Density-Dependent Processes in the Population Dynamics of Armadillidium vulgare (Isopoda: Oniscidae)

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Abstract
(1) Changes in the density of a population of Armadillidium vulgare on a heavily grazed grass heath in the Breckland district of East Anglia from 1973 to 1985 have been analysed. For the first 5 years population densities remained high with annual peaks of 700-1000 m-2. From 1978 to 1981 the population declined steadily and from 1981 to 1985 remained low with annual peaks of 70-100m-2. (2) An annual alternation of low and high density cohorts was superimposed on these long-term changes. We propose that this alternation persists as a result of asymmetrical exploitation competition between 1-year-old sub-adults and first year recruits for limited high quality food. (3) Despite large variations in population density, analysis of mortality rates revealed neither a key factor nor density dependence in mortality during any stage of the life cycle. (4) Yearly changes in population density were significantly related to changes in natality between years. Growth rates of A. vulgare were significantly negatively related to population density. As fecundity is a linear function of body size and growth is indeterminate in A. vulgare, depression of growth rates at high densities had a direct density-dependent effect on natality rates. (5) When an experimental exclosure was erected which prevented rabbit grazing, the availability of high quality foods increased. Isopods within the enclosure grew larger, became more fecund, and consequently increased in density. (6) In isopod enclosure to which high quality food was added, growth rates of isopods also increased. In other enclosures to which sub-adult A. vulgare were experimentally added, growth rates of new recruits decreased. We conclude that intra-specific competition is important in regulating the density of this population and that populations of this macro-decomposer are more likely to be regulated from ''below'' by competing for limited food than from ''above'' by natural enemies. The relaxation of competition at low densities with the consequent positive effects on natality rates provides an effective ''floor'' which reduces the probability of population extinctions.