Dragonflies associated with brooks, and water quality
Many dragonfly species associated with brooks declined in the 20th century, but an improvement in water quality has caused the recent resurgence of some species.
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Trends
In the 20th century the water quality of brooks worsened because of eutrophication and the use of pesticides. As a result, many dragonfly species characteristic of brooks, such as the banded demoiselle and the beautiful demoiselle, have declined. Other factors contributing to this decline are the canalisation and straightening of brooks, which have resulted in brooks acquiring steep banks and thus a changed cross-section.
The banded demoiselle likes sunny brooks and flowing water bodies with a varied vegetation on their banks. Its numbers fell sharply in the 20th century, but the trend was reversed in the 1980s by an improvement in the water quality and by the restoration of meanders and of the original cross-section in some brooks.
The beautiful demoiselle likes small brooks in woods, with cool water and a high oxygen content. If the water is too rich in nutrients, the oxygen content of the water can remain low for long periods (especially in the summer). This is bad for the larvae. The beautiful demoiselle declined throughout the 20th century. Recent observations (not shown in the figure) suggest that a slight recovery is under way.
The beautiful demoiselle is on the Red List of dragonflies.
Technical note
The trend in the 20th century has been calculated using data from the national dragonfly database administered by the Nederlandse Vereniging voor Libellenstudie, EIS-Nederland and the Vlinderstichting. The index is based on the relative abundance per 10-year period. This is the number of km2 grid squares in which a given species has been sighted, expressed as a percentage of the total number of km2 squares in which dragonflies have been recorded.
References
- Wasscher, M., G.O. Keijl en G. van Ommering (1998). Bedreigde en kwetsbare libellen in Nederland. Toelichting op de Rode Lijst. Rapportnummer 30. IKC-Natuurbeheer. Wageningen.
- Groenendijk, D. (2002). Bosbeekjuffer en gewone bronlibel in Nederland: ecologie en bescherming. Rapport VS2002.006. De Vlinderstichting. Wageningen.
