This paper investigates the relationships between competition strength
on individuals. as measured by food absorption variations, and compet
itor body size. To this end I analysed the feeding behaviour of indivi
duals of the freshwater isopod Proasellus coxalis under laboratory man
ipulation of competitor biomass, number of competitors into which biom
ass was partitioned (i.e., degree of biomass concentration) and time l
ag among resource utilizations (i.e., co-occurrence vs temporal separa
tion of competitors). The study was carried out in the home range of s
ingle P. coxalis utilizing P-32-labelled resources. In order to manipu
late biomass concentration, competitors used were smaller mayflies (Ca
enis sp. gr. macrura), conspecifics or larger gastropods (Planorbarius
corneus and Lymnaea truncatula). Under co-occurrence conditions, food
absorption of the target P. coxalis individual did not change signifi
cantly with increasing body size of a single competitor, but it was re
duced to less than 5% with several small competitors. On the contrary,
in conditions of temporal separation, individual absorption rate decr
eased significantly with the increase of the overall biomass of compet
itors which already exploited the resources, but it was not affected b
y biomass concentration. These patterns emphasize the role of spatio-t
emporal size-related constraints on home-range resource harvesting as
a coexistence mechanism in detritivorous guilds.