Consequences for selected high-elevation butterflies and moths from the spread of Pinus mugo into the alpine zone in the High Sudetes Mountains

Ethics statement

Our field survey was approved by the Czech Ministry of Environment and we were allowed to collect species of Lepidoptera at all alpine sites studied, which are part of the Protected Landscape area Jeseníky. The permission was issued for a group of scientists, one of which was Dr. Tomáš Kuras. Permission Nr.: MŽP/13341/04-620/2319/04 from the 21st of July 2004, valid untill the 30th of November 2007.

Study area

The High Sudetes Mts. form a natural border between the Czech Republic and Poland (Jeník, 1961) and are one of the middle-high mountain systems in Central Europe. There are sites in the alpine zone in the High Sudetes Mts. with different degrees of isolation and different Pinus mugo origin; i.e., these mountains are at the edge of the distribution of Pinus mugo with some sites occupied by this indigenous dwarf pine and others where this shrub is absent or was artificially planted. In the Czech Republic, the High Sudetes Mts. consist of three mountain ranges (Fig. 1): the Krkonoše Mts. (Riesengebirge/Giant Mts.), Hrubý Jeseník Mts. (Altvatergebirge) and Kralický Sněžník Mts. (Glatzer Schneegebirge). Pinus mugo occurs naturally in the Krkonoše Mts., but not in the Kralický Sněžník and Hrubý Jeseník Mts.

We also studied sites at a wider regional scale by including the two geographically nearest alpine habitats of different origins, with Pinus mungo present or absent; i.e., Babia Gora, Slovakia/Poland with Pinus mugo and Harz, Germany, without Pinus mugo (Fig. 2).

Historical context

Summits in the High Sudetes usually reach an altitude of not more than 1,600 m and have a narrow belt of treeless arctic-alpine tundra at the summits. Although the insect fauna of middle-high mountains is typically poor compared to that at lower altitudes (Dennis, Shreeve & Williams, 1995; Fleishman, Austin & Weiss, 1998; Nagy et al., 2003), alpine sites in the High Sudetes Mts. host a unique fauna comprised of arcto-alpine and boreo-montane species, plus endemic species and glacial relicts (Mazalová, Kašák & Kuras, 2012). A similar fauna does not occur anywhere else in Central Europe and the sites studied in the High Sudetes Mts. strongly differ in species composition. Therefore, the High Sudetes have a high conservation value. The exceptional nature of these mountains is associated with their location near the northern-most border of Pinus mugo distribution (Hamerník & Musil, 2007). Dwarf pine is indigenous in most Central European mountains (incl. Western Carpathians, Beskydy Mts., Krkonoše Mts./Riesengebirge) and artificially planted in the Hrubý Jeseník Mts./Altvatergebirge and the Kralický Sněžník Mts./Glatzer Schneegebirge (Klimeš & Klimešová, 1991; Rybníček & Rybníčková, 2004). Afforestation of most of the summits in Hrubý Jeseník Mts. and Kralický Sněžník with Pinus mugo started at the end of the 19th century (Jeník & Hampel, 1992) and resulted in it colonizing the area where it currently covers about 63% of what was previously alpine habitat (Treml et al., 2010). On the other hand, this latter afforestation allows us to test recent vs. postglacial effects of Pinus mugo.

Insect groups studied and sampling method

A survey of butterflies was carried out in the Hrubý Jeseník and the Kralický Sněžník Mts.; a group of invertebrates with high conservation and bio-indication importance. Butterflies were captured using yellow plastic traps (“Moericke water traps”), which are widely used for monitoring nectar feeding insects and thus also suitable for butterflies (Barták, 1997; Roháček, Barták & Kubík, 1998; Beneš, Kuras & Konvička, 2000). The traps were made of circular plastic pans (upper diameter: 13 cm, depth: 6 cm) painted yellow inside (Industrol 6200^®). At the study sites the pans were filled to a depth of about 2 cm with a solution of detergent and water (1 ml of detergent per 1 l of water). We placed the traps approximately 20 m apart along transects at each study site, with the number of traps set dependent on the area of the site. The traps were set during the flight period of the butterflies (Kuras et al., 2001b, Kuras et al., 2003; Konvička, Beneš & Kuras, 2002), i.e., from July 15 to September 3 in 2005, and checked every week (Table 1). We also measured the distance of each trap from the tree-line (in m) using software ArcGIS 9 (ESRI, 2004) and the tree-line specification of Treml & Banaš (2000).

