3 million years of a marine latitudinal diversity gradient

Perhaps the largest, most visible, macroecological trend on this planet is that there are generally more species in the tropics (low latitudes) than the poles (high latitudes). This pattern has been observed in both terrestrial and marine systems, across hugely varied groups of plants and animals. Despite the fact that this has been known for awhile now, no unified model exists that explains how this latitudinal species gradient came about or what mechanism(s) act to maintain it (intriguing hypotheses abound, however, including faster speciation in the tropics). What the structure of this gradient was like in the past is uncertain as well; for example, how stable this pattern is through time?

A recent paper in Ecology Letters by Dr. Yashuhara and colleagues uses paleotemperature information and diversity data of tiny planktonic critters called formaminfera (forams, if you’d like fit in) to investigate this latitudinal species gradient at various snapshots through time, over the past three million years.

Planktonic (residing in the water column while alive, rather than at the bottom of the sea in the benthos) forams as a groups are hugely abundant; deep-sea sediment in some parts of the world can comprise of almost 70% of forams, particularly their tests, or tiny calcium carbonate shells. Scientists have been studying forams in depth for some time now, in no small part due to their importance in studying past climate and oceanic conditions, and as a result their taxonomy is well-known. So if you’d like to take a detailed look at the latitudinal diversity gradient in the ocean over time, this is a good model system for that.

Using past datasets consisting of foram diversity and reconstructed sea-surface temperatures, the researchers investigated four specific temporal snapshots: modern, 18,000 years ago (Last Glacial Maximum), 120,000 years ago (Last Interglacial), and 3.3-3.0 millions years ago (in the Pliocene). They found that unimodal (think a relationship with a single hump-shaped peak) relationship between species diversity and latitude for all time points, with diversity generally peaking in the subtropics and falling off rapidly towards the poles. Additionally, used the reconstructed sea-surface temperature dataset, they found that species diversity tracks closely to temperature. For example, diversity was highest in the mid-Pliocene samples when it was the warmest of the time points considered here, and lowest during the last glacial maximum, the coldest. The authors even point out that the latitudinal diversity gradient was steeper during the last glacial maximum (i.e. the diversity in this set of samples decreased faster–at a steeper slope–towards the poles than the other time points), but this is likely due to the fact that the temperature is known to change more drastically with latitude during this time period.

This is important because it indicates that temperature has a big role to play in determining what species (and how many) live where. It also shows, as other studies have, that temperature is an important predictor of diversity in the ocean. This work’s central finding shows that this relationship has been robust at these various time points throughout the last three million years, even when examined at very high taxonomic resolution. Paleoecological work is really interesting and hugely useful in this vein; it can show us how things like climate can affect biodiversity over large timescales, and allow a different perspective of how large ecological patterns work.

Yasuhara, M., G. Hunt, H. J. Dowsett, M. M. Robinson, and D. K. Stoll (2012). Latitudinal species diversity gradient of marine zooplankton for the last three million years. Ecology Letters DOI: 10.1111/j.1461-0248.2012.01828.x