Large-scale fossil study reveals origins of modern-day biodiversity gradient 15 million years ago
IMAGE: DEPICTIONS OF PRESENT-DAY PLANKTONIC FORAMINIFERA FLOATING IN THE DEEP SEA. IMAGE CREDIT: RICHARD BIZLEY, BIZLEYART. view more
CREDIT: DEPICTIONS OF PRESENT-DAY PLANKTONIC FORAMINIFERA FLOATING IN THE DEEP SEA. IMAGE CREDIT: RICHARD BIZLEY, BIZLEYART.
- Today, species richness peaks in equatorial regions but until now there has been no clear explanation for this.
- By analysing fossil records, researchers found that a key driver of the modern biodiversity gradient was a global cooling event that occurred 15 million years ago.
- This produced a steeper temperature gradient between low and high latitudes, and within the water column, causing tropical regions to support a greater number of ecological niches for species to inhabit.
Researchers have used nearly half a million fossils to solve a 200-year-old scientific mystery: why the number of different species is greatest near the equator and decreases steadily towards polar regions. The results – published today in the journal Nature – give valuable insight into how biodiversity is generated over long timescales, and how climate change can affect global species richness.
It has long been known that in both marine and terrestrial systems species (including animals, plants, and single-celled organisms) show a ‘latitudinal diversity gradient’, with biodiversity peaking at the equator. But until now, limited fossil data has prevented researchers from thoroughly investigating how this diversity gradient first arose.
In this new study, researchers at the Universities of Oxford, Leeds and Bristol, used a group of unicellular marine plankton called planktonic foraminifera. The team analysed 434,113 entries in a global fossil database, covering the last 40 million years. They then investigated the relationship between the number of species over time and space, and potential drivers of the latitudinal diversity gradient, such as sea surface temperatures and ocean salinity levels.
Key findings:
- The modern-day latitudinal diversity gradient first started to emerge around 34 million years ago, as the Earth began to transition from a warmer to cooler climate.
- This gradient initially remained shallow, until around 15–10 million years ago, when it steepened significantly. This coincides with a significant increase in global cooling.
- Peak richness for planktonic foraminifera occurred at higher latitudes from 40–20 million years ago. By around 18 million years ago, however, peak richness shifted to between 10° to 20° latitude, consistent with the diversity pattern observed today.
- There was a strong positive relationship between species richness and sea surface temperatures – both when modelled over time at specific locations, or at different locations at a specific time.
- There was also a positive relationship between species richness and the strength of the thermocline: the temperature gradient that exists between the warmer mixed water at the ocean's surface and the cooler deep water below.
According to the researchers, these results indicate that the modern-day distribution of species richness for planktonic foraminifera could be explained by the steepening of the latitudinal temperature gradient from the equator to the poles over the last 15 million years. This may have opened up more ecological niches in tropical regions within the water column, compared with higher latitudes, promoting greater rates of speciation.
To test this hypothesis, the researchers examined the extent to which modern species of planktonic foraminifera live at different depths within the vertical water column. They found that in low latitudes closer to the equator, species today are more evenly distributed vertically within the water column, compared with high latitudes.
This suggests that a key driver of the modern-day diversity gradient was a significant increase in the difference in sea surface temperatures between low- and high-latitude regions, and within the water column, from 15 million years onwards. The warmer waters at the tropics were able to support a broader range of different temperature habitats and ecological niches within the vertical water column, encouraging higher numbers of species to evolve.
This is supported by the fact that the tropics today are richer than the tropics of warmer time periods in the past (such as the Eocene and Miocene) when there was little or no vertical temperature gradient in the oceans.
In addition, cooling sea temperatures at high latitudes likely caused many regional populations of species to become extinct, contributing to the modern diversity gradient.
Planktonic foraminifera originate from the Early to Middle Jurassic period (around 170 million years ago). They are found in oceans all over the world – from polar regions to the equator – and occupy a range of ecological niches in the upper two kilometers of the oceans. Because they produce hard outer shells, they can be preserved in large numbers. The global abundance of planktonic foraminifera and their exceptional fossil record from the last 66 million years made them an ideal group for this study.
