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Supplementary text describing the datasets we used and providing key references. Supplementary figures displaying the multidimensional ecological niches of the 117 plankton species we used in our study.

International audience; Species richness is greater in places where the number of potential niches is high. Consequently, the niche may be fundamental for understanding the arrangement of life and especially, the establishment and maintenance of the well-known Latitudinal Biodiversity Gradient (LBG). However, not all potential niches may be occupied fully in a habitat, as measured by niche vacancy/saturation. Here, we theoretically reconstruct oceanic biodiversity and analyse modeled and observed data together to examine patterns in niche saturation (i.e. the ratio between observed and theoretical biodiversity of a given taxon) for several taxonomic groups. Our results led us to hypothesize that the arrangement of marine life is constrained by the distribution of the maximal number of species’ niches available, which represents a fundamental mathematical limit to the number of species that can co-exist locally. We liken this arrangement to a type of chessboard where each square on the board is a geographic area, itself comprising a distinct number of sub-squares (species’ niches). Each sub-square on the chessboard can accept a unique species of a given ecological guild, whose occurrence is determined by speciation/extinction. Because of the interaction between the thermal niche and changes in temperature, our study shows that the chessboard has more sub-squares at mid-latitudes and we suggest that many clades should exhibit a LBG because their probability of emergence should be higher in the tropics where more niches are available. Our work reveals that each taxonomic group has its own unique chessboard and that global niche saturation increases when organismal complexity decreases. As a result, the mathematical influence of the chessboard is likely to be more prominent for taxonomic groups with low (e.g. plankton) than great (e.g. mammals) biocomplexity. Our study therefore reveals the complex interplay between a fundamental mathematical constraint on biodiversity resulting from the interaction between the species’ ecological niche and fluctuations in the environmental regime (here, temperature), which has a predictable component and a stochastic-like biological influence (diversification rates, origination and clade age) that may alter or blur the former.

International audience; Aim 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. Location The global ocean. 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.