COMPETITION IN PLANT COMMUNITIES
a guide and annotated reading list
Paul A. Keddy
Suggested citation: Keddy, Paul A. 2012. Competition in Plant Communities. Viewed online at www.drpaulkeddy.com, date. Adapted from: Oxford Bibliographies Online: Ecology. Ed. David Gibson. New York: Oxford University Press.
Competition is generally understood to mean 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 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. That 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
Grace, James B., and David Tilman, eds. 1990. Perspectives on Plant Competition. San
Grime, J. Philip. 1979. Plant Strategies and Vegetation Processes.
Harper, John. L. 1977. Population Biology of Plants.
Keddy, Paul. A. 2001. Competition. Dordrecht: Kluwer.
Keddy, Paul A. 2007. Plants and Vegetation.
MacArthur, Robert. H. 1972. Geographical Ecology.
Weaver, John E., and Frederick E. Clements. 1938. Plant Ecology. 2nd ed.
Because the word “competition” has a common usage, it is tempting to assume one knows what it means. In fact, there are many kinds of field and lab experiments that can be used to measure competition, each of which measures a slightly different aspect of competitionGoing back at least to Darwin, it is thought that that competition is most intense between similar species, raising an issue refereed to as limiting similarity. But in plant communities, species of very different sizes and life history compete, so the importance of limiting similarity remains unclear. Moreover, most plants grow surrounded by a large number of species. The tendency to do a pair-wise competition study of two haphazardly selected plant species is one of the greater obstacles to progress. It is necessary to carefully think about the distinction between competition effect and competitive response, between asymmetric and symmetric outcomes, and between monopolistic and diffuse interactions. Each of these distinctions will affect the type of question asked and the type of experiment used. A good example is the classic de Wit replacement series, (Harper 1977), which assumes— indeed requires—that plants be of equal size to avoid changing total biomass among the different mixtures of the two species. Of course, if the plants are nearly identical, it is impossible to study the majority of plant interactions, which are among species of very different sizes. Harper and McNaughton 1962 is an example of the De Wit replacement series in grassland species, Goldsmith 1978 shows a multispecies application to sea cliff plants, while Wilson and Keddy 1986a use the design to construct a multi-species competitive hierarchy. Fonteyn and Mahall 1981 illustrate a two-species removal experiment, while Silander and Antonovics 1982 show how to extend this to many more species. Goldberg 1990 raises the distinction between competitive response and competitive effects; Keddy et al. 1998 illustrates how to assess competitive response across many species. The total effects of all neighbors (which has been called competition intensity and diffuse competition) can be assessed using removal plots with selected species introduced to clearings and intact vegetation (Wilson and Keddy 1986b). This list of types of competition is not exhaustive (for a short introduction to even more possibilities, see Chapter 1 in Keddy 2001 (read here), particularly Figure 1.5. More types of competition are discussed in the sections on Competitive Hierarchies and Asymmetric Competition and Roots and Shoots.
Fonteyn, Paul J., and Bruce E. Mahall. 1981. An experimental analysis of structure in a desert plant community. Journal of Ecology 69: 883-896.
Goldberg, Deborah E. 1990. “Components of resource competition in plant communities.” In Perspectives on Plant Competition. Edited by James B. Grace and David Tilman, 27-49.
Goldsmith, F. Barrie. 1978. Interaction (competition) studies as a step towards the synthesis of sea-cliff vegetation. Journal of Ecology 66: 921-931.
Harper, John L., and
Keddy, Paul A., Lauchlan. H. Fraser, and Irene C. Wisheu. 1998. A comparative approach to examine competitive response of 48 wetland plant species. Journal of Vegetation Science 9: 777-786.
Silander, John A., and Janis Antonovics. 1982. Analysis of interspecific interactions in a coastal plant community—a perturbation approach. Nature 298: 557–560.
Wilson, Scott D., and Paul A. Keddy. 1986a. Species competitive ability and position along a natural stress/disturbance gradient. Ecology 67:1236-1242.
Wilson Scott D., and Paul A. Keddy. 1986b. Measuring diffuse competition along an environmental gradient: Results from a shoreline plant community. The American Naturalist 127: 862?869.
