Latitudinal gradients in diversity intrigue scientists, and various hypotheses have been proposed to explain why the diversity of so many taxonomic groups increases from the poles to the equator. These hypotheses range from null models to environmental factors, and biotic interactions to those that include patterns of dispersal and speciation. Here, we formulate a new theory based upon the concept of the niche sensu Hutchinson and the principle of competitive exclusion, which shows that the latitudinal diversity gradient in the marine environment may result from an interaction between the thermal tolerances of species and climatic variability.
Methods
We design a bioclimatic model that creates pseudospecies from strict stenotherms to large eurytherms and subsequently allows them to colonize a global ocean provided they can tolerate fluctuations in temperature. We test 74 ecologically realistic scenarios that are then correlated with observed patterns of species richness for foraminifers and copepods, two important oceanic planktonic groups.
Results
We found that the model accounted for 96% of the latitudinal gradient in foraminifers and 85% for copepods. Our model both reveals how patterns of biodiversity may develop, and suggests why some taxonomic groups appear not to follow the general pattern. While climate ultimately selected species that could establish in any given habitat, we saw a strong mid-domain effect (MDE) in the niche space. We believe this negates some shortcomings of the MDE that is often assumed to occur in the geographical space.
Main conclusions
By showing the strong effect of temperature on biodiversity and revealing how it enables the development of a planetary gradient in marine biodiversity, our results offer a way to better understand why temperature is so often positively correlated with global patterns in species richness on a global scale.
