Rising seas could create coastal habitat for marine invaders, study projects

A study reported by Nature Portfolio on 29 September projects that rising seas could make newly flooded coastal land suitable for marine animals already established outside their native ranges. Researchers at the South China Sea Institute of Oceanology and collaborating institutions modelled 122 species worldwide. Their findings point to estuaries and several heavily exposed coastlines as places to monitor, while stopping short of predicting that the animals will actually establish there.

The study, published in Nature Ecology & Evolution on 28 September, estimates that 290,000 to 470,000 square kilometres of land newly inundated by the middle and end of this century could provide suitable habitat for at least some of the species examined. Nature Portfolio compares the lower estimate with the area of Italy and the upper estimate with Sweden. These are estimates of potential habitat across future scenarios, not an observed area of invasion.

Where the model finds suitable coastal habitat

Nature Portfolio highlights the eastern United States and Gulf of Mexico, coastlines around the North Sea, southeastern China and northern Australia. The pattern is especially pronounced near estuaries, where rivers meet the sea. The researchers report that the closer newly flooded land lies to an estuary, the greater the area their model identifies as suitable for the animals studied. Those broad regional results do not specify which individual estuaries will be colonised.

The species set includes Chinese mitten crabs, Pacific oysters and Asian tiger shrimp, as well as Atlantic blue crabs, European green crabs and Northern Pacific sea stars. The researchers considered animals already established beyond their native ranges, then asked where rising seas might create additional suitable space next to waters they occupy. Their projection concerns possible expansion from existing populations; it is not a count of new arrivals.

How the sea-level scenario shapes the estimate

Nature Portfolio describes a moderate emissions scenario, SSP2–4.5, under which median global sea level rises by a projected 20 centimetres by 2050 and 56 centimetres by 2100. The research team estimates that about 360,000 square kilometres of coastal land could be inundated by 2050 and about 550,000 square kilometres by 2100 under that scenario. Those totals describe flooded land; the 290,000-to-470,000-square-kilometre range describes land the researchers consider potentially suitable for selected non-native animals.

According to the South China Sea Institute of Oceanology, the team assembled information on the species' geographic distributions, invasion histories and breeding habitats. It combined ecological niche models with projections of future coastal flooding. For each newly inundated area, the researchers used modelled conditions in the directly adjoining sea as a reference for assessing suitability on the flooded land. That approach helps locate potential exposure, but it cannot by itself establish whether an animal will arrive, reproduce and persist at a particular site.

Why estuaries need a closer look

Estuaries provide a connection between marine waters and land newly exposed to tidal flooding, making them a focus of the study's proposed monitoring. The authors say early monitoring and management may help limit possible effects on biodiversity, fisheries productivity and other coastal benefits. Nature Portfolio also reports their caution that newly inundated habitat may not always support rapid colonisation. The model's suitable area should therefore be read as a risk indicator, not a forecast of ecological damage.

Earlier, separate research shows why a map of rising water cannot tell the whole story of coastal habitat change. A 2018 study documented by the U.S. Geological Survey examined wetland migration in 39 Gulf of Mexico estuaries. It found that topography and urban development affect how much room wetlands have to move inland as seas rise. The study identified Charlotte Harbor and Tampa Bay in Florida, and Galveston Bay in Texas, among places where development could strongly constrain that movement.

Another USGS-led study, published in 2022 and covering 166 U.S. estuaries, projected that saline coastal wetlands would move inland even as wetlands at their seaward edges were lost. It estimated that two-thirds of potential wetland migration would replace coastal freshwater wetlands and one-third would replace upland areas, including croplands, forests and pastures. Those figures concern wetland change, not marine-animal invasion. Together, the older studies illustrate how local landscapes can shape the coastal changes occurring alongside rising seas.

The new model adds a possible biological consequence to that changing coastal picture: land that becomes connected to the sea may also become suitable for some animals already established nearby. Which sites are colonised, how quickly populations develop and what effects they have remain open questions. The researchers' proposed next step is to incorporate sea-level rise into invasion-risk assessments and focus early monitoring on exposed coastal areas, particularly around estuaries.