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The tendency for species diversity to be greatest near the Equator developed 34 to 48 million years ago during the Eocene Epoch, Museum scientists have found.
This global distribution pattern of species is a major feature of biodiversity today.
To try to understand the origins of this biodiversity gradient - when and why it became established, the team studied the fossil record of single-celled marine organisms called foraminifera (forams). They provide a unique model for studying how biodiversity has changed over time because of their exceptionally complete fossil record.
The team found that the greater diversity of forams in tropical areas relative to their diversity in temperate regions emerged in the Mid to Late Eocene Epoch.
Before that time there were similar numbers of foram species across latitudes from zero degrees (the Equator) to 60 degrees north and south. This covers an expanse from as far north as Scotland’s Shetland Islands to as far south as the Atlantic Ocean’s boundary with the Southern Ocean.
During the Eocene, the Earth moved from a 'greenhouse' period of global warmth, towards its current 'icehouse' state, with a cooler climate and significant polar ice caps. The team believes that this period of global cooling was a crucial time for the development of the latitudinal trend in diversity that exists today.
'The descent into the icehouse was a turning point for species diversity,' says Museum scientist Dr Isabel Fenton, the lead author of the research. 'By the end of the Eocene, patterns had developed that we still see today.'
For over a hundred years, scientists have reported that species diversity often increases towards the equator - the so-called latitudinal diversity gradient. Yet the reason why this happens is still not fully understood.
Environmental variables, such as temperature and solar energy, are thought to play a role, but there is no easy way to separate their effects out from one another - they all vary with latitude, and there isn't another Earth with a different climate to compare to.
That's what led Dr Fenton and her team to look at the fossil record.
'Because all the variables today are correlated with latitude it's hard to know which could be behind the diversity gradient,' she explains. 'So instead, we studied a time period where the relationship between these variables was still developing.'
From the fossil data available, the team chose planktonic forams - forams that float passively in the ocean. They have one of the most complete fossil records of any group of organisms and are found at a variety of latitudes.
'They're a rare study system,' says Dr Fenton. 'You can estimate their diversity accurately without complex methods, because they're so widespread in the fossil record, both spatially and temporally.'
The team found that during the Early Eocene there were high numbers of foram species even at latitudes of 60 degrees. However, by the end of the epoch, foram diversity peaked at around 35 degrees before falling sharply towards the poles.
Today's forams show a similar pattern in diversity, with the highest number of species found in tropical waters, around 20 degrees north and south.
This suggests researchers looking to understand the formation of the modern latitudinal diversity gradient should look to the Eocene for clues.
'Despite the fact that none of today's species were around in the Eocene, it looks as though the rules that regulate diversity and shape the patterns we see today were already developing then,' says Professor Andy Purvis, a research leader in the Museum's Diversity and Informatics Division, and a collaborator on the study.
'This highlights the importance of the Eocene Epoch in biodiversity research, and gets us another step closer to understanding the latitudinal diversity gradient.'
The research was published in the journal Philosophical Transactions of the Royal Society B, as part of a special issue looking at the regulators of biodiversity over geological timescales. The data used in the study is available on the Museum's Data Portal.