Appearance Of New Species After Mass Extinction Obeys Evolutionary ‘Speed Limit’
How long will it take life on Earth to recover from the current climate and biodiversity crises? According to a new study of planktonic foraminifera fossils, it will take at least 10 million years
How long does it take for life to recover after a mass extinction event? According to a new study, it takes at least ten million years before the diversity of living species achieves a similar level to that seen before the mass extinction event. This is according to findings by an international team of scientists who studied fossils from a group of planktic creatures known as foraminifera, or “forams”.
Forams are single-celled microscopic creatures that ooze their way around their tiny worlds. Paleobiologist Karen Wetmore, who was the invertebrate and microfossil collections manager at the University of California Museum of Paleontology for ten years, estimates there are approximately 4,000 foram species alive today (more here).
Forams are primarily marine creatures: most live on or in seafloor sediments, whilst others float around in the water column at various depths. A few species are found in freshwater or brackish environments, and a very few are non-aquatic soil-dwelling species (ref). Most forams create shells out of calcium carbonate (like clams and other shelled animals) or from sediment particles that they’ve cemented together. The shape of these shells are unique to each foram species and can be quite elaborate and beautiful to behold. It’s these structures that can become fossilized.
In this study, paleobiologist Christopher Lowery, a postdoctoral fellow who studies forams at the Institute for Geophysics at the University of Texas at Austin, and his collaborator, paleobiologist Andrew Fraass, a Research Associate in the School of Earth Sciences at University of Bristol, who also is a foram expert, compared the physical complexity of foram fossils and their overall diversity.
“Foraminifera are useful at the species level because of their superior fossil record, so we’ve been able to look at this process in a closer way than anybody else,” said Dr. Fraass in a press release.
When the dinosaurs were wiped out, foram populations were also devastated, but a few species managed to survive. Dr. Lowery and Dr. Fraass studied foram fossils from a time period covering roughly 20 million years, beginning around the end of the Cretaceous mass extinction and extending through the ensuing recovery, to determine how long it took these minibeasts to recover their species diversity (Figure 1).
Based on their findings, Dr. Lowery and Dr. Fraass concluded that it took at least ten millions years for foram species diversity to recover. They also noted that the general pattern of recovery was that obviously distinct forams popped up first, followed by an increasing array of similar-looking species. Further, they saw that the physical complexity of these creatures recovered long before the overall number of species did.
“From this study, it’s reasonable to infer that it’s going to take an extremely long time — millions of years — to recovery from the extinction that we’re causing through climate change and other methods,” Dr. Fraass said in a press release.
Why must morphological complexity recover before diversity recovers?
Earth has experienced five mass extinction events before today. The most rapid of those was also the most recent; the Cretaceous–Paleogene (K–Pg) extinction event that occurred 66 million years ago. That mass extinction was an accelerated event that resulted from an asteroid strike that triggered sudden environmental and climactic changes that wiped out 75% or more of all terrestrial animal life, including all non-avian dinosaurs. The K–Pg extinction event is comparable to the current climate and biodiversity crises because these modern processes are also occurring with unprecedented speed, compared to previous mass extinction events.
But why does it take so long for species diversity to recover after a mass extinction event?
Animals and plants evolve to fill available niches within an existing ecosystem. But a mass extinction event not only wipes out biodiversity, but it also destroys existing ecosystems and the special niches that species occupy. As a result, brand-new ecological niches must arise from the devastation before new species can evolve to specifically fit them. New species arise by filling broad ecological niches within a recovering ecosystem, and then later evolve innovations that allow them to specialize in filling niche gaps that arise as the ecosystem grows increasingly complex. This process takes time, and that time lag is the source of the evolutionary “speed limit” for recovery after a mass extinction event.
This study provides fossil evidence in support of an argument made by a number of scientists that there was a ten million year — or longer — wait for Earth’s biodiversity to recover after each of the five previous mass extinction events. But most niches lost in previous mass extinctions are not resurrected — entirely new niches develop.
“This should serve as an important reminder: some ecological niches lost due to anthropogenic climate change will never reappear,” the authors wrote in their study. This means that Earth will be radically different after a mass extinction, as it has been after the five previous extinction events.
Further, Dr. Lowery’s and Dr. Fraass’s findings are a powerful warning regarding Earth’s eventual recovery in the aftermath of the escalating climate and biodiversity crises.
At this time, there is still some argument whether we have already entered Earth’s sixth great extinction event, but there is no argument who is to blame: it’s people (ref). Human-driven changes to the planet have been documented as the cause of warming and acidifying oceans, habitat destruction, increasing pollution, increasing introduction of invasive species around the world, and warming climate; all of which are impacting plants and animals by driving them towards extinction.
Already, more than half of all individual animals have disappeared from the planet, according to a recent report by the World Wildlife Fund (report here) — a clear sign we’re teetering on the edge of the sixth mass extinction, if we’ve not already tipped over the edge into an irreversible catastrophe.
Not only will the Earth that we know be radically and irrevocably changed by a mass extinction event, it’s likely that humans will become extinct long before the planet recovers from the damages we’ve caused.
Dr. Lowery and Dr. Fraass have already started examining more forams from the late Jurassic through the present day to gain a deeper understanding of the history of this group as well as the history of Earth’s biodiversity.
“We’re hoping that examining the rest of the planktic foraminiferal record will give us insight into how climate shaped their evolution,” Dr. Fraass said in a press release. “With the past, slower, changes in climate we have in the geological record, we should be able to tease out more details about how climate change might impact these important plankton.”
Christopher M. Lowery and Andrew J. Fraass (2019). Morphospace expansion paces taxonomic diversification after end Cretaceous mass extinction, Nature Ecology & Evolution, published online on 9 April 2019 before print | doi:10.1038/s41559–019–0835–0
Originally published at Forbes on 13 April 2019.