Competition in Plant Communities

by

Paul A. Keddy, James Cahill

• LAST REVIEWED: 03 September 2021
• LAST MODIFIED: 25 September 2018
• DOI: 10.1093/obo/9780199830060-0009

Introduction

Competition is generally understood to refer to the negative effects on plant growth or fitness caused by the presence of neighbors, usually by reducing the availability of resources. Competition can be an important factor controlling plant communities, along with resources, disturbance, herbivory, and mutualisms. 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. Many experiments with both crops and wild species were conducted. 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. Because the word competition has a common usage in English, what it represents in biological systems is frequently assumed, rather than explicitly stated, leading to misunderstanding. Care must be taken in using or interpreting the word without specifying what kind of competition is being investigated, as different forms of competition can have different types of consequences. For example, competition may be looked at from the perspective of an individual, a population, or a species, it may be symmetric or asymmetric, and it can occur among single or multiple species simultaneously. Experimental design carries its own assumptions, which are often not stated in published articles. One of the most difficult tasks in exploring published studies is the need to sift 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 or the experimental 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 new models, may have made some people careless, with the result that basic questions are remaining unaddressed. Ongoing issues of importance include mechanisms of competition, types of competition, effects of competition on plant coexistence, and the intensity of competition under different sets of 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. There is a good deal about plants and plant ecology in Darwin’s work. 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 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). Competition was also included within even the most basic models of ecology, such as the logistic equation, which led to the Lotka-Volterra models for competition, well described in MacArthur 1972. MacArthur’s book also explores how species might escape competition by using different resources (“resource partitioning”), although disagreement remains 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. This book 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, who emphasized that natural habitats lacking in key resources (“stressed” habitats) are very different from the relatively fertile sites favored by agricultural researchers (Grime 2001). Grime introduced the CSR model, which relates plant strategies (competitor, stress-tolerator, ruderal) to two basic gradients: stress and disturbance. The first edition of this book in 1979 was a landmark work, shifting attention away from populations and back to plant traits and environmental gradients. The role of aboveground and belowground resources is explored in Tilman 1982, suggesting that many plants may coexist by exploiting different ratios of above to below ground resources, particularly light and nitrogen. This resource ratio model may be more useful than the Lotka-Volterra model for plants. By 1990 a good deal more thought had been given about how to best explore competition among plants; some of the challenges are illustrated by the array of 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 emphasizes 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. Of course, competition is only one of many factors that affect plant communities: Keddy 2017 provides a shorter introduction to plant competition nested among the other causal factors controlling composition in plant communities.

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

  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. 2001. Plant strategies, vegetation processes, and ecosystem properties. 2d ed. Chichester, UK: John Wiley.

  An early edition of this book (1979) challenged ecologists to consider mechanisms of plant competition in different environments. Plants must invest resources and forage to obtain resources, and this has consequences ranging from plant traits to ecosystem properties.

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

  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: 10.1007/978-94-010-0694-1

  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. 2017. Plant ecology: Origins, processes, consequences. Cambridge, UK: Cambridge Univ. Press.

  Chapter 4, “Competition” (pp. 123–162), provides a contemporary overview of plant competition, set within the context of other forces in plant communities, such as resources, herbivory, and mutualism. Includes two important older models of plant competition by Skellam and Pielou.

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

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

• Tilman, David. 1982. Resource competition and community structure. Princeton, NJ: Princeton Univ. Press.

  Plants may use different ratios of aboveground and belowground resources, and a critical consideration is the lowest levels to which those resources can be reduced, termed r*. The simplest view is that plants often partition gradients along light to nitrogen ratios.

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

  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.