Scientists from the Universities of Oxford, Leeds and Bristol have solved a 200-year-old mystery about why the number of species of living organisms is greatest near the equator and decreases towards the polar regions. The results of the study are published in the journal Nature .
In both marine and terrestrial ecosystems, animals, plants, and unicellular organisms are known to exhibit a latitudinal biodiversity gradient with a peak at the equator. So far, however, limited fossil evidence has prevented researchers from identifying the causes of the gradient.
The scientists analyzed 434,113 records in the global database of Paleogene and Neogene fossils spanning the last 40 million years of the evolution of foraminifera, single-celled shell organisms that make up a large proportion of marine plankton. The relationship between changes in species numbers over time and space and potential factors such as sea surface temperature and ocean salinity was then examined.
The modern latitudinal diversity gradient first began to show about 34 million years ago, when the Earth’s climate was moving from warm to cooler. Initially, this gradient remained weak until about 15-10 million years ago, but then it intensified, which coincides with a significant global cooling. Planktonic foraminifera abundance peaked at higher latitudes 40-20 million years ago, but around 18 million years ago, peak diversity shifted between 10 and 20 degrees latitude.
A strong direct relationship was observed between species richness and sea surface temperature. There was also a positive correlation between species richness and the severity of the thermocline: the temperature gradient that exists between warmer mixed water at the surface of the ocean and cooler deep water below.
The results indicate that the current distribution of species richness of planktonic foraminifera can be explained by an increase in the latitudinal temperature gradient from the equator to the poles over the past 15 million years. This may have created more ecological niches in tropical regions in the water column compared to higher latitudes.