An Allee effect occurs when at low densities there is a positive relationship between density and fitness leading to a higher probability of extinction. One circumstance in which it may arise is when isolated individuals are less effective in modifying their environment in a favourable way than groups of individuals. In this paper we use Porcellio scaber to test the hypothesis that the fitness correlate of growth rate will vary as a function of density not just due to the negative effects of increasing interference competition whilst foraging at higher densities, but also positively at lower densities due to isolated individuals spending more time and energy searching for other individuals with which to aggregate whilst sheltering.

Below densities of 70 m–2 relative growth rates increased as a function of density, above 70 m–2 growth rate decreased with increasing density. Time spent walking and searching increased at lower densities. These behavioural observations, together with the original observations on water loss and oxygen consumption made by Allee [1], are used to interpret the curvilinear growth rate response to density as an example of a component Allee effect, which is discussed in relation to the importance of aggregation as an adaptation to the terrestrial environment in the Oniscidea.