Wednesday, June 25, 2025

Cold winters halt the northward spread of species in a warming climate

Stockholm University

image:
Mats Ittonen, evolutionary ecologist and entomologist, former PhD student at Stockholm University, currently postdoctoral researcher in the Macroecology workgroup at the University of Tartu, Estonia Photo: Mats Ittonen
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Credit: Mats Ittonen

As the climate warms, many species are shifting northward into areas that were previously too cold for them. A new study on the wall brown butterfly, published in the scientific journal PNAS, shows that rapid evolution can aid this process – but only up to a point. Cold winters stop further expansion beyond certain climatic limits.

“Our results show that even though the butterflies adapt their life cycle as they move northwards, there are limits that evolution cannot easily overcome,” says Mats Ittonen, one of the lead authors of the study done by researchers at the Department of Zoology, Stockholm University.

The wall brown (Lasiommata megera), a common European grassland butterfly, has become more common at higher latitudes in Scandinavia, despite declining steeply in Western Europe. During its northward expansion, rapid evolution has helped the butterfly adapt its life cycle to new conditions – evolution helps the species cope with factors that do not change with climate warming, such as daylength. But despite these changes, cold winters still prevent the wall brown from spreading further north.

Two years of field experiments

The researchers did field experiments in which they moved butterflies within and north of the species’ current range in Sweden. Individuals collected in the south (Skåne county) and north (Södermanland, and Uppland counties) were placed in field cages at sites in Skåne, Södermanland, and southern Dalarna – the latter being an area where the species has not yet spread. By comparing how caterpillars of different origins grow and develop under shared environmental conditions, the researchers could investigate how the wall brown has evolved during its northward expansion over the past 20–30 years. The caterpillars were also left in the field over winter to see whether better winter survival has evolved in northern populations – and whether they could persist further north than the species is currently found.

“We wanted to know whether traits like faster growth, properly timed winter dormancy, and improved winter survival have evolved in northern butterflies – traits that could help them establish even further north,” says Karl Gotthard, professor at the Department of Zoology, Stockholm University.

Few caterpillars survived in the north

The study found that butterflies from northern populations grew faster than those from the south, likely as an adaptation to the shorter summers of higher latitudes. They also managed to enter dormancy at the correct time of year. But despite these evolutionary changes, very few caterpillars survived the winter in Dalarna, north of the species’ current range.

“These kinds of limits are important to recognize if we want to predict how far north other species – including pests and disease vectors – will be able to move in a future climate,” says Karl Gotthard.

Winter cold remains a barrier

The findings show that evolution can proceed quickly enough to influence the distribution of insects under climate change, yet that does not necessarily mean that the specific traits that are most critical for survival evolve. For the wall brown butterfly, further northward expansion will still require milder winters, not just longer and warmer summers. This may be true for many species.

“The wall brown is far from alone—many other insect species have also expanded northward over the past few decades, both in Sweden and elsewhere. Understanding both which traits evolve and which ones actually set the limits for where species can live is essential for predicting the future of ecosystems and biodiversity in a warming world,” says Mats Ittonen.

Find the study ”Winters restrict a climate change-driven butterfly range expansion despite rapid evolution of seasonal timing traits” in PNAS.
DOI: 10.1073/pnas.2418392122

Journal

Proceedings of the National Academy of Sciences

DOI

10.1073/pnas.2418392122

Method of Research

Observational study

Subject of Research

Animals

Article Title

Winters restrict a climate change-driven butterfly range expansion despite rapid evolution of seasonal timing traits

Article Publication Date

23-Jun-2025

The wall brown butterfly (Lasiommata megera) is common in large parts of Europe. In Sweden it is often found on beaches and pastures, where it likes to rest on rocks and stone walls. Photo: Mats Ittonen

The wall brown butterfly (Lasiommata megera) is common in large parts of Europe. In Sweden it is often found on beaches and pastures, where it likes to rest on rocks and stone walls. Photo: Mats Ittonen

Caterpillars of the wall brown butterfly live on several common grasses, such as fescue, bluegrass, and orchard grass. Photo: Mats Ittonen

During autumn, the caterpillars were raised in cages at the Tovetorp research station, Stockholm University. Photo: Mats Ittonen

The winter experiment at the Tovetorp research station, Stockholm University. The caterpillars overwintered in jars covered with a plywood sheet as protection against rain and snow. Photo: Mats Ittonen

