Maintenance of melanism in the spider pityohyphantes phrygianus: Is bird predation a selective agent?

Abstract

The sheetweb spider Pityohyphantes phrygianus (Araneae: Linyphiidae) exhibits a continuous variation in dark colouration due to polygenic inheritance of the trait. There are three main phenotype classes: pale, intermediate and melanic. In natural populations, the proportion of melan- ics seems to be stable at a low frequency, usually 3-4 per cent. An hypothesis about possible selective differences between melanics vs. non-melanics was evaluated in a 2-year field experiment. Bird predation was prevented in a large number of spruce branches, the natural habitat of the species, by means of coarse-meshed nets. The experimental branches and the controls were sampled four times (spring and autumn in 2 years). Bird predation was shown to affect negatively the spider abundance. Removal of bird predation increased the P. phrygianus mean density between 2.3 and 10.5 times the control density. However, no difference in the proportion of melanics in experimental vs. control branches could be established. It is suggested that bird predation is not a major selective force affecting the phenotype proportions. Hence, the mechanism of maintenance of melanism in natural populations of P. phrygianus is unclear, but some alternative explanations are discussed. © 1993 The Genetical Society of Great Britain.