Supplemental data from Causal analysis of the temperature impact on deep-sea biodiversity

The deep sea comprises more than 90% of the ocean; therefore, understanding the controlling factors of biodiversity in the deep sea is of great importance for predicting future changes in the functioning of the ocean system. Recently, we have increasing consensus on two plausible factors, (i) seafloor particulate organic carbon (POC) derived from primary production in the euphotic zone and (ii) temperature, as the drivers of deep-sea species richness that have often been discussed in the contexts of the species-energy and physiological tolerance hypotheses. Nonetheless, factors that drive deep-sea biodiversity are still actively debated potentially owing to a mirage of correlations (sign and magnitude are generally time dependent), which are often found in nonlinear, complex ecological systems, making the characterization of causalities difficult. Here, we tested the causal influences of POC flux and temperature on species richness using long-term palaeoecological datasets derived from sediment core samples and convergent cross mapping, a numerical method for characterizing causal relationships in complex systems. The results showed that temperature, but not POC flux, influenced species richness over 103–104-year time scales. The temperature–richness relationship in the deep sea suggests that human-induced future climate change may, under some conditions, affect deep-sea ecosystems through deep-water circulation changes rather than surface productivity changes.