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.
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Trends
Dutch fens changed greatly during the 20th 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 20th 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 20th 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.
Technical note
The figure gives the species group indices for the following nine characteristic species: (index 1901-1920 = 100): Lestes virens, northern damselfly, Irish damselfly, small red damselfly, common hawker, Aeshna osiliensis, downy emerald, white-faced darter and northern white-faced darter.
The indices for the northern damselfly and white-faced darter are shown separately.
The index has been calculated on the basis of the relative abundance per 20-year period, calculated as the number of km2 grid squares in which the given species was recorded expressed as a percentage of the total number of grid squares in which dragonflies were recorded. The data are from the national dragonfly database that is administered by the Nederlandse Vereniging voor Libellenstudie, EIS-Nederland and De Vlinderstichting.
References
- Ketelaar, R. (2001). Verspreidingsgegevens van libellen als instrument bij het herstel van vennen. De Levende Natuur, 102: 166-170.
- Ketelaar, R. (2001). De speerwaterjuffer in Nederland: verspreiding, ecologie en beschermingsperspectief. Rapport VS2001.32. De Vlinderstichting. Wageningen.
- Ketelaar, R. (2002). De status van de speerwaterjuffer Coenagrion hastulatum in Nederland, een karakteristieke libel van niet aangetaste vennen (Odonata). Nederlandse Faunistische Mededelingen, 16: 1-10.