Dr Erin Saupe (Department of Earth Sciences, University of Oxford), lead author for the study, said: ‘By resolving how spatial patterns of biodiversity have varied through deep time, we provide valuable information crucial for understanding how biodiversity is generated and maintained over geological timescales, beyond the scope of modern-day ecological studies.’
Associate Professor Tracy Aze (School of Earth and Environment, University of Leeds), a co-author for the study, added: ‘Although they are small enough to fit on the head of a pin, planktonic foraminifera have one of the most complete species-level fossil records known to science. Our research builds on 60 years of deep-sea sample collection and the diligent counting and recording of hundreds of thousands of specimens by research scientists. It’s fantastic to be able to produce such important results about the drivers of species distributions through time and to do justice this wonderful fossil archive.’
Study co-author Dr Alex Farnsworth, Senior Research Associate at the Department of Geographical Sciences, University of Bristol, said: ‘Understanding why species in ancient history were more diverse and plentiful nearer the equator and less so nearer the poles can give important insights how marine species, such as plankton, might respond in future. These tiny single-celled organisms are a vital link in the marine food chain, so studying their reactions to changing climates may help us better predict how they will likely be affected as temperatures continue to warm with the increasing onset of climate change. This has potentially large implications for marine food webs, such as fish and aquatic mammals like seals and whales, and could be used to inform future measures to protect sea life and preserve biodiversity.’
A scanning electron microscope image of the shell of the planktonic foraminifera species Globigerinella adamsi. This specimen was collected from sea floor sediments in the Southwest Indian Ocean aboard the GLOW Cruise.
A light microscope image of a planktonic foraminifera (bottom right) surrounded by thin strands of its cytoplasm that extend into the surrounding environment. This living specimen had recently been collected from the water in the Southwest Indian Ocean aboard the GLOW Cruise.
Image credit: Tracy Aze, University of Leeds.
Notes to editors:
The paper ‘Origination of the modern-style diversity gradient 15 million years ago’ will be published in Nature.
After the embargo lifts, the paper will be available at: https://www.nature.com/articles/s41586-023-05712-6
For interview requests and to view a pre-embargo copy of the paper, contact Dr Caroline Wood: caroline.wood@admin.ox.ac.uk
A range of images are available of planktonic foraminifera. To request these, contact Dr Caroline Wood: caroline.wood@admin.ox.ac.uk
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JOURNAL
Nature
ARTICLE TITLE
Origination of the modern-style diversity gradient 15 million years ago
ARTICLE PUBLICATION DATE
15-Feb-2023
Late Cenozoic climate cooling biogeographically shifted marine plankton communities
IMAGE: FORAMS ARE SMALL AND SHELLY. THIS FORAM BELONGS TO THE SPECIES DENTOGLOBIGERINA ALTISPIRA, WHICH WENT EXTINCT ABOUT 3 MILLION YEARS AGO, POTENTIALLY DUE TO NORTHERN HEMISPHERE ICE SHEET EXPANSION. view more
CREDIT: ADAM WOODHOUSE
Studying changes in marine biogeographic patterns, and the factors impacting these patterns over geological time, can help scientists understand current responses in organisms due to human-driven climate change. For instance, researchers know that marine organisms are shifting geographically towards the Earth’s poles in response to human-driven climate change. However, predicting the extent to which the species will shift and how such shifts are intertwined with extinction events has not been easy to discern.
One group of organisms though, the planktonic foraminifera, have recently helped researchers answer how the Late Cenozoic climatic events restructured global marine plankton communities, which may aid in predicting Earth’s current climate change’s impact on all ocean organisms.
Planktonic foraminifera are unicellular marine eukaryotic organisms that have calcareous (calcium carbonate) shells. Their calcitic shells help to preserve these tiny organisms, which get buried on the seafloor as micro fossils. Their physiology, including their shells, are sensitive to alterations in their surrounding environment, making their calcareous remains useful as climatic tracers of both past and present environmental conditions within the ocean’s water column.
In a new study in Nature, researchers examined planktonic foraminifera (forams) fossils and found a global clade-wide shift in marine latitudinal zones towards the Equator, likely driven by the development of bipolar ice sheets. The study further showed that the shift was not tied to a coupling of functional traits and species diversity, but rather the combination of ecological and morphological traits of the organisms.
