Effects of Weather and Habitat Structure on the Population Dynamics of Isopods in a Dune Grassland

Abstract

Samples of 14 0.1 m2 turves were taken from 4-10 times a year between 1968 and 1975 from a dune grassland at Spurn, England and the woodlice extracted using a heat and light extractor. The annual mean density of three species of woodlice combined ranged from 334 in 1968 to 673 per m2 in 1970. Individual species showed greater variation. .Philoscia muscorum decreased from 577 in 1970 to 8 per m2 in 1975, Armadillidium vulgare increased from 24 in 1968 to 350 per m2 in 1973 and Porcellio scaber increased from 22 in 1968 to 82 per m2 in 1974. The degree of aggregation was greatest in P. scaber, intermediate in A. vulgare and least in P. muscorum. All species showed a sharp increase in aggregation in 1974 and 1975. Both the long term changes in abundance and aggregation coincided with habitat changes and not with weather. The abundance of A. vulgare increased with disturbance caused by rabbit grazing. Density of P. muscorum decreased and of P. scaber increased with sand accumulation. The increase in aggregation was also associated with sand accumulation. The role of water availability and habitat structure in affecting mortality and aggregation were investigated in the laboratory and the results supported the interpretation of field observations. Mortality and aggregation were higher in sand covered cultures, under all watering regimes, compared to cultures with easily accessible surface litter. Low water availability caused increases in mortality and aggregation mainly when sand covered the surface. All species survived equally well in cultures without surface sand when the litter was 'wet' or 'moist', but mortality of P. muscorum was greater than of the other two in dry and sandy conditions.