Jesenik Mts Geometry of Habitats Affects Population Structures

With timberline (formed by sparse growths of Norwegian spruce) at about 1,250 m a.s.l. and with the highest summit (Praded Mt., at 1,495 m), Jesenik Mts. do not contain alpine habitats sensu stricto. The flat plateaux above the timberline harbors 12 km2 of subalpine grasslands, dominated by Nardus stricta, Calamagrostis villosa, and Festuca supina. These Nardeta of the Jesenik plateau represent essentially stabilized habitats not affected by succession (German: Dauergesellschaften). Steeper, wind-shielded valley headwalls affected by periodic avalanches harbor structurally diverse and species-rich tall-herb formations (Jenik 1961; Jenik et al. 1980).

The relic Erebia species of this mountain area differ in habitat use. E. epiphron occurs in high abundance at these stable subalpine grasslands, but rarely descends to elevations near the timberline, whereas E. sudetica is limited to the islets of timberline-adjoining tall-herb formations, which are transitional habitats in a gradient of natural succession (Fig. 1). The two remaining Erebia species (E. euryale and E. ligea) inhabit openings within mountain forests, ranging from ca 700 m to growths of dwarfed spruce at about 1,300 m.

Although these patterns were already noted by Stiova (1988), the first quantitative support originated from a trapping study (Benes et al. 2000). This study used flower-mimicking water-filled traps, an efficient method for capturing Lepidoptera of subalpine habitats. A total of 35 traps exposed for 665 trap-days captured 3,948

Fig. 1 Map showing the position of Jesenik and Krkonose Mts. in Central Europe (lower left corner), extend of subalpine habitats, and the distribution of two alpine butterflies, Erebia epiphron and E. sudetica. For E. epiphron, the entire distribution extend is shown, whereas for E. sudetica, the map shows only the larger colonies in Jesenik Mts. The northernmost Jesenik colonies shown are those in Ramzova saddle, which separates Jesenik (South) and Rychlebske Mts. (North)

butterflies and moths, including 3,731 Erebia individuals. The tall-herb formations near the timberline yielded the highest species diversity, corroborating patterns known for vegetation (cf. Jenik 1961, 1998). There was a significant habitat separation among woodland-preferring E. ligea and E. euryale, timberline-preferring E. sudetica, and grassland-preferring E. epiphron (Kuras et al. 2000).

The summit grasslands form several islets in Jesenik. E. epiphron inhabits the two largest units, main ridge (area 10 km2) and Mravenecnik (area 1.2 km2), both separated by 4 km of forested saddles (Fig. 1). In contrast, the tall-herb formations inhabited by E. sudetica form an archipelago of valley headwalls and springs. Kuras et al. (2003) used the mark-recapture method to illustrate that the differences in spatial configuration of habitats determine the population structures of both species. The main ridge population of E. epiphron, studied by mark-recapture in 1996, was really huge: For a total of 4,034 marked individuals, only 236 individuals were recaptured. This was likely due to rather high local density, as the estimated numbers were about 20,000 males for 1 km2 of habitat, corresponding to hundreds of thousands of individuals for the entire main ridge (female numbers were not estimable), and the unsuitable mountain weather conditions not permitting daily field work. For the lateral Mravenecnik population, marking for 8 days in 1997 yielded 2,542 captured and 440 recaptured individuals, giving an estimate of 4,500 individuals.

Fig. 1 Map showing the position of Jesenik and Krkonose Mts. in Central Europe (lower left corner), extend of subalpine habitats, and the distribution of two alpine butterflies, Erebia epiphron and E. sudetica. For E. epiphron, the entire distribution extend is shown, whereas for E. sudetica, the map shows only the larger colonies in Jesenik Mts. The northernmost Jesenik colonies shown are those in Ramzova saddle, which separates Jesenik (South) and Rychlebske Mts. (North)

In the case of E. sudetica, one large and one small population were studied in 1998, the large one yielding 2,171 captured and 893 recaptured individuals, the small one yielding 253 captured and 80 recaptured individuals. The estimates were 5,000 and 300 individuals, respectively, suggesting a population size for the entire main ridge area of around 10,000 individuals.

E. epiphron individuals were considerably poorer dispersers than those of E. sudetica. The lifetime probability of moving 1 km was 0.5-1.2% for the former and 2.2-6.4% for the latter, the wide range being due to differences among sexes and populations (Kuras et al. 2003). The difference became even more dramatic over longer distances: the probability of crossing 5 km was 1 x 10-10 for E. epiphron and 1 x 10-7, thousand times higher, for E. sudetica. The latter species is thus more mobile, indicating that populations inhabiting relatively unstable environments, such as disturbance-dependent timberline sites, should profit from occasional migrations, while those inhabiting stable and extended environments, such as the summit grasslands, may do without them.

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