The Population Dynamics of Trichoniscus pusillus and Philoscia muscorum (Crustacea, Oniscoidea) in Limestone Grassland

Abstract

1. The population dynamics of the woodlice Philoscia muscorum (Scopoli) and Trichoniscus pusillus pusillus (Brandt) living in limestone grassland in Wytham Woods, Berkshire, were studied between March 1964 and March 1966. 2. Densities of over 2000/m2 were found for Trichoniscus at the height of the breeding season. Minimal figures before recruitment were 830 in 1964 and 495 a year later. Figures for Philoscia were generally very much less than these. 3. The total live weight of Trichoniscus was greater than that of Philoscia in the spring of 1964 and 1966, but the position was reversed in 1965. 4. Size structure analysis showed the Trichoniscus population to have been severely affected by the drought of 1964. Growth and breeding almost ceased. Initially in the following year recruitment was very poor, because few animals had matured, but later good growth and recruitment largely re-established the pre-drought size structure, numbers and biomass. Philoscia showed none of these reactions to drought conditions. 5. The two breeding seasons in Philoscia were short and did not vary. In Trichoniscus they were long and very different, with an April breeding peak in 1964 and twin peaks in May and August during 1965. 6. Embryo production in Trichoniscus was 5847/m2 in 1964 and 2390 in 1965. Figures estimated for Philoscia were 219/m2 and 284 respectively. 7. Mortality of Trichoniscus juveniles was greater in 1964 than 1965, but the drought was not the cause. In this species life expectancy fell sharply on completion of breeding. Juvenile mortality was severe in both species. Mortality decreased exponentially in Philoscia, but in Trichoniscus the pattern was more complex. 8. Trichoniscus survived the drought by vertical migration deep into the soil, while Philoscia stayed within sampling range, seeming to have a physiological tolerance of extreme conditions. Size alone cannot explain this difference in behaviour. 9. In Trichoniscus there are indications of compensatory mechanisms tending to maintain a characteristic size structure and some stability of numbers and biomass. Philoscia showed numerical stability but biomass fell by 50% during the period of study.