Dragonflies and the effects of water draw-down and of acidification in fens

Dragonfly species characteristic of fens are declining. The most important reasons are acidification and eutrophication.

Trends

Dutch fens changed greatly during the 20^th century. The dragonfly fauna characteristic of pristine fens declined sharply, as can be seen from the mean indices of nine characteristic species.

Two species serve as illustration. The northern damselfly is characteristic of fens that are neither acidified nor eutrophied and of the edges of raised bogs. The white-faced darter is a species of slightly acid undisturbed fens in which raised bogs are developing. In the 20^th century the distributions of both species shrank. The northern damselfly is now known from only 16 locations in the Netherlands.

Causes

The decline in the 20^th century was caused by acidification, often in combination with eutrophication. Certain plant species disappeared as a result, and the change in the vegetation structure led to a decline in the dragonflies. In the case of the northern damselfly, for example, the structure of the vegetation of the sedges growing in the water and of the floating pondweeds is crucial. Competition with other species of dragonfly that can survive in acid water has probably aggravated the decline of this species.

Both the northern damselfly and the white-faced darter are on the Red List of dragonflies.