MI8 Mire butterflies

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Development

Eight Finnish butterfly species can be regarded as mire specialists. Over 74 000 observed individuals of these species have been recorded into the National Butterfly Recording Scheme in Finland (NAFI) since it was started in 1991. These come from a total of 964 observation squares, each 10 x 10 km in size.

One or more mire species was reported from 577 squares in the 1990s (annual average 128 squares) and from 509 squares in the 2000s (annual average 116 squares). Between these periods the numbers fell by approximately 10%. This development has continued during the four first years of the 2010's (annual average 102 squares).

The distribution area of Finnish mire butterflies appears to have shrunk especially in southern Finland where mire butterflies are now reported from only approximately half of the squares of the early 1990s. Partially, the declining trend may also be  explained by changes in observation activity.

Although the stronghold of mire butterfly populations is in the northern parts of the country, less observations accumulate from there compared to more southern districts. Of the approximately 500 annual squares 14% are located in northern Finland, 26% in central Finland and 60% in southern Finland. The declining trend in the distribution area of mire butterflies in northern Finland is therefore more uncertain than the trends in central and southern Finland. In recent years the trend in northern Finland has started to rise.

Mire butterflies

Eight of the altogether 122 day-active Finnish butterfly species occur predominantly on mires. These species can be roughly grouped into two based on their distribution area and abundance. The first group consists of four relatively common mire species, which are still quite widespread also in central and southern Finland. These are Bog Fritillary, Cranberry Fritillary, Baltic Grayling and Large Heath.

The second group consists of species with a more northern distribution. The most abundant of these are Zig-zag Fritillary and Willow-bog Fritillary, which are absent from the southern coast and Åland Islands. The Lapland Ringlet flies solely in odd years in the southern parts of its range. The Grizzled Skipper is clearly the rarest of these species although it has been recorded almost in the whole country. The species in this group have been observed to decline most rapidly after the draining of pristine mires.

Some other butterfly species also occur on pine mires and fen edges, but these are more generalist in their habitat preference. The most common are Moorland Clouded Yellow, Green Hairstreak, Cranberry Blue and Pearl-bordered Fritillary. Especially in the northern parts of the country these species also thrive in woodlands and other habitats. As the draining of mires increases tree growth these species may even benefit from draining activities. The Disa Alpine, which flies only in northern Finland, is another typical species occurring on mires with tree cover.