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On the Analysis of Competition Within Two-Species Mixtures of Plants

L. G. Firbank and A. R. Watkinson
Journal of Applied Ecology
Vol. 22, No. 2 (Aug., 1985), pp. 503-517
DOI: 10.2307/2403181
Stable URL: http://www.jstor.org/stable/2403181
Page Count: 15
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On the Analysis of Competition Within Two-Species Mixtures of Plants
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Abstract

(1) Replacement series experiments have been used extensively in studies of interactions between two species. These are usually carried out at a single total density, and, as a result, the effects of intraspecific competition cannot be isolated from those of interspecific competition. These factors can only be separated if both freqency and total density are varied. (2) A model to describe competition in two-species mixtures is presented which can be used to estimate survival, mean yield per plant and yield per unit area for each species at any combination of frequency and density. This model is more succinct and easier to interpret than the replacement series analysis of de Wit (1960). (3) The model is used to analyse data from an experiment in which Agrostemma githago and Triticum aestivum were grown in monocultures and 1:1 mixtures at five total densities. Surviving Triticum plants responded to the average Agrostemma plant as if the latter were equivalent to 1.63 Triticum plants, whilst Triticum was equivalent to 0.41 Agrostemma plants. (4) Data previously published by Marshall & Jain (1969) are reanalysed to show that the competitive superiority of Avena fatua over A. barbata was underestimated by replacement series analysis. (5) It is argued that this model should be used in preference to replacement series analysis whenever both frequency and total density vary, for example in weed-crop systems and in intercropping situations.

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