THEORETICAL POPULATION BIOLOGY 37, 291-319 (1990) Intraspecific Competition and Components of Niche Width in Age Structured Populations HANS REDLEF SIECISMUND Institute qf Population Biology, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen 0, Denmark VOLKER LOESCHCKE Institute of Ecology and Genetics, University of‘ Aarhus, Ny Munkegade, DK-8000 Arhus C, Denmark AND J~~RCEN JACOBS Zoological Institute, University of Munich, Seidlstr. ZS, D-8000 Miinchen 2, Federal Republic of‘ Germany Received December 1, 1986 The interaction of competing species is commonly framed in Lotka- Volterra type equations. Parameters needed to describe the population dynamics are the intrinsic rates of increase, the carrying capacities, and competition coefficients. In species with exploitative competition for common resources such as food, MacArthur and Levins (1967) defined the competition coefficients in terms of the relative overlap in resource utiliza- tion. This approach allows problems such as the density or packing of species along resource gradients to be addressed. MacArthur and Levins (1967) concluded that, under the specific assumptions of this model, exploitative competition results in a limit to the similarity of competing species: the distance between the means of the resource utilization functions of competing speciesshould exceed about one standard deviation of the utilization functions (MacArthur and Levins, 1967 ; May and MacArthur, 1972; Christiansen and Fenchel, 1977; Roughgarden, 1979). Differences in the food niche often reflect differences in body size. On the basis of published field data Hutchinson (1959) suggested that a difference 291 0040-5809/90 $3.00 Copyright c 1990 by Academic Press, Inc All rights of reproduction m any form reserved.