Sex ratio and mode of reproduction in Collembola of an oak-beech forest

Summary Sex ratios of 31 species of Collembola of a temperate oak-beech forest were investigated in 2 months intervals during 1 year. For the estimation of population dynamics, the abundance and dominance of taxa in the litter (OL,F,H), the 0–3 and 3–6 cm soil layer (Ah) were assessed. A total of six species, including the abundant Mesaphorura machrochaeta, Parisotoma notabilis, Neanura muscorum and Isotomiella minor formed pure female populations suggesting that they reproduced by parthenogenesis. In three species, Lepidocyrtus cyaneus, Orchesella flavescens and Tomocerus longicornis, the mode of reproduction remained uncertain. The remaining 22 species including the widespread Folsomia quadrioculata, Protaphorura fimata and Lepidocyrtus lignorum formed bisexual populations, suggesting that sexual reproduction predominates in Collembola of the studied forest. Sex ratios of sexual species ranged between 33% and 90%. In five species the sex ratio was balanced (ca. 50%), in eight species males made up about one third of the population and in seven species males were rare constituting less than 25% of adults. Females generally comprised 77% of adult Collembola. Densities were highest in late summer and autumn. Except for M. machrochaeta, the density of Collembola deeper in soil (3–6 cm) was low. Consistent with earlier studies parthenogenetic species predominated deeper in the soil (euedaphic species), but some parthenogenetic species were hemiedaphic. The sex ratio of bisexual Collembola species in the litter layer generally was more female biased than that in 0–3 cm of the mineral soil. Presumably, females concentrate at sites with high density of resources whereas males are relatively more abundant at sites more favourable for spermatophore placement. Neither climatic factors (i.e. season) nor population parameters (i.e. density) correlated with the sex ratio of Collembola.