Species on east-west coastlines are more likely to go extinct than those on north-south shores – new study

As the Atlantic warms, many fish along the east coast of North America have moved northwards to keep within their preferred temperature range. Black sea bass, for instance, have shifted hundreds of miles up the coast.

In the Mediterranean, the picture is very different. Without an easy escape route towards the poles, many species are effectively trapped in a sea that is warming rapidly. Some native fish are even being replaced by more heat-tolerant species that have slipped in through the Suez Canal.

It’s a process affecting coastal species around the world: without a continuous pathway to cooler waters, many are in trouble. Escape becomes difficult where coastlines run east–west or are broken into enclosed basins and islands. In these settings, species have to move huge distances just to gain a few degrees of latitude – the so-called “latitudinal trap”.

It’s also a process that has repeated throughout history. When we analysed 540 million years of fossil data for a recent study published in the journal Science, we found that species along east-west coastlines were more likely to go extinct than those with easier movement north-south.

We hypothesised that the shape and orientation of coastlines could help species escape – or trap them. If coastlines provide direct, continuous pathways to move north or south, species should be able to better track shifting climates. But, where species have to travel a long way for minimal latitude gain, their extinction risk is raised during episodes of environmental change.

Coastlines themselves are not fixed. Over millions of years, plate tectonics rearrange continents, sometimes producing long north-south coasts, like those of the Americas today, and at other times sprawling east-west seaways such as during the Ordovician a bit over 400 million years ago.

This means climate shocks can produce very different extinction outcomes depending on the layout of continents at the time.

To test this hypothesis, we analysed fossil data for about 13,000 groups of related shallow-marine invertebrate species, such as clams, snails, sponges and starfish, spanning the last 540 million years. We then paired these records with reconstructions of ancient geography.

For each fossil, we estimated how difficult it would have been for that species to shift its latitude along shallow coastlines. We measured this as the shortest number of steps to travel 5°, 10°, or 15° latitude north or south. (For context, Great Britain covers about 9° from top to bottom). Short distances imply a relatively direct escape; long distances imply a long or maybe impossible escape route.

We found that, over the last 540 million years, extinction risk was consistently higher for marine animals with long escape routes.

Geography amplifies catastrophe

This pattern intensified during Earth’s five mass extinction events. In our models, species with longer distances showed increases in extinction risk of up to 400% during mass extinctions, compared with about 60% during other intervals, highlighting that geography becomes far more consequential when climate change intensifies.

Although our analyses focused on geologic timescales, our results help us understand how shallow marine species may respond to climate change today. Species living in the Mediterranean or the Gulf of Mexico or other regions with semi-enclosed geography, or around the margins of islands, may have more difficulty as the ocean warms.

Coastline geometry may matter less for species that are good at dispersing themselves, however, especially those that have a long planktonic larvae phase where they drift around the ocean before becoming fixed in place. The survival of those species depends more on factors like ocean currents than coastline orientation.

Estimating whether a species is at risk of extinction is typically done with reference to attributes such as body size or geographic range size. But our work shows that extinction risk also depends on geography. Survival during climate upheaval depends not only on a species’ biology – but on whether the map itself offers an escape.

Erin Saupe receives funding from the UK Natural Environment Research Council (NERC) and the Leverhulme Trust.

Cooper Malanoski does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.