Imagine species moving house as the world warms. Some will move toward the poles, dive deeper, or climb higher to find their ideal environmental conditions. These relocations are known as species range shifts. Range shifts occur worldwide and are especially fast in the oceans, thus leaving ecologists wondering: are redistributions helping species persist in the face of climate change? Well, it is complicated. Some species thrive, but others struggle. Similar to moving – sometimes it is a good idea, but sometimes the move is stressful and messes up your life ultimately proving to be a bad decision. Ecologists seek to understand this connection better as it can help predict how species will fare in the future and guide efforts to protect them. Our new study  attempts to figure out if redistribution is good or bad for species in a warming world. The answer will be key for managing wildlife conservation as our planet changes.

Understanding how local changes in species’ population size are affected by shifts in geographic range presents a multifaceted challenge. For species predicted to benefit from climate change (‘winners’), basic models assume their overall abundance remains stable after a range shift. This implies a seesaw where populations in the equatorward side of the species range decline as their range contracts, while populations in the poleward side of the species range grow due to expansion. A recent study in marine species corroborates this, demonstrating abundance increases in cooler, poleward populations alongside declines in warmer, equatorward ones. However, range shift velocities vary considerably, with some species sprinting poleward at up to 100 kilometers per year while others barely budge. We thereby ask:

How does diversity in migration pace influence population trends?

What may be the fate of rapid range shifters?

We propose two central outcomes that frame the consequences of rapid poleward range shifts: ‘shift advantageous’ and ‘shift and collapse’. For the first outcome, rapid poleward movement might indicate species' adaptability, perhaps through broad environmental tolerance. In the simplest scenario, assuming overall stable abundance throughout the species range, the population increase in the cooler edges would be balanced by a decline in the warm edge (as in the figure above). Further, under this outcome, species that do not shift may indicate poor abilities to cope with climate change and hence are predicted to show negative population trends across much of the species range. In contrast, the ‘shift and collapse’ scenario portrays a bleaker picture. Here, rapid poleward range shifters will be associated with population declines. One explanation for this may be limited tolerance to novel environmental constraints, both biological (e.g., competition, mutational load) and environmental (e.g., extreme climatic events in the leading edge). However, this scenario has never before been examined and may act as a ‘canary in the coal mine’ for species persistence in the face of climate change. Elucidating the connection between species-specific range shift velocities and population trends is vital for identifying the true ‘winners’ and ‘losers’ of our changing climate, allowing us to effectively manage and protect biodiversity in the face of ongoing global changes.

What we did:

To investigate if range shift velocities help species persist under climatic changes, we used 2,572 local fish abundance time series spread worldwide from the BioTIME database belonging to 146 species and joined them with estimates of range shift velocities using the BioShifts database. Further, we calculated the spatial position of each population with respect to the entire species' latitudinal range using occurrence data from the Global Biodiversity Information Facility. This enables the separation of poleward populations from equatorward populations that may have unique abundance responses.

What we found:

Fast-moving fish were not exactly thriving. The faster species were shifting toward the poles the greater their populations were declining. This was especially strong for populations that were located in the cold and poleward portion of the species range.

What is the meaning of our findings?

It seems that the rapid shifts are not enough to overcome the hurdles of climate change. This pattern matches what we call the ‘shift and collapse’ outcome, where poleward movement does not safeguard them from warming. Even if species manage to adjust themselves by moving towards the poles, their populations still suffer from decline. Hence, even at the colder, poleward-facing edges, things seem tough for fish under the Anthropocene. This means we need to pay extra attention to fish populations on the move, both for conservation and fisheries. Just because they are shifting range does not mean they thrive.

We share tremendous gratitude to the BioTIME and BioShifts teams for opening these unique datasets to enable the execution of this study.