“In modern ecology we consider species diversity and functional traits synonymous,” said co-lead author Anshuman Swain, postdoctoral researcher in the Department of Organismic and Evolutionary Biology and Junior Fellow of the Society of Fellows at Harvard University. “But, looking back in time we found that this correlation breaks down after two million years, so our assumption that we can use that to predict future climate change might be misguided.”
Swain and co-lead author Adam Woodhouse, postdoctoral researcher at the Institute for Geophysics, The University of Texas Austin, examined the fossil data of Late Cenozoic planktonic forams, specifically the last eight million years, to see how their relative distribution changed in response to climatic events. Rather than focus on species diversity though, Swain and Woodhouse classified the data by ecological characteristics of ecogroups (where they live in the water column) and morphogroups (morphological categories of their shells).
There are benthic and planktonic types of forams. Planktonic forams are found floating in the upper reaches of the ocean. This placing is important as the global distributions of many other organisms correlate with forams due to their low placement in the food chain. Many marine organisms (such as predatory fish, squid, krill, sharks, and cetaceans) rely on stable food chains, so how the forams respond to climate change can be a predictor for these and other organisms. Another bonus of studying these organisms is the incomparable quality of fossil data available. The researchers applied network science methods to Triton, a global dataset of planktonic foraminiferal records with more than 500,000 individual species occurrences. The specimens were collected by the International Ocean Drilling Program from across the Earth’s oceans during more than 50 years of scientific ocean drilling.
“The fossil record of the planktonic foraminifera represents an incredible biological archive, and exhibits a better Cenozoic species-level record than even the best genus-level record of any macroinvertebrate group – making them the perfect solution for our study,” said Woodhouse.
The researchers looked at seventeen morphogroups and six ecogroups of forams. Most studies examine how species are emerging and changing. For this study, the researchers asked how are the organisms responding to climate change and environmental factors ecologically. “Ecogroups and morphogroups are more consistent groups throughout the Cenozoic era,” said Swain, “so they have advantages over species studies, which are inconsistent groups. This makes it easier to make predictions from traits rather than species.”
They gathered a large dataset of traits and plotted the biogeographical distribution patterns in the ecogroups and morphogroups during the late Cenozoic (0-15 million years). Their findings showed a global latitudinal shift towards the Equator regions within clade-wide communities in both ecological and morphological groups, especially during the past eight million years.
“Once we saw the results we said this is wild,” said Swain. “Before this shift, everything was kind of random, there was no discernible strong pattern. But then, there was a strong shift that coincided with the formation of the ice sheets.”
The study showed dynamic biogeography among planktonic foraminifera in the last eight million years, including large-scale spatial rearrangements of biodiversity patterns that appear to be coupled with the emergence of bipolar ice sheets. The expansion of polar ice caps impacted the latitudes where the ecological groups were most happy, causing them to shift due to a number of factors including where oxygen was most available. Surprisingly, this trend was not visible when looking at only species data.
“We don’t know the exact reason for this,” said Swain, “but you can have an equal abundance of species without having a sense of the different ecosystems. What we did see was that ecogroups showed this trend. Meaning this climatic event affected the distribution of foraminifera and in turn the distribution of other organisms. The foram’s correlation with anthropogenically important marine animal groups may lead us to predict more alterations to their ranges and community structure driven by ongoing climate change.”
"Earth’s current biosphere has slowly evolved over millions of year to be adapted to a world of ice ages,” Woodhouse said, “so the trends we document are potentially worrying because if human-driven climate change suddenly switches us to an Earth of eight million years ago [before glaciation], we may be detrimentally restructuring the marine communities of the entire ocean."
The left panel illustrates the shift of the latitudinal region where all groups were equally abundant and were present in the highest diversity (in blue). The bands shifted from high to mid-latitudes (8 million years ago) to low latitudes today due to increased particulate organic carbon and oxygen availability with depth as well as the presence of bipolar ice sheets, which increased the gradient of temperature between high and low latitudes. (SML in the right panel refers to the surface mixed layer, where the turbulence is generated by winds, and processes such as evaporation or sea ice formation cause an increase in salinity).
CREDIT
Anshuman Swain
JOURNAL
Nature
ARTICLE TITLE
Late Cenozoic cooling restructured global marine plankton communities
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