Raised bogs are extreme and azonal ecosystems with a characteristic hydrological balance, microclimatic conditions and a specific flora and fauna. Recently, these ecosystems have increasingly become the focus of scientific and general attention because of their important ecosystem roles in the face of global warming and providing biodiversity refuges. From a biogeographical and evolutionary context, the peat bogs of the European Lowlands serve as palaeorefugia, acting as cold, edaphic island habitats for arcto-alpine or boreo-montane insect species in temperate biomes. Analysing 105 peat bog sites in the northern lowlands of Central Europe, we compare the diversity and geographic distribution pattern of a subset of six butterfly species, which appear to be tyrphobiontic or tyrphophile mire specialists. We demonstrate a decrease in mean species number in the European Lowlands on a gradient from the east (Northern Belarus, about 4 species) to the west (Northern Germany, about 1 species), and suggest that the decreasing species number may be mainly caused by human impact in the past. The individual distribution pattern shows a nearly complete gap in occurrence of the sensitive bog specialist species Colias palaeno and Boloria eunomia in Northern Germany and an increasing presence of those species in peat bogs of eastern Europe. Boloria aquilonaris shows a different pattern, which, in contrast to C. palaeno, is continuously distributed in all sampled regions and seems to be the more tolerant of tyrphobiontic butterflies in the face of human impact on peat bogs. In the light of other recent findings our results also suggest that Boloria aquilonaris and Plebejus optilete may serve as target species reflecting success in ecological restoration of peat bog ecosystems.

Raised bogs are extreme and azonal ecosystems with characteristic hydrological balance, microclimatic conditions and a specific flora and fauna (Mikkola and Spitzer 1983; Sushko 2012). The initiation of peat bogs in northern European young moraine regions began during the Early Holocene and the development of raised bogs occurred during Mid- and Late Holocene climatic periods (Mauquoy and Yeloff 2007; Ruppel et al. 2013). Raised bogs and fens have important ecological functions in terms of hydrological balance, as well as storage of carbon dioxide and methane on local and global scales (Edom 2001). Whereas in Western Europe many countries have lost far more than 90% of intact peatland ecosystems due to human impacts, several countries in Eastern Europe have maintained between 15–50% of their natural peatland heritage (Bragg et al. 2003; Sushko 2012; Bonn et al. 2016). Recently, in light of numerous scientific studies regarding the role and function of peatlands in the global carbon and water cycles (e. g. Dorrepaal 2009; Kasimir et al. 2018 and references therein), there have been increased bog restoration efforts in many European countries; many of which have been supported by the European Union (Bonn et al. 2016). From a biogeographical point of view, raised bogs in Central Europe provide refugia for boreo-alpine species in a large scale temperate landscape and support the habitat preference of boreal or subarctic species mainly by climatic (cold) and edaphic (acidic and nutrient poor) conditions (Spitzer and Danks 2006; Sommer et al. 2015). During the climatic development from the Early to Late Holocene peat bogs remained “cold-lands” within the temperate biome and established patchy and isolated refugial habitats for cold adapted biota (Spitzer and Danks 2006; Sommer et al. 2015). The term “glacial relicts” is frequently used for boreo-montane species from peat bogs, especially by lepidopterists (Turlure et al. 2009, 2010). However, as there is no obligatory connection between these species and Pleniglacial or Late Glacial environments (Sommer et al. 2015), we suggest following Spitzer et al. (1999) and Spitzer and Danks (2006) in classifying peat bogs with cold adapted and tyrphobiontic insect species as “palaeorefugia” which indicate the colonization origin of this species and establishment of populations during past millennia of the Holocene. In this sense the butterfly Boloria aquilonaris and dragonfly Aeshna subarctica are typical cold-adapted species; isolated relict survivors in peat bog palaeorefugia (Sommer et al. 2015). As documented, in northern Germany, during the 19^th and 20^th centuries peatlands and raised bogs were increasingly degraded and drained by humans (Joosten and Couwenberg 2001; Heinecke et al. 2011; Thiele et al. 2015). For example, in northern Germany several tyrphobiontic butterfly species such as B. aquilonaris or Plebejus optilete experienced a strong decline or went extinct, as did Colias palaeno in historic times, due to peatland degeneration (Reinhard et al. 2014; Thiele et al. 2015, 2016; Meineke 2020; Caspari et al. 2020). These species also nearly disappeared from neighboring areas in Poland, i. e. from Western Pomerania (Buszko and Masłowski 2015). Accordingly, it has been suggested that cold adapted stenotopic butterfly species will increasingly suffer with future rising global temperatures (Settele et al. 2008; Thiele and Hoffmann 2017). Thus, in light of current climate change and efforts to restore peatlands (Noreika et al. 2016) it is important to have both current and historical information on environmentally sensitive target species in order to predict further developments of species distribution by modelling or adaptation of conservation strategies locally and globally. Here we analyse the distribution pattern of mire specialist butterflies along a west-east gradient in the Central European lowland regions.

