Tracking marine bioregions through a million years of climate change

Marine bioregions capture major discontinuities in the spatial distribution of species assemblages, reflecting the interplay among temperature, nutrient supply, circulation patterns, and other environmental factors. Planktonic foraminifera provide an exceptional archive for investigating these dynamics, as their continuous, globally distributed, and taxonomically well-resolved fossil record enables spatially explicit reconstructions across climatic oscillations. While modern and late-Holocene datasets reveal short-term migration of foraminiferal bioregions, the geographic migration of these regions on longer timescales remains poorly characterized. Characterizing such bioregions and their dynamics helps us understand how marine ecosystems change in response to climate warming.

Using the Triton compilation of globally distributed Pleistocene–Recent assemblages, we reconstruct foraminiferal bioregions over the past 1 million years, using relative abundances of bioregion-diagnostic (indicator) species based on the modern data. We used this approach to overcome spatially uneven sampling that increases deeper in time.  We then evaluate bioregion temporal stability, shifts in dominant assemblages, and patterns of latitudinal migration. Using the relative abundance of indicator species, we quantify bioregion composition at both the site level and the global level, calculate mean paleolatitudes through time, and compare bioregion dynamics through glacial–interglacial cycles.

Our results reveal substantial, climate-linked restructuring of bioregions. Bioregions generally migrated equatorwards during cooler intervals, whereas migration was generally polewards during warmer periods - a pattern consistent with thermal niche tracking. However, bioregions exhibit heterogeneous sensitivities, with some displaying strong, directional responses to temperature changes and others showing muted or asymmetric behavior. These findings demonstrate that bioregions do not act as uniform units under climate forcing; instead, their mobility and resilience vary depending on their species composition and environmental context.

By contextualizing contemporary and future bioregional changes within a million-year perspective, this work underscores the importance of spatially explicit biodiversity baselines for conservation planning. The results provide empirical insights relevant to implementing the Kunming–Montreal Global Biodiversity Framework, particularly Targets 3, 4, and 8, which emphasize spatial conservation prioritization, species persistence, and climate change mitigation.