The yellow plastic traps provide a simpler and quicker way of acquiring large quantitative data sets than monitoring transects or capture-recapture. Using yellow water traps avoids human errors as the same principle determines the catch at all sites, which are all caught at the same time. This method is also very effective for faunal surveys of alpine habitats (including butterflies) as very few plants are in flower above the tree-line during the vegetation season (Beneš, Kuras & Konvička, 2000).

This method is non-selective as different taxa of nectar feeding insects were caught. The contents of the traps were separated into different taxa, with the species and sex of the butterflies recorded (Table 2) and other insect groups conserved for future use. During the trapping period, only 15 species were caught: Aglais urticae, Boloria dia, Coenonympha pamphilus, Erebia epiphron silesiana, Erebia euryale, Erebia ligea, Erebia sudetica sudetica, Gonepteryx rhamni, Inachis io, Issoria lathonia, Lycaena hippothoe, Pieris brassicae, Pieris napi, Pieris rapae and Polygonia c-album. The species richness recorded is rather low, consisting mostly of species that migrate into alpine zones from lower altitudes. A total of 7,651 individuals were caught during the summer of 2005. Erebia spp. were the most abundant (94.2% of the individuals sampled), mainly because of their exclusive association with alpine and montane habitats and low probability of occurring at lower altitudes than the other species captured, which are generalists and good migrants. Therefore, we used only four species of Erebia (i.e., E. epiphron silesiana, E. euryale, E. ligea, E. s. sudetica) in the analysis and testing of the effects of environmental factors on autochthonous alpine butterfly assemblages.

Classification of the vegetation

Vegetation at the alpine sites in the Hrubý Jeseník Mts. was classified according to the Habitat Catalogue of the Czech Republic (Chytrý et al., 2010), where habitats are specified in terms of the dominant, diagnostic and other plant species present. There are five types of habitat at the alpine sites studied: (1) Alpine heathland dominated by Calluna vulgaris; (2) Subalpine Vaccinium vegetation dominated by Vaccinium myrtillus and Vaccinium vitis-idaea; (3) Alpine grassland dominated by Avenella flexuosa, Nardus stricta and Festuca supina; (4) Subalpine tall-herbaceous vegetation dominated by Molinia coerullea; and (5) Pinus mugo scrub dominated by dwarf pine. These habitats occurred at all the sites studied. We recorded the percentage vegetation cover made up of each type of habitat within an area of 10 m radius around each trap. The percentage cover of Pinus mugo was obtained from aerial photographs taken in 2005 with a grid of 0.3 m pixel resolution (Wild, 2005) (Table 3).

Local effect of Pinus mugo

We used a unimodal direct gradient analysis, i.e., a CCA (Canonical Correspondence Analysis) to test the association of the abundances of the different species of Erebia with the environmental variables measured (Ter Braak & Šmilauer, 1998). Vegetation types (Calluna, Vaccinium, Avenella, Molinia and Pinus), area of the alpine sites (ha) and distance to the tree-line (m) were considered as explanatory variables. There was little difference in the altitudes of the sites and as it had an insignificant effect it was not included in the model. The covariates of the model were selected by forward selection and were time of year (i.e., number of days from 1st July) and degree of isolation of each alpine site (i.e., distance to the nearest alpine site, in km). We configured the CCA model with an axis scaled in terms of inter-species distances. Species abundances were log transformed and results for males and females were identified in the analysis. Significance of canonical axis and explanatory variables was tested using a Monte-Carlo permutation test (5,000 restricted permutations). The permutation test was restricted by the split-plot design (time series and linear transect as a split-plot and freely exchangeable whole plot). Relationship between species abundance and cover of Pinus mugo was tested using a Generalized Linear Model (GLM) and a Poisson distribution. We tested both sexes of butterflies for possible differences in their behaviour (occupancy and usage of the habitat), especially for Erebia spp. (Kuras, Beneš & Konvička, 2001a; Konvička, Beneš & Kuras, 2002).

Regional effect of Pinus mugo

Faunal structure and its historical development in the presence and absence of Pinus mugo was tested using methods based on diagnostic species of both butterflies and moths. Diagnostic species are alpine and boreal species with an exclusive association with alpine habitats or generally unlikely to be found below the tree-line (Patočka & Kulfan, 2009). From an eco-zoogeographical point of view (sensu Krampl, 1992) it means species with euboreal, boreo-alpine, arctic-alpine and subalpine distributions with disjunct and very narrow dispersal ranges within Central European mountains. All literature sources related to these mountain ranges were critically reviewed with regard to recent findings (Soffner, 1960; Krampl, 1992; Jahn, Kozlowski & Pulina, 1997; Liška, 1997; Liška & Skyva, 1997; Liška, 2000; Kuras et al., 2009). In these publications, the following species were consistently claimed to be exclusively associated with the alpine environment and are therefore referred to as “diagnostic” in this paper (Table 4): Argyresthia amiantella, Blastesthia mughiana, Callisto coffeella, Catoptria maculalis, Catoptria petrificella, Chioniodes viduellus, Clepsis rogana, Clepsis steineriana, Elachista dimicatella, Elachista kilmunella, Elophos operarius, Epichnopterix ardua, Erebia epiphron, Erebia sudetica, Glacies alpinatus, Incurvaria vetulella, Kessleria zimmermanni, Lampronia rupella, Olethreutes obsoletanus, Psodos quadrifarius, Rhigognostis senilella, Sparganothis rubicundana and Xestia alpicola. Pinus mugo cover, nearest distances and areas of the alpine sites were estimated using aerial photographs and GIS analysis.

