The Comparative Biology of Closely Related Species Living in the Same Area

S ummary Seeds of Papaver rhoeas, P. dubium, P. lecoqii, P. argemone and P. apulum were sown in garden plots at a range of densities in pure stands and mixtures of two species. The plants reacted to density by a reduced chance of establishment and a plastic reduction of plant weight and capsule numbers—the species did not differ in their reactions to density in pure stands. In mixed populations the chance of establishment of a species was reduced when its own density in the mixture was increased—the density of the associated species exerted a less significant influence. The species in lower proportion in a mixture gave the more successful establishment from seed—and it is argued that the persistence of mixed populations is therefore favoured in comparison with pure stands.