The theory of evolution by natural selection is an ecological theory-founded on ecological observation by perhaps the greatest of all ecologists. It has been adopted by and brought up by the science of genetics, and ecologists, being modest people, are apt to forget their distinguished parenthood. Indeed, Darwinian plant ecology has been largely neglected and a changeling child nourished and brought o adulthood by Schimper and Warming who asked geographical questions about vegetation, and answered the questions by demonstrating correlations between climate and soils on the one hand and comparative physiology on the other. By contrast with the 'vegetationalist' and his concern to describe and interpret areas of land, Darwin's ecological observations and the questions he asked were based on a consideration of individuals and populations-a preoccupation with numbers. 'Look at a plant in the midst of its range, why does it not double or quadruple its numbers?' '. . . if we wish in imagination to give the plant the power of increasing in number, we should have to give it some advantage over its competitors, or over the animals which prey on it'. 'Look at the most vigorous species; by as much as it swarms in numbers, by so much will it tend to increase still further' (Darwin, Origin of Species, Chapter III).* These quotations, with their emphasis on numbers, pose problems of population biology-of a demography which has never gained a momentum in plant ecology, although it has played a vertebral role in animal ecology. Two interlinked properties of higher plants have seriously hindered the development of plant demography-plasticity and vegetative reproduction. Darwin found a 26-yearold pine tree on heathland which 'had during many years tried to raise its head above the stems of the heath and had failed'. It is clearly not fair to count such a plant as a unit equal to a full grown tree in a population census. A mature plant of an annual weed such as Chenopodium album may produce four seeds or 100 000 seeds, depending on the nutrient and water status of the soil. It can therefore be argued that a statement about numbers of plants implies very little about the real nature of the population. Vegetative reproduction is a further obstacle to census making, because the vegetative offspring remain to some extent a part of the parent, often for a long period. When is such a ramet to be counted as an individual? Arbitrary decisions have to be made if plant populations are to become numerable and the arbitrariness of the decisions has often discouraged attempts to count plants. These problems are, however, not peculiar to plants and have had to be faced in animal demography where they arise in only a slightly less acute form. Plasticity in individual size and reproductive capacity have to be taken into account in population studies of fish and even of Drosophila. Vegetative reproduction in Hydra, where the 'ramets' slowlv develon indenendence. has not Drevented its use in model population studies. * All quotations in this paper are from Chapters III and IV of The Origin of Species (1859), the text being the Everyman edition of 1928. Italics are mine. 247

https://www.jstor.org/stable/pdfplus/2401336.pdf

A darwinian approach to plant ecology

Summary 1 Mathematical indices, formed by combining several primary measures, can help researchers summarize, interpret and display results from plant competition experiments. This essay compiles and discusses more than 50 indices that have been used in studies of plant competition. 2 Indices allow researchers to quantify and express several attributes of plant competition, including competition intensity and importance, competitive effects and responses, and the outcome of competition. 3 Several qualities should be considered when selecting a competition index for use. These include specificity and clarity of meaning, mathematical and statistical properties, density dependence, versatility and freedom from size bias. 4 Many indices are based on comparisons of plant performance in mixtures compared with pure stands or control plots. Additional measures of competition are also available from yield-density functions, size distributions and neighbourhood analysis. 5 Indices reflect the consequences of competition. In order to appreciate competitive processes, indices have to be used in conjunction with studies of the chronology, circumstances and composition of plant associations.

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Indices of plant competition

Competition indices of common vetch and cereal intercrops in two seeding ratio

A simple competitive ratio (CR) is proposed as a measure of intercrop competition, to indicate the number of times by which one component crop is more competitive than the other. Intercropping data show that this CR term could be useful in (i) comparing the competitive ability of different crops, (ii) measuring competitive changes within a given combination, (iii) identifying which plant characters are associated with competitive ability, and (iv) determining what competitive balance between components is most likely to give maximum yield advantages.

/pdf/a-competitive-ratio-for-quantifying-competition-between-37nv6q9956.pdf

A competitive ratio for quantifying competition between intercrops.

Wheat and chickpea intercropping systems in an additive series experiment: Advantages and weed smothering

Biologists have, for some time, been studying the effect of growing one plant species in close proximity to another. In some cases the yield of a species is increased over its yield in monoculture and in other cases decreased. A species is regarded as a good competitor if its yield generally increases when grown with other species. This paper presents an analysis of an experiment in which p species are compared by growing pure stand and mixtures in pots or plots at equivalent total density of planting. Each species is grown in monoculture and each combination of two grown in equal proportion. It is assumed that only one harvest is made and that each species is competing either for just one nutrient or for the same nutrients with no differentiation being made between competition for differing nutrients. Such an analysis has previously been presented by K. Sakai [1961] and Williams [1962] and, although the analysis is essentially similar to that of Williams, a different parametrisation is used which corresponds more closely to easily defined biological concepts.

C. A. McGilchrist

Papers

Analysis of Competition Experiments