Extraordinary Diversity on Land Is Not a Recent Phenomenon, Says New Study
According to new research published in the journal Nature Ecology & Evolution, land animal diversity has been similar for at least the last 60 million years, since soon after the extinction of the dinosaurs.
“Our work provides an example of the combined power of the fossil record and modern statistical approaches to answer major questions about the origins of modern biodiversity,” said University of Birmingham’s Professor Richard Butler, senior author of the study.
“By understanding how biodiversity has changed in the past, we may be able to better understand the likely long-term impact of the current biodiversity crisis.”
Previously, many scientists have argued that diversity increased steadily through geological time, which would mean that biodiversity today is much greater than it was tens of millions of years ago.
But building an accurate picture of how land diversity was assembled is challenging because the fossil record generally becomes less complete further back in time.
Professor Butler and colleagues examined how the diversity of land vertebrate species living in local ecosystems (also known as ecological communities) changed over the last 375 million years.
The researchers analyzed nearly 30,000 fossil sites that have produced fossils of tetrapods, land vertebrate animals, such as mammals, birds, reptiles (including dinosaurs) and amphibians.
They found that the average number of species within ecological communities of land vertebrates have not increased for tens of millions of years.
Their results suggest that interactions between species, including competition for food and space, will limit the overall number of species that can co-exist.
“Scientists often think that species diversity has been increasing unchecked over millions of years, and that diversity is much greater today than it was in the distant past,” said study lead author Dr. Roger Close, also from the University of Birmingham.
“Our research shows that numbers of species within terrestrial communities are limited over long timescales, which contradicts the results of many experiments in modern ecological communities — now we need to understand why.”
One reason why diversity within ecological communities does not increase unchecked on long timescales could be because resources used by species, such as food and space, are finite.
Competition for these resources may prevent new species invading ecosystems and lead to a balance between rates of speciation and extinction.
After the origins of major groups of animals, or large-scale ecological disruptions like mass extinctions, though, increases in diversity may happen abruptly — on geological, if not human timescales — and are again followed by long periods where no increases occur.
“Contrary to what you might expect, the largest increase in diversity within land vertebrate communities came after the mass extinction that wiped out the dinosaurs, 66 million years ago, at the end of the Cretaceous period,” Dr. Close said.
“Within just a few million years, local diversity had increased to two or three times that of pre-extinction levels — driven primarily by the spectacular success of modern mammals.”
Roger A. Close et al. Diversity dynamics of Phanerozoic terrestrial tetrapods at the local-community scale. Nature Ecology & Evolution, published online February 18, 2019; doi: 10.1038/s41559-019-0811-8