Interference in Pure and Mixed Populations of Agrostemma githago

Abstract

(1) The responses to density of the annual Agrostemma githago were investigated over a wide range of densities in pure populations and also in the presence of a constant density of wheat. In monoculture a 2500-fold variation in the input of seeds gave only a 1.6-fold range of seed output, whilst in the presence of wheat, yield varied to a much greater extent. (2) Considerable self-thinning occurred in both sets of populations at high densities. Agrostemma responded to increasing density not only through an increase in mortality but also through a plastic reduction in the shoot dry weight of survivors, the number of capsules per plant, the number of seeds per capsule and mean seed weight. (3) There was a clear negatively density-dependent relationship between fecundity and density in both pure and mixed populations of Agrostemma. Simple mathematical models based on a reciprocal equation provided a very good fit to these data. (4) It is suggested that the models provide the basis for an analysis of additive interference experiments in which both density and frequency are varied. (5) The biological interpretation of the model parameters is discussed in relation to the seed production of isolated plants, ecological neighbourhood areas, the efficiency of resource utilization and equivalence between species. (6) A discrete (difference) population dynamics model is used to show that pure populations of Agrostemma can exhibit damped oscillations towards equilibrium whilst populations grown with wheat show monotonic damping.