Species diversity increases from the poles to the equator. Two studies of fossil marine plankton now show what factors contributed to this latitudinal gradient in marine diversity. Accordingly, vertical temperature differences between the sea surface and deeper water layers play an important role. The latitudinal gradient was created around 15 million years ago when the poles froze and biodiversity increasingly shifted towards the equator. A look into the past also provides indications of how marine biodiversity could change in the face of current climate change.
At the beginning of the 19th century, the German naturalist Alexander von Humboldt identified a latitudinal gradient in species diversity: the closer to the equator, the greater the diversity. However, the reasons for this are still not fully understood. According to studies, the simple assumption “the warmer, the more species” does not go far enough. Because in this case, the diversity of species should have been higher in warm phases of the past than in cold phases. But that is not always the case.
Fossil foraminifera reveal displacement
Two research teams have now independently contributed new pieces of the puzzle to solve the question. To do this, both evaluated different aspects of a large data set of fossil marine organisms, the so-called planktonic foraminifera. These are unicellular organisms with a calcareous shell that still form an important basis for marine food webs today. Because the shells of foraminifera are preserved as microfossils in marine sediments, there is detailed fossil evidence going back to the Cambrian, around 560 million years ago. The Triton dataset, released in 2021, includes more than half a million foraminiferal species whose fossils have been collected from internationally coordinated marine drilling in all of the world’s oceans. “The fossil record of planktonic foraminifera represents an incredible biological archive,” says Adam Woodhouse of the University of Texas at Austin.
Together with his team, he analyzed the dataset with the question of how the diversity of the foraminifera in different latitudes has changed over the course of millions of years. The researchers categorized the individual species according to their external and ecological characteristics. “Ecogroups and morphogroups are consistent groups across the Cenozoic period,” explains co-author Anshuman Swain of the University of Maryland. “So they have advantages over species studies that are based on patchy groups. That makes it easier to make predictions based on traits rather than species.” For 17 morphogroups and six ecogroups, the researchers used fossils to track how they have been geographically distributed over the past 15 million years.
They found that the groups were initially distributed to a similar extent across the planet. Over time, however, diversity has shifted ever closer to the equator—particularly over the past eight million years. This shift coincided with the formation of the polar ice sheets and, according to the researchers, was probably triggered by this change in climate: Ecological and morphological groups at the poles became extinct or migrated towards the equator, while diversity at the equator increased through migratory and emerging communities.
Temperature differences in the water column promote biodiversity
Isabel Fenton from the University of Oxford and her colleagues come to a similar conclusion. They combined Triton data from the last 40 million years with paleoclimate models. With their analysis at the level of the individual species, they found that the gradient in species diversity from lower to higher latitudes first became apparent around 34 million years ago and increased significantly around 15 to 10 million years ago – accompanied by a phase of global cooling. This coincides with the timing determined by Woodhouse and his team.
However, contrary to the intuitive assumption that heat allows for higher species diversity, the analysis by Fenton and her colleagues shows that species diversity increased precisely with cooling. The reason from the perspective of the researchers: “Due to the cooling, the vertical temperature gradient in equatorial waters increased, i.e. the temperature differences between the sea surface and deeper water layers. This could have created more ecological niches and thus new opportunities for the emergence of new species.” These temperature differences are amplified by the global circulation, in which cold water from the poles pushes under the warm water masses of the tropics.
Estimating future developments
Both research teams agree that the new findings can also help to better assess future responses to the current global warming. It can currently be observed that the distribution areas of marine organisms are shifting towards the poles again due to climate change. “By understanding why species were more diverse and abundant near the equator than near the poles in early history, we can gain important insights into how marine species might respond in the future,” says Fenton’s co-author Alexander Farnsworth from the University of Bristol.
Woodhouse adds: “Earth’s current biosphere has slowly adapted to an ice age world over millions of years. Therefore, the trends we have documented are potentially worrying because if human-caused climate change suddenly takes us back to an Earth eight million years ago, before the Ice Age, we could adversely restructure marine communities across the ocean.”
Sources: Adam Woodhouse (University of Texas at Austin) et al., Nature, doi: 10.1038/s41586-023-05694-5; Isabel Fenton (University of Oxford) et al., Nature, doi: 10.1038/s41586-023-05712-6