Population size and structure
In terms of adult demography, the studied E. aethiops population shared several features with mountain Erebia congeners occurring in the Czech Republic (Cizek et al., 2003; Kuras et al., 2003) as well as with a mountain E. aethiops population from the Alps (Loertscher, 1991). In all these cases, adult recruitment is protandrous, with the maximum abundance and balanced sex ratio in the middle of the flight period. A difference in relative timing of male and female emergence at different altitudes was reported in some butterflies with broad altitudinal range, such as in Euphydryas aurinia (Rottemburg, 1775), whose lowland populations are protandric (Zimmermann et al., 2011), whereas sexes appear synchronously at high altitudes (Junker et al., 2010). In contrast, Erebia butterflies seem to be protandrous across altitudes (Loertscher, 1991; Cizek et al., 2003; Kuras et al., 2003; Slamova & Klecka unpubl. data). The studied E. aethiops population contained approximately 1400 individuals, similar to a population inhabiting a network of mountain meadows in Switzerland (Loertscher, 1991), whereas populations containing tens of thousands of individuals may exist at large areas of suitable habitat in England (Kirkland, 1995). Apparently, the population sizes reflect the available habitat area.
Habitat requirements and mobility
E. aethiops adults utilise both open and wooded patches, and its sexes differ in habitat use. Adults reach the highest densities on small open enclaves within forests and the lowest densities at large meadows and pastures with no scattered trees. Female densities were higher than male densities at the open grasslands with solitary trees, and male densities were higher at shrubby sites (Fig. 6). At the open enclaves surrounded by forest, the resources essential for males (shade, mating substrates) and females (nectar and larval sources) overlapped spatially, in contrast to large pastures or meadows where especially male resources were scarce.
Figure 6. (a) Grassland patch with nectar supply of Origanum spp. and Centaurea spp. preferred by females and (b) abandoned pasture with solitary trees, habitat favoured by males of Erebia aethiops (photo I. Slamova).
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The intersexual differences in habitat use reflect the associations of individual activities with vegetation structures, investigated by Slamova et al. (2011). Males stay most of the time at the partially shaded clearings to avoid overheating, whereas females nectar, bask and oviposit at grasslands with solitary trees and a high litter accumulation. Mating takes place on shrubs. Females tolerate warmer ambient temperatures, probably because they periodically cool down while laying eggs at bases of grass tussocks (Slamova et al., 2011); they also likely profit from lighter colouration and higher weight (cf. Heinrich, 1986; Konvicka et al., 2002). The female tolerance for warmer microclimate, and resulting association with warmer sites, is likely advantageous during cooler days of adult flight, enhancing their fertility by prolonging oviposition time (cf. Gossard & Jones, 1977; Karlsson & Wiklund, 2005). It also provides an advantage for larvae, which should profit from the warmer microclimate at such sites in autumn and spring. There is a trade-off involved, however, due to desiccation risk at the warmest spots of the calcareous locality. E. aethiops larvae require a humid and stable microclimate, best provided by a litter in older grass tussocks (Leopold, 2006).
Sex-related differences in habitat use were previously reported for at least two other woodland butterflies. In a cage experiment with artificial vegetation, Leimar et al. (2003) observed that females of L. achine and P. aegeria preferred open parts of the cage, whereas males preferred shaded ones. Such intersexual differences in habitat perception might represent a common but overlooked phenomenon in butterflies living in structurally diverse habitats and utilising different structures for different activities (Dennis et al., 2003; Vanreusel & Van Dyck, 2007). Different motivations affect female and male habitat choice. Females strive to maintain an optimum body temperature (e.g. Karlsson & Wiklund, 2005), maximise nectar intake to increase fecundity (e.g. Mevi-Schutz & Erhardt, 2003) and locate egg-laying sites. Males invest mainly in mating activities, such as locating virgin females and defending them against rivals (e.g. Rutowski, 1991).
