Diurnal butterflies mainly inhabit open and semi-open areas and are sensitive to changes in their environment. They are popular bioindicators because they are attractive and relatively easy to collect. Due to their sensitivity to climatic factors, among others, diurnal butterflies are good indicators of the effects of climate change. Observing long-term trends allows conclusions to be drawn about the impact of climate change on the species' distribution ranges.
Since 1990, thermophilic species have increased significantly in Switzerland. The cold-loving (psychrophilous) high mountain species, such as the Alpine Grayling, are in decline. As a result of global warming, species that prefer higher temperatures tend to expand their spatial distribution, while species adapted to lower temperatures are in decline. In the medium term, this trend – replacement of alpine species by more widespread lowland species – will lead to uniformisation and impoverishment of species communities.
Unlike the Swiss Bird Index, which measures changes in population size, the butterfly index is based on the distribution of species (presence or absence in a given area). Ecologically, this process is much slower. Even a small change in the index already illustrates significant changes in the species populations.
The assessment of the indicator focuses only on the evolution of psychrophilous species, as these species are the most threatened by the effects of climate change.
The indicator is similar to the EU Grassland Butterfly Indicator in its calculation method. However, the two indices differ in the number, choice and ecology of the butterfly species considered and are not directly comparable.
The indicator is based on the joint evaluation of data from the Biodiversity Monitoring (BDM) database and the Info Fauna database. It assesses the variation in the species distribution. The average of the distribution ranges for the years 2003 to 2007 was given an index value of 100. In order to take into account the heterogeneity of the data, the different sources of data (e.g. censuses carried out within the framework of the red list of butterflies or spot observations) are considered separately. This compensates for the fact that each data source does not cover all species in the same way. These differences were taken into account to estimate the evolution of the species concerned, making it possible to estimate trends for 170 of the approximately 200 butterfly species observed in Switzerland and to have them validated by experts. The species' site preferences are taken from the German manual used for the analysis of faunistic data, Fauna Indicativa. The trends for cold-loving and thermophilic species were then aggregated to obtain the curves presented here.
|Targeted trend||Initial value||Final value||Variation in %||Observed trend||Assessment|
|Stabilisation||Average 1990-1992||Average 2019-2021||-23.32%||Decrease||negative|
|Basis: Psychrophilous species|