Spatially heterogenous range dynamics since the Last Glacial Maximum underpin the modern bimodal latitudinal diversity gradient in planktonic foraminifera

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Abstract
The modern latitudinal diversity gradient (LDG) of planktonic foraminifera is bimodal, with a distinct depression near the equator surrounded by mid‐latitude diversity peaks. This pattern emerged after the Last Glacial Maximum (LGM; 19–23 ka), but it remains unclear how species' spatial dynamics contributed to its formation. Here, we investigate how species range dynamics, namely trailing‐edge contractions (extirpations) and leading‐edge expansions (colonisations), shaped the modern bimodal pattern. We analysed taxonomically standardized LGM and modern foraminiferal assemblage datasets to characterize changes in species diversity at multiple spatial scales, from the global ocean to within basins. We quantified species' range shifts and estimated site‐level richness change, temporal turnover, and the net imbalance between colonisations and extirpations (NICE). We found no evidence of systematic trailing edge contractions, indicating that equatorial extirpations did not generate the bimodal LDG. Instead, leading‐edge expansions were evident in the Atlantic Ocean, suggesting that the modern bimodal LDG of planktonic foraminifera arose primarily through mid‐latitude colonisations. In the Pacific Ocean, however, biodiversity changes were highly spatially heterogeneous. Given the overall warming trend since the LGM, we expected cold‐adapted species to be at greater risk of local extinction; consistent with this expectation, species' thermal optima weakly predicted the number of extirpations. Together, our findings show that climate‐driven biodiversity change can arise from spatially heterogeneous, species‐specific range dynamics, with important implications for monitoring and protecting marine biodiversity.

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