Butterfly survivors on Europe’s vanishing raised bogs trace an east-west decline

Raised bogs are among the strangest landscapes in temperate Europe. These waterlogged, acidic domes of peat support a cold-adapted flora and fauna that belongs, in a sense, to another climate entirely, persisting as azonal islands of subarctic and boreal conditions within a temperate biome. A new analysis of 105 raised bog sites across the northern lowlands of Central Europe now shows that the butterflies dependent on these extreme habitats tell a strikingly coherent story: species richness falls steadily from east to west, and the pattern closely mirrors the history of human destruction of peatlands.

The study, published in the journal Nota Lepidopterologica, focused on six butterfly species classified as mire specialists, meaning tyrphobiontic species that are essentially restricted to peat bogs or tyrphophilous species that preferentially occur there. The selected species were the Moorland Clouded Yellow (Colias palaeno), the Cranberry Fritillary (Boloria aquilonaris), the Bog Fritillary (Boloria eunomia), the Cranberry Blue (Plebejus optilete), the Large Heath (Coenonympha tullia) and the Jutta Arctic (Oeneis jutta). The researchers compiled presence and absence records from raised bogs in northwest and northeast Germany, northern Poland, Lithuania and northern Belarus, drawing on two decades of expert field surveys conducted along transect lines between roughly 1998 and 2018.

Every site was visited during three to five different years, typically in the June to August flight period, and transect lengths of one to three kilometres covered the most important habitats at each locality. By relying on presence and absence data rather than abundance counts, the authors ensured comparability across regions that differed in survey intensity. Because the underlying records were collected over many seasons by experienced specialists, the authors argue that the broad picture of which species occur where should be highly reliable, even without standardised quantitative sampling.

The headline result is a clear gradient. Mean species number per site dropped from about four species in northern Belarus and just over three in Lithuania, to roughly 2.3 in northeastern Poland, to around 1.1 in both northwest and northeast Germany. Individual species showed equally revealing patterns. Colias palaeno and Boloria eunomia were entirely absent from northwest German sites and nearly absent from the northeast, while their occurrence soared eastwards: Colias palaeno was found at more than 90 percent of sampled bogs in Lithuania and northern Belarus, and Boloria eunomia reached 100 percent occupancy in northern Belarus. The boreal Oeneis jutta followed a similar eastward-increasing trajectory, reaching two-thirds of Belarusian bogs but never occurring in Germany.

One species broke the mould. The Cranberry Fritillary, Boloria aquilonaris, was continuously distributed across all sampled regions, present at 50 percent of northwest German sites and between roughly 32 and 75 percent elsewhere. Unlike the more sensitive specialists, it appears to be the most tolerant of the tyrphobiontic butterflies in the face of human impact. The authors note that it persists in smaller bogs with partial forest cover and recolonises open patches of former peat-cutting holes rapidly after hydrological restoration. Its sensitivity is nonetheless real: about half of the historically occupied peat bog sites in northwestern Germany have suffered local extinctions of this species.

The Large Heath told an unexpected story in the opposite direction. Although long considered a bog specialist, Coenonympha tullia was absent from all raised bogs surveyed east of the river Vistula in Poland and in Lithuania and Belarus, despite its Palaearctic range covering the entire study area. Previous research offers a likely explanation: the species can reach twentyfold higher densities in managed but undrained fens dominated by broad-leaved bog-cotton, and its larval food plants, sedges of the genus Carex, are far more abundant in fens than in raised bogs. Where pristine, traditionally mown fens survive in Eastern Europe, the raised bogs nearby may simply be suboptimal refuges. In Western Europe, where fenland has largely vanished, peat bogs increasingly serve that fallback role.

From a biogeographical perspective, the peat bogs of the European Lowlands function as palaeorefugia, isolated cold and nutrient-poor habitats in which boreo-montane and arcto-alpine insects established populations during the Holocene. Raised bogs began forming in the young moraine landscapes of northern Europe during the Early Holocene and developed through the Mid and Late Holocene, remaining cold islands within a warming temperate biome. The authors favour the term palaeorefugia over the familiar label glacial relicts, because these species do not have an obligatory connection to Pleniglacial or Late Glacial environments; rather, they colonised bogs over past millennia as the mires themselves developed.

The west-to-east decline in butterfly species richness, the study suggests, is mainly a consequence of human pressure rather than natural biogeography. Western European countries have lost far more than 90 percent of their intact peatland ecosystems, while several Eastern European countries have retained between 15 and 50 percent of their natural peatland heritage. The figures cited for remaining natural peatland are stark: roughly 1 percent in Germany, 15 percent in Poland, 40 percent in Lithuania and about 45 percent in Belarus. The geographic pattern of species depletion documented in the 105 bogs roughly coincides with this gradient in remaining habitat, and the almost complete gap in occurrences of Colias palaeno and Boloria eunomia in northern Germany, where Colias palaeno went extinct in the first half of the twentieth century, fits the same picture.

Ecological flexibility also varies regionally, which complicates simple categories of bog dependence. Colias palaeno is strongly tyrphobiontic in most of Central Europe but uses dwarf shrub communities above the treeline in the Alps. Boloria eunomia exists as two distinct ecotypes in the eastern part of its range, one feeding on Bistorta major on wet meadows and the other on Vaccinium oxycoccus on raised bogs, with recent findings suggesting the meadow form is ancestral. In Finland, some of the six species fall into the highest class of mire dependence, with over 95 percent of populations in mires, while others are less tightly bound. The lesson is that a species’ degree of association with peat bogs is not fixed across its range but shaped by the surrounding landscape and the availability of alternative wetlands.

Beyond diagnosis, the study offers practical guidance for restoration. The authors propose Boloria aquilonaris and Plebejus optilete as target species whose presence can signal success in peat bog restoration across the western part of the northern lowlands. Both are tyrphobiontic in the strict sense, threatened by drainage, degradation and afforestation, and yet ecologically flexible enough to respond positively to rewetting, deforestation and the return of vegetation heterogeneity. Boloria aquilonaris colonises open peat-cutting holes quickly after hydrological restoration, while Plebejus optilete tolerates a wider spectrum of larval host plants and restricted flowering resources. Because both are among the most widely distributed mire specialists across the entire region, their re-establishment in the few remaining German bogs, where only five sites currently host both together, could provide a living measure of whether Europe’s efforts to bring its cold islands back from the brink are working.

Subject of Research: Distribution patterns of mire specialist butterflies in raised bogs of the Central European lowlands

Article Title: The distribution pattern of mire specialist butterflies in raised bogs of the northern lowlands of Central Europe

Article References: Sommer, R. S., Thiele, V., Sushko, G., Sielezniew, M., Kolligs, D., & Dapkus, D. (2022). The distribution pattern of mire specialist butterflies in raised bogs of the northern lowlands of Central Europe. Nota Lepidopterologica, 45, 41-52. https://doi.org/10.3897/nl.45.75182

Image Credits: AI Generated

DOI: 10.3897/nl.45.75182

Keywords: raised bogs, butterflies, tyrphobiontic species, peatland restoration, Boloria aquilonaris, Colias palaeno, Plebejus optilete, biogeography, palaeorefugia, species richness gradient, Central Europe, biodiversity