THE POPULATION-DYNAMICS OF A WOODLOUSE, ARMADILLIDIUM-VULGARE - AN EXAMPLE OF BIOTIC COMPENSATORY MECHANISMS AMONGST TERRESTRIAL MACRODECOMPOSERS

The hypothesis that in communities dominated by stochastic perturbatio ns, persistence of component populations is promoted by biotic compens ating mechanisms is tested with a member of the soil macrofauna commun ity in grasslands subject to large and stochastic fluctuations in vert ebrate grazing intensity. Populations of the terrestrial isopod Armadi llidum vulgare (Latreille) fluctuated in density by over an order magn itude on each of three grassland sites differing markedly in habitat s tructure. Density and survivorship were negatively correlated with rai nfall but not significantly with mean litter layer temperatures. An in teraction between temperature and rainfall accentuated the negative ef fects on excess moisture. Tussocks in lightly grazed swards provide an important source of shelter from excessively wet conditions and from extremes of diurnal temperature fluctuation. Reduction of spatial hete rogeneity in habitat structure caused by heavier grazing pressure resu lted in increased mortality and decreases in population density. Short er, more heavily grazed, swards have a higher biomass of dicotyledonou s plants, the leaf litter of which forms a high quality food resource for which competition in lent: swards leads to reduced growth rates, f ecundity, and natality rates in A. vulgare. When availability of this food source was experimentally increased on the site which already had the highest levels of dicotyledonous litter production, marked increa ses in growth and development rates and fecundity occurred, despite de creases in litter layer temperatures. We conclude that population regu lation by intra-specific competition for limited food resources occurs throughout the density ranges observed but that the levels of this re source, which determine the carrying capacity of these habitats for th is species, are subject to frequent changes due to variation in rabbit grazing intensity which results in dynamic equilibrium levels for the populations. Stochastic changes in the availability of shelter can re duce densities to levels at which competition for food is reduced. Und er these conditions, near to the 'density floor' of the populations, e xtinction is avoided and recovery accelerated by the increases in grow th and natality rates that result from reduced competition for food. T his density dependent process therefore does act as a biotic compensat ing mechanism increasing the probability that the populations will per sist throughout stochastic environmental perturbations.