Life cycle and population dynamics of Trochosa terricola Thorell (Araneae: Lycosidae) in a Norfolk grass heath

Abstract

Abstract. 1. The population of the lycosid Trochosa terricola Thorell was sampled from April 1973 to August 1975 at Weeting Heath NNR, a Breckland grass heath.2. Four sampling methods were compared for efficiency. Hand searching gave density estimates between 38.3 and 70.1% of heat extraction.3. The temperature range in the sward at +1 cm was –5°C to 39°C with January and July means of 3.2°C and 17.4°C.4. Eight male and nine female instars were determined and the life cycle extended over 2 or 3 years.5. Adults were nocturnal but the juveniles diurnal. An annual diplochrone activity pattern was observed for adult males.6. The horizontal distribution within the sward was aggregated, the structure and microhabitat being important determining factors. The population density was greater in moist, young Festuca spp. tussocks. An equation relating population density to habitat characteristics was derived.7. The overall population density ranged from 14.0 m‐2 to 76.0 m‐2 and was maximal in autumn after breeding. The population biomass was greatest during autumn (291.2 mg d.wt m‐2).8. Mean number of juveniles emerging from an egg sac was 77.3 (first sac) and 38.0 (second sac). The natality in 1973 was 66.8 individuals m‐2.9. The survivorship curve until maturity varied between types I and III in different years.10. The population dynamics were compared and are discussed in the light of other data. The variable population characteristics suggested that Den Boer's “spreading of risk” theory applied to the T.terricola population.