Did the Evolution of Biodiversity Hit a Ceiling?

The traditional view is that species have increased in diversity continuously over the past 200 million years, particularly in the last 100 million, leading to more diversity now than ever before. But some recent studies suggest biodiversity has tended to stay largely the same, with only occasional surges.

‘Our findings strongly contradict past studies that suggested unbounded diversity increases at local and regional scales over the last 100 million years,’ said a fresh study on terrestrial species. It found no evidence of a rise in diversity in the past 66 million years, following a brief two- to three-fold increase over a couple of million years after the mass extinction of the dinosaurs at the end of the Cretaceous Period and as mammals began to thrive.

https://royalsocietypublishing.org/doi/10.1098/rspb.2020.0372

The apparent exponential radiation of Phanerozoic land vertebrates is an artefact of spatial sampling biases

There is no consensus about how terrestrial biodiversity was assembled through deep time, and in particular whether it has risen exponentially over the Phanerozoic. Using a database of 60 859 fossil occurrences, we show that the spatial extent of the worldwide terrestrial tetrapod fossil record itself expands exponentially through the Phanerozoic. Changes in spatial sampling explain up to 67% of the change in known fossil species counts, and these changes are decoupled from variation in habitable land area that existed through time. Spatial sampling therefore represents a real and profound sampling bias that cannot be explained as redundancy. To address this bias, we estimate terrestrial tetrapod diversity for palaeogeographical regions of approximately equal size. We find that regional-scale diversity was constrained over timespans of tens to hundreds of millions of years, and similar patterns are recovered for major subgroups, such as dinosaurs, mammals and squamates. Although the Cretaceous/Palaeogene mass extinction catalysed an abrupt two- to three-fold increase in regional diversity 66 million years ago, no further increases occurred, and recent levels of regional diversity do not exceed those of the Palaeogene. These results parallel those recovered in analyses of local community-level richness. Taken together, our findings strongly contradict past studies that suggested unbounded diversity increases at local and regional scales over the last 100 million years.

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It’s frequently been proposed that the Phanerozoic diversity curve is an artifact of sampling in some way or other, and spatial or geographic sampling is one of the ideas. It’s also been proposed that species diversity follows a logistic curve, with both upper and lower asymptotes. However, the potential problem with that study is that it translates regional diversity to world diversity. But it seems likely that the provinciality of life has changed over time, with the current biota at a high point. Then again, these sorts of studies, and the usual Phanerozoic diversity curve, have mostly been done with marine invertebrates. One wonders if the questions are at all comparable.

Incidentally, John Alroy has long been a proponent of a relatively constant species carrying capacity for mammals, at least since I knew him in grad school. He’s done a lot of good work. He and Matt Carrano were both students when I was. U.C. rules! (That’s University of Chicago, not any other U.C. you may imagine.)

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