MECHANISMS RELATING BIOMASS CONCENTRATION IN INDIVIDUALS TO INTERSPECIFIC RESOURCE COMPETITION

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.