Cold winters halt the northward spread of species in a warming climate
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. The publication in PNAS: ”Winters restrict a climate change-driven butterfly range expansion despite rapid evolution of seasonal timing traits” DOI:10.1073/pnas.2418392122
