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 competitor body size. To this end I analysed the feeding behaviour of individuals of the freshwater isopod Proasellus coxalis under laboratory manipulation of competitor biomass, number of competitors into which biomass was partitioned (i.e., degree of biomass concentration) and time lag among resource utilizations (i.e., co-occurrence vs temporal separation of competitors). The study was carried out in the home range of single P. coxalis utilizing P-32-labelled resources. In order to manipulate biomass concentration, competitors used were smaller mayflies (Caenis 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 significantly with increasing body size of a single competitor, but it was reduced to less than 5% with several small competitors. On the contrary, in conditions of temporal separation, individual absorption rate decreased significantly with the increase of the overall biomass of competitors which already exploited the resources, but it was not affected by biomass concentration. These patterns emphasize the role of spatio-temporal size-related constraints on home-range resource harvesting as a coexistence mechanism in detritivorous guilds.