Karl Gotthard, professor of Animal Ecology at the Department of Zoology, Stockholm University, Sweden Phto: Rickard Kilström/Stockholm University

Study finds early signs of widespread coastal marsh decline

Colorado State University

Marsh deterioration with sea-level rise graphic — image:
Declines in belowground biomass can indicate marsh vulnerability to loss. This diagram illustrates how a healthy salt marsh (left) can convert first to a vulnerable marsh (middle) before loss of the vegetated marsh (right) as a result of increased inundation intensity, which is driven by sea-level rise. Diagram by Kyle Runion
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Credit: Kyle Runion/Colorado State University

Researchers have revealed the declining health of coastal marshes several years before visible signs of decline, providing an early warning and opportunity to protect an ecosystem that serves as the first line of defense against coastal flooding.

Scientists from Colorado State University, the University of Georgia and the University of Texas at Austin developed a model to detect early signs of marsh decline using satellite observations. The model identified vulnerable marshes along Georgia’s coast by locating declining root production – a harbinger of marsh failure.

The research, published in June in Proceedings of the National Academy of Sciences, is the first to predict broad trends in marsh deterioration using remote sensing.

“Our findings show widespread belowground decline over the past decade and suggest this is an early warning sign of marsh deterioration and loss,” said lead author Kyle Runion, a research scientist at the University of Georgia who was at UT Austin’s Marine Science Institute during the study. “By pinpointing where belowground loss is happening, we can get a head start on conservation and restoration projects to more effectively prevent marsh loss.”

Coastal marshes filter water, store carbon, act as habitat for wildlife and provide food and fishing livelihoods for people. Marshes also absorb storm surges and sea-level rise, preventing worse flooding inland.

"These marsh areas might be in someone's backyard,” said senior author Jessica O’Connell, a CSU professor and Runion’s adviser. “It’s important to have this early warning and a chance to do something before you lose these special landscapes that people have an economic or emotional connection to.”

O’Connell added, “Conserving the natural landscape is a lot cheaper than losing it and then trying to come up with an environmentally engineered way to do the things that the land used to do.”

The study focused on the marsh grass Spartina alterniflora, which is found along much of the U.S. coast. The researchers examined the entire Georgia coast – an area for which they had extensive field data – and found belowground biomass has declined across 72% of Georgia’s coastal marsh since 2014, with nearly 30% suffering substantial loss.

They are now working to make the model universally applicable to all marsh plants and coastlines.

Declines in belowground biomass indicate marsh vulnerability to loss

The research team developed the Belowground Ecosystem Resiliency Model (BERM) using more than a decade of field data gathered through the Georgia Coastal Ecosystems Long Term Ecological Research Program at the University of Georgia. The program, led by co-author Merryl Alber and funded by the National Science Foundation, collected data on marsh grass leaves and stems aboveground, along with roots and rhizomes belowground.

This rigorous fieldwork detailed the complicated – and sometimes contradictory – relationship between above- and belowground marsh grass growth. The study found that marsh grass might appear to be healthy or even thriving aboveground, while the roots could be receding.

Coastal marshes are adapted to salt water and fluctuating water levels. Periodic flooding or slow sea-level rise can boost plant growth by flushing out excess salt buildup and bathing plants in nutrient-rich sediment, which contributes to the soil in which they grow.

“Coastal wetlands such as salt marshes historically have responded dynamically to keep pace with sea-level rise in part by developing extensive root networks, which expand the soils with organic-rich material,” Runion said. “This accumulation of ‘blue carbon’ drives elevation gain and enables the marsh surface to maintain elevation relative to rising sea levels.”

However, if the water stays too high, marsh vegetation can drown as roots are deprived of oxygen. Root system decline is the first sign that a marsh is failing.

“The declines in vegetation were closely related to flooding pressure, which will only worsen with the rapid acceleration of sea-level rise that the southeast U.S. is experiencing,” said Alber, a UGA professor and director of UGA’s Marine Institute. “That is why this work is so important.”

BERM predicts both above- and belowground biomass based on plant characteristics that are observable from space and environmental factors, such as elevation, sea level and climate. The researchers applied it to all Georgia locations of Spartina alterniflora, mapped by co-author Christine Hladik, a geography professor at Georgia Southern University.

The study was also co-authored by Deepak Mishra at the University of Georgia and Mark Lever at the University of Texas at Austin Marine Science Institute. Research was funded by the National Aeronautics and Space Administration, the National Oceanic and Atmospheric Administration and the National Science Foundation.