A Darwinian Approach to Plant Ecology

Abstract

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