Ecology Competition in Plant Communities
by
Paul A. Keddy
  • LAST REVIEWED: 19 May 2015
  • LAST MODIFIED: 25 November 2014
  • DOI: 10.1093/obo/9780199830060-0009

Introduction

Competition is generally understood to refer to the negative effects caused by the presence of neighbors, usually by reducing the availability of resources. Competition is one of the most important factors controlling plant communities, along with resources, disturbance, grazing, and mutualism. Since all plants require a few basic elements, the resource involved is generally light, water, nitrogen, or phosphorus, depending upon the species and the location. The effects of competition are widespread and easily observed in mixtures of crops and managed forests, which is why weeding and thinning are practiced. Competition is also widespread in native habitats, from deserts to wetlands, and is known to have important—indeed crucial—effects upon recruitment, growth, and reproduction. In the late 1800s, Darwin wrote extensively about the importance of competition in nature, particularly its role in driving natural selection. Thereafter, interest in the phenomenon grew. There were many experiments with both crops and wild species, most now overlooked. Models of competitive interactions were also constructed, with the number and size of the models increasing rapidly with the advent of computers in the 1970s. Hence, simple, early models are now often overlooked. Because the word competition has a common usage in English, it is frequently taken for granted, and therefore misunderstood. Care must be taken in using or interpreting the word without specifying what kind of competition is being investigated. For example, competition may be looked at from the perspective of an individual, a population, or a species, and it may be asymmetric or involve multiple species simultaneously. Experimental design carries its own assumptions, which are often not stated in published articles. One of the most difficult tasks in reading the literature is sifting through large numbers of experiments in which investigators have haphazardly selected (a pair of) species and grown them in mixture, without adequately justifying their choice of species and study design. Another difficult task is distinguishing between models that, at least in principle, have measurable inputs or make measurable predictions (or both) and those that do not and cannot be tested. Overall, the very ease of growing plants in mixture, as well as the ease of making models, has made people careless, with the result that basic questions are remaining unaddressed. Ongoing issues of importance include mechanisms of competition, types of competition, and the intensity of competition under different conditions.

General Overviews

Darwin’s On the Origin of Species contains a good deal about competition, usually competition between species operating as the force of natural selection. Of course, Darwin was greatly influenced by the English economist Thomas Malthus, who wrote about resources and population growth, including the famous Essay on the Principle of Population. The first major work of the 20th century in this area was Weaver and Clements 1938, a frequently overlooked volume with a wealth of competition experiments. Perhaps the book is overlooked because of its extensive discussion of succession, as well as the many new terms introduced under this topic (one is advised to try reading the book as a treatise on competition, skipping the other parts). Although plant ecologists continued with studies of competition, by the 1970s work by zoologists was ascendant, as illustrated by MacArthur 1972. Models such as Lotka-Volterra had become standard tools. MacArthur 1972 also illustrates the concern with how species might escape competition by using different resources (“resource partitioning”), although there is still disagreement about how applicable this concept is to plants, which share a common set of resources. Harper 1977 can be considered very influential for refocusing attention upon plant populations and plant life cycles. The work summarizes a vast number of studies on plant populations, including studies in agriculture and forestry. The emphasis on populations and agricultural systems was challenged in the 1970s by Grime 1979, which emphasized that habitats lacking in natural resources (stressed habitats) are very different from the relatively fertile sites favored by agricultural researchers. Grime introduced the CSR model, which relates plant strategies (competitor, stress-tolerator, ruderal) to two basic gradients: stress and disturbance. Grime 1979 was a landmark work, shifting attention away from populations and back to plant traits and environmental gradients. By 1990 the situation had become confused, as illustrated by the many views, ideas, and data sets in Grace and Tilman 1990. As one referee noted, the book shows primarily how little agreement there was about what the word competition meant, how it should be measured, and how even common experimental designs should be interpreted. Keddy 2001 (originally published in 1989) emphasized that some of the confusion was the result of there being many different components of competition—intraspecific/interspecific, symmetrical/asymmetrical, and diffuse/monopolistic, to name just three. This book explicitly addresses some of the work done since the era of Clements, as well as the advances, such as the different types of competition and the different types of experiments necessary to measure each type; the 2001 edition includes additional and more recent examples compared to the original 1989 version, while a chapter in Keddy 2007 provides a shorter introduction to the field.

  • Grace, James B., and David Tilman, eds. 1990. Perspectives on plant competition. San Diego, CA: Academic Press.

    E-mail Citation »

    It is instructive to read the book as a historical snapshot. Weaver and Clements, for example, are largely forgotten. Grime is beginning to challenge the status quo—but does not appear as a contributor. It is also useful to compare and contrast the definitions of competition and the sources of evidence used in each chapter.

  • Grime, J. Philip. 1979. Plant strategies and vegetation processes. Chichester, UK: John Wiley.

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    This book played an important role by challenging ecologists to rethink their understanding of competition. It emphasizes an active role for competition, suggesting that plants must invest resources and forage to obtain resources, and describes how other (environmental) factors such as stress and distribution interact with competition. It stimulated decades of research and debate.

  • Harper, John L. 1977. Population biology of plants. London and New York: Academic Press.

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    Chapters 6–11 are must reading for someone planning to study plant populations and competition. Most of the other chapters are relevant as well, since competition may occur at different stages in life history.

  • Keddy, Paul A. 2001. Competition. 2d ed. Dordrecht, The Netherlands: Kluwer.

    DOI: 10.1007/978-94-010-0694-1E-mail Citation »

    Chapters 1 and 2 introduce kinds of competition as measured by different kinds of experiments. The book was originally published in 1989 by Chapman and Hall, and many more examples were added to this second edition. Chapter 9 introduces many models, including models by Skellam and Pielou, for competition among patches, and along gradients, respectively. Chapters 1 and 9 are available online from the author.

  • Keddy, Paul A. 2007. Plants and vegetation: Origins, processes, consequences. Cambridge, UK: Cambridge Univ. Press.

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    Chapter 6 provides a contemporary overview of plant competition, including newer examples, set within the context of other forces in plant communities, such as stress, herbivory, and mutualism.

  • MacArthur, Robert H. 1972. Geographical ecology: Patterns in the distribution of species. New York: Harper & Row.

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    This book begins with general issues affecting plant and animal distributions. It has a lucid description of the two-species Lotka-Volterra model.

  • Weaver, John E., and Frederic E. Clements. 1938. Plant ecology. 2d ed. New York: McGraw-Hill.

    E-mail Citation »

    A classic that all plant ecologists should own, and read, before planning their own work. Inexpensive copies of this book can still be found second-hand. For the study of competition, it is best to ignore the other main theme of this book, succession, which is an entirely different topic (see also the Oxford Bibliographies entry on Succession). This book also introduces the use of phytometers; that is, using an easily grown species to compare habitats from a plant’s perspective.

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