Spatial variation in favourability of a grass heath as a habitat for woodlice (Isopoda: Oniscidea)

Isopods were sampled from seven sites varying in the intensity of rabbit grazing, and six sites which had been artificially disturbed, on a grass heath in the Breckland district of East Anglia, UK. Densities of three species of isopod were related to four characteristics of sward structure, three of sward composition and to rates of rabbit faecal pellet production, used as an index of grazing intensity. Armadillidium vulgare was the commonest isopod, with peak densities of 490 m(-2) on sites subject to medium grazing intensities where Porcellio scaber was also most abundant. P, scaber was however more widespread on short, heavily grazed and disturbed swards than either of the other two species. Philoscia muscorum was absent from these more open sites and most abundant in the densest, least grazed swards. Size frequency analysis was used to distinguish cohorts of Armadillidium vulgare and calculate mortality and growth rates for each of the undisturbed sites. Mortality varied by over 6.6 times between sites, was greatest on those which were most heavily grazed and negatively correlated with proportion of tussock in the sward. Growth varied by 5 times between sites and was positively correlated with the percentage cover of dicotyledonous plants. The proportion of the sward forming tussocks was negatively correlated with percentage cover by dicotyledonous plants. There was thus a negative relationship between growth and survivorship on these sites. A graphical model of this trade off is used to predict optimal sward characteristics and the level of rabbit grazing required to produce them. This model, derived from size frequency data is supported by the density data which show that abundance peaks at the predicted level of rabbit grazing. These observations of field populations reveal that a wide range of outcomes for the trade off between survivorship and growth can result in viable population densities. They also show that the outcome is strongly influenced by the phylogenetic constraints imposed by differences in morphology, physiology and behaviour between species. It is postulated that highly stochastic temporal heterogeneity in habitat favourability for isopods has led to selection of exceptionally high phenotypic plasticity in growth and survivorship traits. This facilitates persistance under a wide spatial range of habitat favourability. The management implications of species-specific optima in habitat characteristics for invertebrates are discussed. It is concluded that manipulating rabbit populations to produce a mosaic of different grazing intensities is both feasible and desirable to further a range of conservation objectives.