Seasonal patterns in abundance of water-beetles belonging to the Hydrophiloidea (Coleoptera)

Populations of Hydraena britteni Joy., Limnebius aluta Bedel (Hydraenidae), and Helophorus brevipalpis Bedel, H. strigifrons Thoms., Hydrobius fuscipes L. and Anacaena limbata Fabr. (Hydrophilidae) were studied from the break up of the ice until autumn or freezing at three different shores of a lake near Stockholm, Sweden. The beetles were generally found to frequent the zone immediately at the water's edge. Both larvae and imagines inhabit the uppermost 2 or 3 cm of the substrate. Two patterns in the seasonal abundance were found. (a) Maximal abundance during spring, minimal during summer, and sometimes a new, smaller maximum during late summer and autumn (Hydraena britteni, Helophorus strigifrons, Hydrobius fuscipes, Anacaena limbata, and perhaps Limnebius aluta). (b) An abrupt peak in abundance during July-August and very low abundance at other times (Helophorus brevipalpis). Type (a) is probably the general one in this region and type of habitat. It fits a univoltine life cycle, where the beetles breed during early summer and hibernate as adults. In other climates this life cycle seems to be modified, partial second generations may occur or development is retarded and a new generation does not emerge until next spring. Type (b) is shown by a univoltine species much disposed to flight. The peak coincides both with its dispersal phase and the emergence of the new generation. The species is thought not to belong to a closed, stable habitat but to ephemeral waters in open landscapes. Its ecology is quite different to that of H. strigifrons, which is considered to belong to more closed, stable habitats. Patterns similar to type (a) are also shown by Laccophilus spp. (Dytiscidae) and Haliplus spp. (Haliplidae) according to the literature. This may be an adaptation to the probable danger of being trapped in ice during winter. Like the water-beetles these two beetle groups also mostly leave the water margin during autumn. The pattern of type (a) of most Hydrophiloidea seems to be geared to the amounts of plant remnants in the littoral, imagines and some larvae being most abundant when the amounts are maximal, i.e. during spring. The plant material probably offers food and cover. It disintegrates rapidly during summer and a new maximum is reached next spring. The pond-like habitat, which may develop behind reeds is compared with ephemeral ponds. Both may be regarded as unstable habitats, each passing through a series of predictable changes. The difference is the final stage, often disastrous when a pond dries out, but seldom so in a lake when the water line moves outwards in the littoral during summer.