Population Density in Adult Dragonflies (Odonata-Anisoptera)

Abstract

Counts of dragonflies by the Portbury River, Somerset and Stoke Bishop pond, Bristol, were made from 1949 to 1952 inclusive. Majority of [male] [male] in populations of Brachytron pratense, Libellula quadrimaculata, L. depressa, Anax imperator. Orthetrum cancellatum and Sympetrum striolatum are present by water at about noon on fine days. Total number of [male]dragonflies per 100 yd. of stream on 7 adjacent stretches of Portbury River very rarely exceeded 13 per 100 yd., whether 1, 2, 3, 4 or 5 spp. were present. It is suggested that daily density of [male] dragonflies by water is regulated by their reproductive behavior. Degree of coincidence of seasons of spp. studied is shown to vary considerably from yr. to yr. This probably affects density of the spp. concerned.