Climate change leads to hidden biodiversity loss in Flemish forests
A 25-year study of ground beetles in Flemish forests shows that climate change has a much greater impact on biodiversity than previously assumed. "We expected species communities to reorganize as a result of warming, but what we actually observe is a net loss of biodiversity," says lead author San Yin Leemans of the Institute of Natural Sciences and the Forest & Nature Lab at Ghent University.
The effects of climate change are becoming increasingly visible. Species that were previously found mainly in Southern Europe, such as the wasp spider, the European tree cricket, and the great egret, are now being observed more frequently in Flanders. These striking newcomers may create the impression that climate change is enriching nature. However, the new research paints a less positive picture. Behind the expansion of a few warm-adapted species lies a much larger decline of species adapted to cooler conditions.
Cold-adapted species are disappearing
For this research, 54 forest sites were studied using exactly the same sampling methods in both 1997 and 2023. Comparing ground beetle communities over a 25-year period provides a unique insight into the effects of climate change on forest insect species. Over recent decades, average temperatures in our region have gradually increased, while air humidity has declined. As a result, species better adapted to warmer conditions have gained a relative advantage.
At 83 percent of the surveyed sites, researchers observed a clear shift toward warm- adapted species. This phenomenon, known as thermophilization, accounted for approximately 22 percent of the changes in species composition. However, the study shows that this shift is mainly driven by the disappearance of cold-adapted species rather than by a substantial increase in new warm-adapted species. "It is not the case that new species are increasing en masse; it is mainly the cold-adapted species that are disappearing," says Leemans.
Exceptionally strong decline
The total number of captured ground beetles nearly halved between 1997 and 2023. At the same time, overall species richness declined from 97 to 83 species. Cold-adapted species were particularly affected. "Their populations declined by as much as 70 percent, while we observed hardly any increase among warm-adapted species," says Leemans. "Such a large decline is exceptional over such a short ecological timescale." Several cold-adapted species that were formerly common, such as Patrobus atrorufus and Bembidion mannerheimii, were no longer found at all in the surveyed forests.
Forest fragmentation
The researchers also found that forest fragmentation amplifies the effects of climate change. Cold-adapted forest species with limited dispersal ability are particularly affected. Extreme drought and heat can cause local populations to disappear, and because many forests are isolated, these species often fail to recolonize suitable habitats afterward, even when conditions improve. The exceptionally warm and dry conditions experienced this summer may further intensify these negative effects.
Without targeted measures, many cold-adapted species are likely to continue disappearing from the Flemish landscape. The researchers therefore advocate stronger protection of existing forests and improved connectivity between natural areas. "If we do not tackle climate change and biodiversity loss together, we risk irreversibly losing species and habitats," concludes Leemans.
The study was published in the scientific journal Oikos.
The research was conducted within the FOURCAST project, funded by the Belgian Science Policy Office (BELSPO) under the BRAIN-be 2.0 programme. It is a collaboration between the Royal Belgian Institute of Natural Sciences (RBINS), the Research Institute for Nature and Forest (INBO), Meise Botanic Garden, the Royal Meteorological Institute of Belgium (RMI), and the Forest & Nature Lab of Ghent University.
