THE IMPACT OF FORESTRY ON DRAGONFLY DIVERSITY IN CENTRAL SWEDEN

Abstract

A survey of 32 lakes for dragonfly larvae, aquatic plants and forestry regime in the surrounding boreal forests was performed. The highest diversity was found in undisturbed forests. Lakes rich in aquatic plants were shown also to be rich in dragonflies. A rich plant community is proposed to provide a wider range of microhabitats thereby increasing dragonfly biodiversity. If the forest surrounding a lake has been logged, a decrease in the species-richness of dragonflies with partivoltine life-cycles can be observed after a 5 year "lag phase." Increased fluctuations in water temperature and leakage of nutrients into the water are two possible causes. Univoltine species are not affected and appear to be less dependent on constant water conditions. The water plant community is only moderately affected, but a slight decrease in the number of species can be observed. A return to more species-rich conditions can be observed after more than 15 years, but whether the original community is restored or replaced with more "trivial" species is an open question. Table I. Dragonfly species found in the 32 surveyed lakes. Numbers in brackets indicate the number of lakes the individual species were found in. Lestes sponsa* (22) L. dryas* (9) Erythromma najas (5) Coenagrion armatum (1) C. hastulatum (21) C. lunulatum (4) C. johanssoni (2) C. puella/pulchellum (12) Enallagma cyathigerum (6) Aeshna caerulea (I) A. juncea (25) A. osiliensis (I) A. grandis (22) A. viridis (2) Brachytron pratense (I) * = univoltine species. Cordulia aenea (8) Somatochlora metallica (I) S. flavomaculata (5) Epitheca bimaculata (I) Leucorrhinia albifrons ( 4) L. dubia (8) L. rubicunda (19) L. pectoralis (3) Libellula quadrimaculata (26) Orthetrum cancellatum (I) Sympetrumflaveolum* (6) S. danae