FA10 Farmland butterflies

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Farmland butterflies have been monitored in Finland by means of transect counts since 1999. During this time the annual fluctuations of butterfly populations have been great. Most of the fluctuation is explained by weather conditions during midsummer. Because of annual variation and shortness of data series very clear conclusions of trends of farmland butterflies cannot yet be drawn.

Sufficient data for a statistical population trend analysis has so far accumulated of 45 species (graph B). Over the monitoring period altogether 19 species underwent either a strong or moderate population decline, 10 species remained stable and 9 species increased moderately or strongly.

On the whole, the populations of farmland butterflies can be estimated to have decreased a little. The trend is not very clear, however, and a couple of good butterfly summers can reverse the situation. Conditions during the summers of 2014 and 2015 were particularly poor for day active butterflies.

The sections below introduce three different ecological groups of farmland butterflies. The populations trends of these groups are not shown, however, since there has been more variation within the groups than among them over the past years.

Grassland butterflies

Semi-natural grasslands are the single most important habitat for Finnish butterfly species. Most of the species preferring grasslands declined considerably during the 20^th century. It has been estimated that the distribution areas of even 70% of the species have decreased. Most of the red-listed day active active butterly species belong to this group, including the False Heath Fritillary and Clouded Apollo.

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.

Several grassland butterflies continue to decline. Since the turn of the 21st century, for example, the populations of Scarce Copper and Small Heath have fallen by more than 90%. The species that have fared better include species that feed on some common plant species or a wider group of plants. Examples of such butterfly species include the Essex Skipper and Lesser Marbled Fritillary.

Butterflies of open field margins

Only few Finnish butterfly species can reproduce and occur relatively abundantly 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.

The populations of Small Tortoiseshell and Peacock have fluctuated steeply in the 2000s. This is probably due to the natural density variation of the parasites living on their larvae. Apart from Green-veined White, all the species in this group migrate to Finland every summer from the south. The populations of such species tend to vary a lot. If there are warm southern air currents at the right time in the summer, millions of individuals of these species migrate to Finland.

Butterflies of forest edges and clearings

Many of Finnish 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.

Altogether 25 forest edge and clearing species are included in the indicator. During the last 50 years 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.

Several of the species in this group have become more common in the 2000s. Such species include the Map and Silver-washed Fritillary. In addition to these also Holly Blue and Brimstone have extented their range. The Orange Tip has increased all over Europe. A few new edge species, including Purple Emperor and Lesser Purple Emperor, have established in Finland lately. On the other hand, there are also species in this group that have declined, especially the Speckled Wood and Northern Wall Brown. The causes of their decline remain unknown.