on the other hand, were vastly different.

After the mass extinction, it took about nine million years for diversity to rebound throughout western interior North America. By that time, diversity in Patagonia was already up and running, reaching pre-extinction levels five million years earlier.

This research supports an emerging idea that the southern hemisphere did, in fact, suffer major extinctions, just like the northern hemisphere – but it was able to recover much more quickly.

Perhaps the asteroid impact did have different effects around the world, the researchers write.

Anne-Marie Tosolini, a palaeontologist at the University of Melbourne in Australia, believes the work “helps to show that we’re starting to see some differences in the timing of the extinction, and certainly the timing of recovery after the extinction.

“That’s [already] been shown in New Zealand with pollen and spore studies, and also now with this study into plant-insect interactions through the damage types on the leaves.”

Nicholas Longrich, an evolutionary biologist from the UK’s University of Bath, points out that more areas need to be sampled to see if this pattern held up around the world.

But, he adds, sampling the southern hemisphere is a start.

Studies such as this are important, Tosolini says. Recent investigations on plant-insect interactions in another geological time period – the end of Palaeocene, the first epoch of the Palaeogene – provided valuable information on how dramatic increases in carbon dioxide affected those biological systems.

“Looking at the past […] also really helps to understand what might happen in the future,” Tosolini says.