Effects of density and spatial heterogeneity on foraging behaviour and fitness correlates of Armadillidium vulgare (Isopoda Oniscidea)

Abstract

The encounter rate of focal individuals of Armadillidium vulgare in experimental arenas increased with increasing density of other isopods. This increase was only significantly more than would be predicted from a random encounter model when the additional animals were of a different species, Porcellio scaber. Increasing density of both species resulted in focal individuals spending a higher proportion of time walking while in high quality food patches, and a smaller proportion of their time feeding on the high quality food. This was only significant when the high quality food was most heterogeneously distributed and was more significant following encounters with P. scaber than with other A. vulgare. At higher densities focal individuals alternated turns less while searching in the low quality food matrix

Relative growth rates and survivorship were both higher at higher population densities (278 m–2) than in the low density treatments of 28 m–2. We suggest that at low densities, which are below those observed for three field populations over twenty years, the advantages of having additional individuals with which to aggregate while sheltering may outweigh the costs of increased interference competition during foraging.

The effects of spatial heterogeneity of food resources on interference competition between species differing in aggressiveness are discussed. The possible advantages of aggregating with another species, that has different morphological, physiological and behavioural adaptations to the terrestrial environment, while sheltering are examined in relation to the Allee effect of the positive fitness consequences of increasing population densities from very low levels.

Keywords:

• Allee effect
• feeding behaviour
• foraging traits
• growth rate
• survivorship