>>Background information

Development

Butterflies have been monitored in Finland by means of transect counts since 1999. The monitoring scheme has concentrated on agricultural habitats and species.

Because of annual variation caused varying weather conditions and shortness of data series no definite trends can yet be observed. However, due to favourable conditions – most importantly warm summers – during the past couple of years more species have increased rather than decreased over the whole monitoring period.

Out of the 33 species for which a TRIM-analysis yielded a trend indication, 15 species showed increasing and 11 species decreasing trends. For 18 out of 51 species the data were too scanty for reliable analysis. In terms of ecological groups the recent increasing trend appears to be strongest in the case of species associated with grasslands and forest edges.

Grassland butterflies

Semi-natural grasslands are the single most important habitat for Finnish butterfly species, and most of the species preferring grasslands have declined considerably during the 20^th century. It has been estimated, that the distribution areas of ca. 70 % of the species have decreased. Grassland butterflies benefit from traditional farming practices, such as cattle grazing on natural pastures. However, Finnish agriculture has become increasingly intensive during the last 50 years, and consequently the areas of grasslands and pastures have decreased drastically.

During the last ten years the populations of the most common grassland butterflies have remained rather stable, but annual variation has been considerable. There have also been differences between species, as the populations of for instance Small pearl-bordered Fritillary (Boloria selene) have increased, while Small Heath (Coenonympha pamphilus) has declined.

Butterflies of open field margins

Only few Finnish butterfly species are relatively abundant in the open, windy margins surrounding arable fields. Three of these are the Whites of genus Pieris (napi, rapae and brassicae), which use a wide variety of wild and cultivated crucifers as larval food plants. Also the nettle-eating Nymphalids and Vanessa –species are common in open farmlands.

Butterflies of forest edges and clearings

Many of our butterfly species require the shelter and warmth offered by sunny, half-closed forest edges and small forest clearings. These species typically avoid crossing larger open areas, such arable fields or pastures, but they cannot survive in mature, closed forests either. During the last 50 years or so the distribution areas of these species have remained mostly stable, in contrast to the grassland butterflies. This is likely to be a consequence of the modern large-scale forestry, which continuously creates new suitable habitat patches, such as edges surrounding clear-cuts for these species.