We selected a subset of six species that are mire specialists after Noreika et al. (2016). These species (Colias palaeno, Oeneis jutta, Boloria aquilonaris, Boloria eunomia, Coenonympha tullia and Plebejus optilete) show a regularly presence in peat bogs in the young moraine areas of northern Germany, northern Poland, Lithuania and northern Belarus and have mainly a boreo-montane distribution pattern. In Central Europe these species show a strong association with peat bogs. Several of these species, for example B. eunomia or C. palaeno appear to have relict distributions, restricted to palaeorefugia in Central and Northern Europe (Nève et al. 2009; Krzysztofiak et al. 2010; Turlure 2010; Sommer et al. 2015). The Bog Fritillary B. eunomia is not restricted to raised bogs but also inhabits pine bogs, fens and marshes and the ecotone regions between bogs and fens (Nève et al. 2009; Noreika et al. 2016; Sielezniew et al. 2019). We analysed a dataset of records of the six selected mire specialist butterflies from 105 raised bogs in the European Lowlands (Fig. 1 and Table 1). The studied region for the countries mentioned above is mainly a function of the research activities on Lepidoptera by the authors of this paper. In these regions, over the last 20 years, peat bog sites with a potentially promising insect fauna (including Lepidoptera, Coleoptera and dragonflies) were selected and regularly recorded along transects lines. Every site was visited during three to five different years regularly during the flight period in June to August. The transect lines (1–3 km in length) represent the most important habitats for each site, and accordingly differ in size. Only the species inventory of insect populations is compared between the sites and no quantitative information is given. We compiled presence/absence records for the six selected species (from peat bog site) (Table 2, Suppl. material 1: Table S1 and Fig. 1). Percentage frequencies of occurrence were calculated to evaluate the consistency of species in the sites in each focused region. We used absence/presence data for our data assessment to ensure comparability (because of differences in the number of field surveys and extent of sampled transects). However, as our data are based on numerous samplings from each site in different seasons, over the last 20 years, taken by very experienced experts in the field. Thus, the broad picture of presence/absence should be reliable. We determined a gradient in species richness from west (NW Germany, NE Germany) to east (NE Poland, Lithuania, N Belarus) in the northern lowlands of Central Europe (Table 1 and Fig. 1). Additionally, we assessed the local presence of species to determine the differences in species patterns and the depletion of fauna from a geographical point of view. From our large sampling area and available regional context information, we reviewed the selected species in the light of a possible geographical gradient in the degree of association with peat bogs. The observed regional degree of association with peat bogs is assigned to the different species in four different categories that are displayed in Table 3: 1. tyrphobiontic sensu stricto (exclusively restricted to peat bogs), 2. tyrphobiontic sensu lato (tyrphobiontic to tyrphophilous), 3. tyrphophilous (preferentially occurs in bogs but also other habitats) and 4. frequently present in but not obligatory associated with peat bogs. For calculating mean species richness per site, frequency of occurrence, standard errors, standard deviation and data plots we used the software program “Past”, version 3.25, 05/2019 (Hammer et al. 2001).

Figure 1. 

Geographic covering of sample sites (raised bogs) in the northern lowlands of Central Europe and number of selected tyrphobiontic/tyrphophile butterfly species. For selected species see Table 2.

The inventory of six selected tyrphobiont species from 105 raised bogs in our investigation area shows remarkable differences in individual species composition and species richness along a gradient from western to eastern Europe (Fig. 1, Table 1). In Western Europe the mean number of species per site was low (NW Germany 1.12 and NE Germany 1.14) in contrast to Eastern Europe (northern Poland 2.27, Lithuania 3.03 and Belarus 4.00) (Table 2, Fig. 2b). Two to three species are regularly recorded from NW and NE Germany, whereas up to six are recorded from the eastern European countries (Table 2). Fig. 2a displays the number of recorded species from each site and shows highest values for NW and NE Germany of up to three species per site, whereas there are 4–6 species in NE Poland, Lithuania and Belarus (Figs 1, 2a). The individual species patterns are given in Table 2 and Fig. 3a–f. The pattern of the tyrphobiontic species C. palaeno and B. eunomia, extinct in Northern Germany, shows a very similar trend with increasing occurrence from west to east (Table 2. Fig. 3a, c). Oeneis jutta is absent from Germany but shows increasing frequency along a west to east gradient (s. Table 2, Fig. 3f). In contrast, the other strongly tyrphobiontic species, B. aquilonaris, is present in all regions (Fig. 3b), although in NE Germany the frequency is below 50% (Table 2). In Northern Germany only two of the bog-associated species show an uninterrupted, continuous occurrence: the tyrphobiont B. aquilonaris and the tyrphophilous C. tullia (Table 2).

Figure 2. 

Number of sampled peat bog sites and recorded species per site. (a) Number of peat bogs surveyed per study area (y-axsis). The number of species from single sites are keyed with different colors in the bar plots (see legend at the right margin). (b) Mean species number (whisker represent the standard error) from raised bogs in different regions.

We are grateful to Torsten Foy for creating Figures 1 and 2a–f. We thank Tony Stuart, Thilo Storm and David Lees for improving the English text and comments on the manuscript. Thanks to Matthias Nuss and two anonymous reviewers for helpful suggestions and comments on an earlier version of the manuscript.