Despite different microhabitat preferences of E. aethiops sexes, both sexes display similar range size and movement probabilities to longer distances. The majority of individuals stayed within a subsite or crossed only short distances (Fig. 2), similar to the mountain congeners E. epiphron (Knoch, 1783) and E. sudetica (Staudinger, 1861) (Kuras et al., 2003). The closest locality occupied by another large E. aethiops population is within the Boletice military area, 5 km westerly from our study system, well within the range of predicted movement distances. The ability to reach more remote locations, however, likely depends on permeability of the landscape matrix (Hanski, 1998; Ricketts, 2001; Baguette & Van Dyck, 2007; Lange et al., 2010). We did not detect, however, any effect of subsite area or borders on E. aethiops movement patterns. We admit, however, that we delimited habitat boundaries somewhat deliberately, which is often unavoidable in complex environments, particularly if the animals use diverse structures, change activity patterns in time (Cizek & Konvicka, 2005; Vanreusel & Van Dyck, 2007) or, as in E. aethiops, differ in space utilisation between sexes.
Relating mobility to habitat structure, both sexes tended to stay at subsites with sparsely growing trees, and females at subsites with accumulated litter. In addition, males more frequently emigrated from flowery patches, perhaps due to increased intrasexual interactions with other males, or while alternating between sun-exposed and shaded patches (cf. Slamova et al., 2011). While marking, we often noticed males crossing forest along narrow paths. Females left some subsites isolated by forest too, but they probably did so by direct flight, because we observed only a few female individuals within these shady areas.
Reserve management and conservation
Abandoned grassland patches, containing solitary trees, scattered shrubs and high accumulation of litter, supported the highest butterfly densities, in line with the requirements for oviposition (litter-rich tufts) mating (shrubby structures) and overheating prevention (shade). Grassland abandonment, however, is not a viable management option in the long term, as unmanaged grasslands eventually turn to scrub. Occasionally mown or lightly grazed patches hosted the second-highest E. aethiops densities, and alteration of these management options with temporary abandonment appears as a suitable compromise, keeping the habitat open while still supporting the study species. Intensively mown grasslands, on the other hand, supported the lowest densities, likely due to the absence of litter, temporary nectar shortages, and perhaps direct mortality caused by mowing (Dover et al., 2010; Cizek et al., 2012). All the above observations point to E. aethiops' requirements for a highly diversified vegetation structure with alteration of shaded, half-shaded and sun-exposed spots, with both managed grassland patches offering nectar and neglected patches with litter-rich grass tussocks for oviposition. Such habitats traditionally existed in open woodlands, which have virtually disappeared from many regions of Europe (Spitzer et al., 2008; Warren & Bourn, 2011).
At least two other open woodland butterflies, Hamearis lucina (Linnaeus, 1758) and Leptidea sinapis (Linnaeus, 1758), have locally retracted from shady woodlands to abandoned shrub-encroached meadows (Fartmann, 2006; Clarke et al., 2011). This also applies to the studied E. aethiops population, as open woodlots are scarce within the Vysenske kopce Reserve, and the species reaches the highest densities at shrubby grasslands. On the other hand, shrub-encroached grasslands are unsuitable for several other priority butterflies occurring in the reserve and inhabiting short-sward grasslands, such as Polyommatus coridon (Poda, 1761) and Pyrgus trebevicensis (Warren, 1926) (Hanc, 2005). Therefore, instead of allowing the grasslands to turn into scrub, E. aethiops population should be supported by managing the forests towards more open conditions (see Turner et al., 2009; for an analogous situation). Gradual formation of small coppiced panels within local forests would create a continuity of diverse successional stages not only for butterflies, but also for other invertebrates, birds, plants and fungi (Warren & Bourn, 2011). To increase the connectivity with other E. aethiops localities on a regional scale, stepping-stone sites could be formed by managing wider transitional zones between forests and grasslands in the largely cultivated landscape.