We used CCA to determine the effect of area of the alpine site, Pinus mugo cover and its origin (naturally occurring/planted) for all summits in the High Sudetes Mts., plus two geographically nearest and isolated alpine sites, i.e., Babia Gora Mts. (Slovakia) and Harz Mts. (Germany). Significance of the canonical axis and explanatory variables was tested using a Monte-Carlo permutation test (5,000 unrestricted permutations).

Next, we tested the similarity in the species composition at all the sites. Diagnostic species were allocated values 1/0 (present/absent) and tested using the UPGMA (un-weighted pair group using arithmetical average) clustering method (Everitt & Hothorn, 2006). Significance of each cluster was estimated and possible subjectivity of the diagnostic species list decreased using bootstrap methods, so that a dissimilarity matrix of the alpine sites was generated for each cluster and bootstrap probabilities calculated using multi-scale bootstrap re-sampling, in order to acquire AU (Approximately Unbiased) p-values.

Local effect of Pinus mugo

After filtering out the effects of covariates, the CCA model explained 15.0% (adjusted explained variation 13.5%) of the variance of the results for this species. As shown in the ordination graph (Table 5 and Fig. 3), most of the variability is accounted for in terms of the distance to the tree-line, area of the alpine site and vegetation cover of Avenella and Pinus. With increase in the area of the site we recorded an increase in the abundance of Erebia epiphron, a typical alpine species. Other Erebia spp. are negatively correlated with the area of the site.

Using the CCA model, we also tested each parameter separately. Each environmental variable was included in the model individually and remaining parameters acted as covariates. We recorded that individual environmental parameters (after subtracting summarized variability of the covariates) accounted for the variability as follows: time of year 11.9% , distance from the tree-line 7.1%, distance to the next nearest alpine site 2.7%, area of alpine site 3.1%, vegetation cover of Calluna 0.5%, Avenella 0.9%, Pinus 0.9%, Molinia 0.4% and Vaccinium 0.4%.

The association between species abundance and area covered by Pinus mugo was further tested using a Generalized Linear Model. The mountain specialists (Erebia spp.) are affected more than the generalists (Aglais urticae and Pieris rapae) by Pinus mugo (Table 6). The abundance of the two most abundant specialists (Erebia epiphron and Erebia euryale) is considerably reduced in areas where there is a relatively low cover of Pinus mugo (Fig. 4). We also tested the association of both sexes of these butterflies with the area covered by Pinus mugo, which revealed no significant difference between males and females in spite of the higher numbers of males caught.

Regional effect of Pinus mugo

CCA accounted for 61% of the variance in the species’ environment (i.e., area of alpine site, percentage cover and origin of Pinus mugo) (Table 7, Fig. 5). The effect of Pinus mugo origin and area of alpine site were significant, but not the percentage cover of Pinus mugo.

Relationship between species and environmental variables identified by CCA was further tested using a similarity analysis (Fig. 6) and three different groups of alpine sites were identified. The first cluster is of alpine habitats with naturally occurring Pinus mugo (Krkonoše-West and Krkonoše-East), which are clearly distinguished from other sites (Multi-scale Bootstrap Re-sampling, AU = 98, p = 0.01). The second cluster includes alpine summits in the Hrubý Jeseník Mts., i.e., Vysoká hole, Praděd and Malý Děd, which is presumably associated with the presence of planted Pinus mugo and their large areas. Furthermore, we can also consider the cluster that includes Kralický Sněžník, Harz, Keprník, Červená hora, Šerák and Mravenečník-Vřesník (last four are in the Hrubý Jeseník Mts.), which are small areas with a dense cover of planted Pinus mugo. These two clusters were separated at a level of significance AU = 92, p = 0.07. The position of the Babia Gora Mt. in the dendrogram seems to be ambiguous (AU = 86, p = 0.11) due to a combination of small species pool, small alpine area and the presence of naturally occurring Pinus mugo.

