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

Abstract

Studies were made of the growth of populations of Lemna minor, L. polyrrhiza, L. gibba, and Salvinia natans under controlled laboratory conditions. The intrinsic exponential growth-rates of the clones were determined in un-crowded cultures, and the changes in growth-rate of self-crowding cultures were measured and interpreted in terms of an initial exponential growth-rate followed by a phase of arithmetic increase in weight and followed in turn by a phase in which the death of submerged and shaded fronds caused a decline from the arithmetic rate of growth. Mean frond weight declined in self-crowding cultures (except of L. gibba). Mixed cultures of two species were examined under self-crowding conditions and changes in the proportions of the species were followed. Whereas the total weight of mixed cultures remained very constant between replicates, there was wide variation in the proportions of components. The variation in the two components was most closely correlated (negatively) when the struggle for existence was most evenly balanced. The mean frond weight of the losing component declined during the experiments. The order of decreasing vigour of species measured by various parameters was as follows: Relative (intrinsic) growth-rate: L. minor > S. natans > L. gibba > L. polyrrhiza Arithmetic growth-rate when crowded: S. natans > L. polyrrhiza > L. gibba > L. minor Asymptotic yield per culture: L. polyrrhiza > L. minor > S. natans > L. gibba Success in mixed cultures: $S.natans>L.gibba>L.polyrrhizaor>=L.minor$ The success of a species in mixture could not be predicted from the parameters of growth in pure culture. Morphologic features such as the gibbosity of L. gibba