Study Area: the communities of butterflies were studied along two altitudinal transects identified within the municipality of Saint Marcel. The choice of the sites was made in such a way as to reflect as far as possible the following criteria: presence of homogeneous habitats covering at least one hectare; proximity, at least for some of the sites, to MeteoReseau stations. The first transect was located in the Valley of Mount Corquet, with three sampling stations (1550 m, location Laychèr; 1970 m, location Bren; 2280 m, Mount Corquet). The second was located along the Saint Marcel torrent, following the path that leads to the plateau of the Gran Chaux hunting lodge, with three sampling stations (1870 m, location Roullaz; 2300 m, peat bog a wetland below the Gran Chaux; 2400 m, Gran Chaux, a rhododendron patch adjacent to the Gran Chaux).
Protocol: the technique adopted is that of the time transect over a limited surface area, using a semi-quantitative sampling technique which, in anticipation of the repetition of monitoring over a period of time, satisfies some basic requirements: it's simple to apply, economical and easy to standardize, in addition to already having been widely tested and applied (Pollard 1997, Pollard & Yates 1993). An operator has to traverse a specific surface area in a given time span (30 minutes in our case), covering it wholly in a zigzag pattern and identifying all the specimens on the way within a hypothetical square measuring 5 m per side. Monitoring operations were conducted on a weekly basis from June to August, for a total of 10 weekly monitoring walks per season during 2010-2011. The field activities were conducted by two operators and in a time band between 10 a.m. and 4.30 p.m.
Community composition: 72 species of butterflies were identified in all, with 31 found to be new sightings for the area (Balletto 2007) and 4 at risk according to the criteria of the Habitats Directive (92/43/EEC) and the IUCN. They are: Parnassius apollo (Ann. IV, NT), Euphydryas aurinia glaciegenita (Ann. II, LC), Maculinea rebeli (NT), Maculinea arion (Ann. IV, EN).
The composition of the community of the individual sites showed variability from year to year. This variability is, however, much less in areas located at higher altitudes (Mount Corquet and the Gran Chaux, as shown in the figure, from the bond distance revealed by Cluster Analysis carried out on the composition of the community of single areas and years of monitoring). The areas located at higher altitudes were then found to be at the same time made up of extremely specialized elements and with a low dispersion capacity, and characterized by a composition of communities that was more stable in time.
Phenology: in both years, the highest number of species and individuals was sampled during July. The number of new species sampled gradually grew until it reached the asymptote during the second half of August (8th sampling session): hence during September it is rare to sample species not previously identified. The weighted average of the period of flight, namely peak flight, is reached during the 5th sampling (23-29 July). In 2011, we observed a slight anticipation of peak flight (non-significant differences between years; t-test for paired data; t = 1285; df = 4; p = 0.268), but a significant increase in the standard deviation for this value (t -test for paired data, t = -3141; df = 4; p = 0.035), indicating a longer flight time and less synchronization between the species present in the community. Not all species are in fact show the same variability in phenological parameters: in particular we observed that the species that overwinter as pupae and the species most closely linked to high altitudes are those that showed, by comparing 2010 with 2011, a greater variability in the flight period.
Advantages. Butterflies are suitable for use as biological indicators. They respond quickly to environmental and climatic changes. Variations in their communities reflect and anticipate responses in other animal groups. The relative ease of sampling (economical and easily standardized techniques) makes for wide use of this taxon as a case study. Furthermore, the fact that they are holometabolous, i.e. have a series of larval stages, whose development depends on the weather and climatic conditions, as well as the availability and quality of food (food plants for caterpillars), means their phenology is closely linked to environmental conditions.
Disadvantages. Given the brevity of the life span of individual specimens and the relatively short period of flight of some species, phenological studies call for frequent sampling (weekly). In addition, the close tie between flight behavior and meteorological variability makes it difficult to identify phenological trends caused by climate change. Many interesting factors related to the link between communities of butterflies, their phenology and meteorological-climatic variability, especially in high-altitude environments, however, still have to be explored and deserve further study.