Local effect of Pinus mugo

In a local context, we tested for the recent effect of Pinus mugo on the butterfly assemblage, namely it’s overgrowing of alpine sites and competing with native vegetation. We tested the response of males and females separately to the vegetation cover because previous research showed that males fly more frequently in search of food than females, which mainly bask and feed on nectar (Kuras, Beneš & Konvička, 2001a; Konvička, Beneš & Kuras, 2002). However, our analyses did not indicate any significant differences between the sexes of Erebia spp. except for the greater probability of catching males.

The presence of Erebia spp was most significantly associated with the area of an alpine site and its distance to the tree-line, followed by vegetation dominated by Avenella (a characteristic species of alpine tundra) and Pinus mugo. Correlation of E. epiphron abundance with the area of the sites supports an alpine origin for this species as does its association with alpine grassland where vegetation dominated by Avenella and Calluna prevail. At small sites there are fewer E. epiphron and E. sudetica (Kuras et al., 2003), species with a high association with treeless sites, or these species do not occur there (Kuras et al., 2001b). Thus, the area of an alpine site alone seems to be of greater importance for the butterfly assemblage than its heterogenity in terms of different types of alpine vegetation. The congeneric species, E. Euryale and E. Ligea, also occur at alpine sites, but mainly at lower altitudes, i.e., in mountain taiga (Kuras, Beneš & Konvička, 2000), or like E.s. sudetica near springs close to the upper tree-line (Kuras et al., 2001b). This fact accounts for the significant effect of distance to the tree-line on the species studied. The relatively small effect of site heterogenity can be due to the similarity of the different types of vegetation and high mobility of the butterflies (Chytrý et al., 2010).

Even though adult butterflies are highly mobile Pinus mugo serves as a barrier to their dispersal (Kuras, Beneš & Konvička, 2001a; Konvička, Beneš & Kuras, 2002). This is also supported by Fig. 4. Dwarf pine overgrows alpine vegetation and in this way is changing the characteristics of this habitat (food available for adults and quantity of suitable plants for caterpillars—see Zeidler et al., 2012) and decreasing the area of native grass-herbaceous vegetation at alpine sites. Over the last 30 years the extent of the cover of planted dwarf pine has increased by 63% and the percentage cover at small alpine sites in the Hrubý Jeseník Mts. is currently more than 30% and is continuing to increase (Treml et al., 2010).

Regional effect of Pinus mugo

Pinus mugo significantly affected the fauna and flora of alpine sites in the Central European Mountains during their postglacial development (Jeník, 1961; Gutierrez, 1997; Čížek et al., 2003). We found a significant effect of the presence of Pinus mugo, but not the area covered by this species at alpine sites. We suppose this is a consequence of the postglacial development at the sites studied. The tree-line shifted during the postglacial era and the area of treeless sites obviously changed (Treml & Banaš, 2008). During the Holocene climate optimum, the conditions for alpine species were even less favourable than now because the mountain summits were overed with dwarf pine or spruce (Rybníček & Rybníčková, 2004). Therefore, species confined to treeless habitats could survive only at very few localities, e.g., glacial cirques, stone debris fields, rocks or wind swept summits.

We also recorded significant differences in the Lepidopteran fauna at the alpine sites studied in the High Sudetes Mts., which were associated with differences in the historical occurrence of Pinus mugo. Our results (Fig. 6) are clearly similar to the effect that Pinus mugo had on the fauna of alpine sites during the Holocene. Alpine sites with naturally occurring Pinus mugo (Krkonoše Mts, Babia Gora Mt.), which most probably completely covered or at least a large part of the summits of these mountains in the past, host predominantly species of Lepidoptera that are able to feed on Pinus or those that survived in glacial cirques or stone debris fields. Alpine sites where dwarf pine was not present (all summits of the Hrubý Jeseník Mts., Kralický Sněžník Mt. and Harz Mt.) were covered with dwarf spruce, which does not form such dense formations as dwarf pine. Thus, species of Lepidoptera associated with grassy tundra could survive there until colder conditions resulted in the tree line occurring at lower altitude.

These changes selected the species assemblages currently recorded at the different alpine sites and the most critical time was when the summits were either completely or nearly covered by trees during the Holocene. Recent climate changes favoured pine, which became a more prominent element in the landscape again and so reversed a natural multicentennial or even millennial trend in tree line decline and recession (Kullman, 2007). Global warming is now favouring a similar spread of planted Pinus mugo into alpine sites where it previously did not occur. Even though its current coverage at these sites is not very extensive, Pinus mugo has the potential to change these treeless sites and increase their degree of isolation (Bílá et al., 2012).