Scientists have traditionally described long-term changes to Earth’s marine ecosystems by measuring biodiversity—the number of different species that show up in ancient rock samples.
Until now, no one had measured how marine biomass—the sheer amount of organic material—fluctuated over hundreds of millions of years. A new study published in Current Biology does just that, using limestone samples to show for the first time that marine biomass and biodiversity trends aligned over the past 541 million years. The results may help answer questions about how ecosystems evolve over geologic time and how humans are driving a mass extinction in the modern world.
“[Biomass] patterns really followed the biodiversity curve, at least on macroevolutionary timescales.”
“[Biomass] patterns really followed the biodiversity curve, at least on macroevolutionary timescales,” said Pulkit Singh, a paleobiologist at Stanford University and coauthor of the new study. Singh’s graduate research forms the basis of the new study.
“This provides a new type of data that allows us for the first time to test some very influential ideas about the causality of long-term biodiversity changes,” said Seth Finnegan, a paleobiologist at the University of California, Berkeley, who was not involved in the new study.
Counting Skeletons and Shells
As organisms living in shallow marine environments die and settle to the seafloor, their calcium carbonate shells and skeletons are preserved as fossil-filled limestone. The successive layers of this limestone serve as an inventory of the diversity and abundance of life in the oceans over millions of years and are especially valuable to paleontologists because of their high shell content as well as the fact that limestone deposition rates likely stay stable over time, even in the absence of shells and skeletons.
To get a comprehensive picture of biomass over the Phanerozoic eon, Singh and the research team collected troves of data from previous studies that included counts of skeleton and shell fragments in marine limestone samples. In all, the team found data for more than 7,000 samples from 111 studies and conducted point counts for 73 new samples, too.
The data collection required a lot of “intellectual courage” from Singh, said Jonathan Payne, a paleobiologist at Stanford University and coauthor of the new study. “It took a lot of hard work with no guarantee that we’d get anything informative in the end.”
The gamble paid off: Results showed that “shelliness,” as Payne calls it—a proxy for biomass—generally increased over the past 541 million years alongside recorded trends in marine biodiversity, with dips in biomass aligning with known major extinction events.
The study “provides a link that has been missing until now” that connects long-term biodiversity processes to biomass trends, Finnegan said. The data appear to confirm an idea many paleobiologists expected but had not had the data to demonstrate—that marine animal biomass and biodiversity aligned over Earth’s history, he said.
Singh and the team performed a series of analyses to ensure the trends they were seeing weren’t due to other factors such as depositional environment, latitude, ocean depth, and ecosystem type. No matter how they sliced up the data, the results showed the same trends.
“It’s really rare to get the first chance to document a pattern about life across long histories of time,” Payne said. “There’s theory, but in the end, theory is meaningful when you can compare it to real data.”
The patterns the team uncovered in the limestone were reflected, too, in language past researchers used to describe their samples: An analysis of nearly 16,000 abstracts including descriptions of sedimentary carbonate rock over geologic time showed that the “shelliness” of words used to describe limestone samples increased alongside biomass trends. Words like “skeletal” and “fossiliferous” showed up at higher ratios compared to nonskeletal words in descriptions of samples from times in Earth’s history when biomass was higher.
“It was an interesting, independent confirmation of the rest of the study,” Payne said.
What Biomass Tells Us
Biomass indicates how much energy is available in an ecosystem. For animals, the ultimate source of that energy is created via the primary productivity of photosynthetic organisms such as plants and algae. Understanding the relationship between biomass and biodiversity can provide insight into how ecosystems evolve, how diversity arises and collapses, and what the ultimate factor that limits biodiversity in an ecosystem is.
“When there is more stuff to eat at the base of the food chain, ecosystems can support more and larger individuals, and maybe they can also support more different kinds of organisms.”
“It has been suggested for a long time that the long-term increase in biodiversity is a response to higher primary productivity,” Finnegan said. “When there is more stuff to eat at the base of the food chain, ecosystems can support more and larger individuals, and maybe they can also support more different kinds of organisms.”
In the ecology of the modern world, scientists have evidence that this is true. But modern scientists live in a “thin little time slice” in which any observations of ecosystems occur on very short timescales relative to Earth’s history, Finnegan said.
Scientists don’t know whether ecosystems work the same now as they did for all of Earth’s history. Long ago, biodiversity may have dictated biomass instead, or the relationship may have been a feedback loop. “Really understanding biodiversity processes means understanding them on the million-year timescale,” he said.
Since humans started to dominate ecosystems, biodiversity has plummeted. Biomass, however, has increased significantly, mostly as a result of animal husbandry and pet ownership. “We have a lot of humans, and a lot of cats and dogs, but not a lot of diversity,” Singh said. The world’s oceans are also “very likely in the early stages of a significant extinction event,” Finnegan said.
Deeper knowledge of how biomass and biodiversity relate over geologic time could help scientists better understand the effects of human-caused ecosystem changes and the drivers of this sixth mass extinction. Humans are altering the planet in a “massive experiment,” Payne said. And the only way to understand planetary-scale experiments is to use the geologic record, he said. “It is the only source of information at the same temporal and spatial scales.”
At least during the Phanerozoic, biomass and biodiversity seem to have been coupled, according to the new study. The results provide a coarse, but robust, picture, Payne said, though “there’s a lot more to learn.”
—Grace van Deelen (@gvd.bsky.social), Staff Writer