Population ecology of a terrestrial isopod in two breckland grass heaths

(1) Two superficially similar chalk grasslands in the Breckland of East Anglia provided very different habitats for the abundant terrestrial isopod Armadillidium vulgare (Latreille). At Lakenheath Warren the site was a tussocky grassland, ungrazed, with Festuca spp. covering 65-80% of the area, and most of the isopod population was found in the litter layer (5 cm deep) of the large tussocks throughout the year. At Weeting Heath National Nature Reserve, smaller Festuca tussocks comprised 25-30% of the site, and the isopods moved between the tussocks and the intervening grazed sward. (2) Mean annual population density of A. vulgare on the grazed site at Weeting was 1.4-1.6 times larger than that of the ungrazed site at Lakenheath, which increased to a maximum of 2.8 times at other periods. (3) Differences in population structure between the two sites were detected in terms of age structure, generation distribution and cohort composition. (4) At Weeting each cohort bred once; whilst at Lakenheath each cohort participated in two annual breeding seasons. (5) There was a high mortality of new born young at both sites, and survivorship curves were similar being type 3 (Deevey 1947). (6) Large variations occurred in the number of young produced per year at Weeting, whilst annual recruitment was similar at Lakenheath. An alternation of high and low density generations occurred as a result at Weeting. (7) Changes in the structure of the two populations of A. vulgare are discussed in relation to habitat structure, grazing effects and environmental heterogeneity