Space-for-time substitution reveals partial transferability of marine biodiversity-temperature relationships

To understand how life on Earth might shift as the planet warms, scientists often face a difficult choice: they cannot wait decades to watch the future unfold, so they must look for clues in the present. One common strategy is to compare different places at the same moment. If a warm ocean today holds fewer species than a cold one, researchers assume that warming a cold ocean in the future will cause a similar drop in species. This method, known as space-for-time substitution, treats geography as a stand-in for time, assuming that the rules governing life across a map are the same as the rules governing life through history. It is a practical necessity, given that detailed records of how ocean life has changed over thousands of years are incredibly rare. Yet, the fundamental question remains whether a snapshot of the world today can truly predict how that world will change tomorrow.

A team of researchers decided to test this assumption directly using the tiny, ancient shells of planktonic foraminifera. These microscopic organisms drift in the ocean and leave behind layers of fossilized remains on the sea floor, creating a continuous record of life that stretches back through deep time. By combining modern data on where these creatures live today with fossil records from specific locations, the scientists could compare two different ways of measuring change. They looked at how species composition shifts across the globe at a single point in time, and they compared that to how composition shifts at a single location over thousands of years as the water temperature changed. They also examined how the total number of species, known as alpha diversity, responded to temperature in both scenarios.

The results revealed a subtle but important distinction between the two approaches. When the researchers modeled how species turnover—the swapping of one group of organisms for another—responded to temperature, the steepness of the relationship was similar in both space and time. However, the starting points were different. The model based on geography consistently predicted a higher rate of species turnover across the full range of temperatures than the model based on history. Even when the scientists narrowed their geographic comparison to match only the specific temperature ranges found in the fossil records, the mismatch in the starting point remained, though it became smaller. This suggests that while looking at different places gives a good sense of the general direction life will take, it tends to overestimate the magnitude of the change.

For the total number of species living in a specific spot, the geographic model performed better, closely matching the historical record of how diversity changed with temperature. This indicates that the space-for-time approach captures the broad direction of biodiversity change but struggles to predict exactly how much change will occur over time. The study does not discard the method entirely, but it clarifies its limits. For the timescales examined in this research, using the present map to forecast the future timeline provides a reliable compass for direction, but it is less precise when measuring the distance of the journey.