From plankton to fish: 21st-century redistribution of marine biodiversity and the changing role of rare species

Climate-driven shifts in marine biodiversity are reshaping ecosystems globally, yet most projections focus on charismatic or commercially important species. This leaves major gaps in understanding how climate change affects whole food webs, particularly through changes in rare species that can disproportionately influence biodiversity and ecosystem resilience. We address this gap by providing the first multidimensional projections of climate-driven biodiversity change across co-occurring phytoplankton, zooplankton, benthos and fish assemblages in the Northeast Atlantic. Using Hill numbers to quantify α-, β-, and γ-diversity, we systematically capture changes in richness, composition, and rarity across trophic levels. Leveraging extensive long-term survey data and Bayesian Additive Regression Trees, we project biodiversity trajectories to 2100 under multiple emissions scenarios, with detailed results shown for RCP 4.5.

Our projections reveal divergent responses across the marine food web. Fish and benthos show widespread increases in γ-diversity driven by poleward range expansion, whereas phytoplankton and zooplankton exhibit widespread declines, indicating potential disruptions to energy transfer from lower to higher trophic levels. By comparing Hill numbers of different orders, we show that shifts in rare species are central to these patterns: climate-driven arrivals of newcomers increase biodiversity in some regions, while the loss of locally rare species signals decline in others. These dynamics cannot be captured by conventional species distribution models focused on individual taxa.

Together, these results demonstrate that climate change will restructure marine biodiversity in ways that vary markedly across trophic levels. To support management under shifting baselines, our work provides a suite of complementary biodiversity indicators capable of assessing current status and projecting future trends across assemblages. These indicators provide early warning of biodiversity change and offer a pathway toward embedding predictive, preventive tools into decision-making and long-term conservation strategies. Our framework identifies emerging hotspots of biodiversity change, highlights regions at risk of food-web disruption, and contributes a scalable approach for tracking progress toward international biodiversity targets in a rapidly warming ocean.