Are there traits that consistently are associated with higher competitive ability in plants? And if so, what are they? We can use the word “trait” in the broadest possible sense, from morphological and physiological traits to life history traits. There is a very long list of candidate traits (Grime 1977, Weiher et al. 1999, Westoby and Wright 2006), and some discretion is needed in selecting them. There are vastly more studies using morphological traits (e.g. height) than using physiological traits (e.g. relative growth rate), not because the former are more important, but because they are easier to measure. There is also a good deal of confusion about the fundamental distinction between functional traits and phylogenetic traits—much of the current work on phylogeny, while important for understanding plant evolution, does not contribute to the understanding of plant functional traits that affect competition (Keddy 1990). In general, height seems to be the most important trait, since taller plants can intercept light for shorter plants—and indeed, woody plants have evolved multiple times in response to this pressure to grow higher (see Keddy 2001, cited under General Overviews). Lycopods, gymnosperms, as well as both major groups of angiosperms have woody clades. The other traits are less obvious, and it is not clear if they vary with habitat. Below-ground competition, and the traits associated with it, are particularly poorly-understood, except for the general principle that plants best able to lower resources to neighbors are likely to be superior in the long run. Hence, in reading the literature, there are two issues to keep in mind. First, at the larger scale, height is likely of over-riding importance, reflecting the general observation that most plants shade one another. Indeed, back in 1933, Clements 1933 summarized the many field experiments completed by this time (most of which are now forgotten), noting that taller plants have “a decisive advantage” over shorter ones. There are also more recent studies that specifically examine height as a predictor of competitive ability (Givnish 1982, Gaudet and Keddy 1988). One general trait which is difficult to measure, but of possible importance, is the ability to lower the resource supply to neighbors (Tilman 1982). Below-ground interactions are demonstrably important in some cases, but the traits that provide this competitive success are difficult to measure, except perhaps efficiency of nutrient uptake (see also Roots and Shoots). There are many other possible traits that may become important under certain conditions, including litter accumulation, shoot thrust, rhizome aerenchyma, foraging ability, and even mycorrhizae. Large data sets of plant traits provide an emerging tool for linking plant traits to ecological interactions and ecosystem processes (Westoby and Wright 2006).
Clements, Frederick E. 1933. Competition in plant societies. News Service Bulletin, Carnegie Institution of
Gaudet, Connie L., and Paul A. Keddy. 1988. Predicting competitive ability from plant traits: a comparative approach. Nature 334: 242-243.
Givnish, Thomas J. 1982. On the adaptive significance of leaf height in forest herbs. The American Naturalist 120: 353–81.
Grime, J. Philip. 1977. Evidence for the existence of three primary strategies in plants and its relevance to ecological and evolutionary theory. The American Naturalist 111: 1169–1194.
Keddy, Paul A. 1990. “The use of functional as opposed to phylogenetic systematics: a first step in predictive community ecology.” In Biological Approaches and Evolutionary Trends in Plants. Edited by Shoichi Kawano, p. 387-406.
Tilman, David. 1982. Resource Competition and Community Structure.
Weiher, Evan, Adrie van der Werf, Ken Thompson, Michael Roderick, Eric Garnier, and Ove Eriksson. 1999. Challenging Theophrastus: a common core list of plant traits for functional ecology. Journal of Vegetation Science 10: 609-620.
Westoby, Mark and Ian J. Wright. 2006. Land-plant ecology on the basis of functional traits. Trends in Ecology and Evolution 21: 261-268.
ALONG NATURAL GRADIENTS
As Chris Pielou once observed in her characteristic acerbic tone, ecologists spend too much time looking for mythical homogeneous environments. The alternative, as she vividly illustrated with her zonation studies (e.g., Pielou 1975), was to accept that natural gradients exist and use them as research tools. Natural environmental gradients provide many advantages – not unlike a prism, they make patterns in plant communities obvious. Gradients therefore allow for quantitative tests for pattern against null models. They allow changes in environment factors to be quantified, making it possible to construct testable hypotheses as to causation and mechanism. They provide the opportunity for removal experiments, Here is a set of studies that illustrate how you can take a gradient and use it to your advantage to study competition. A general overview to the opportunities provided by gradients as a research tool, which could be treated as an amplification of Pielou’s observation, is found in Keddy 1991. Possibility the earliest experiment on competition in native species (Tansley 1917); although it uses only two environments, it clearly addresses a natural pH gradient in the English environment. More recent experimental work taking advantage of natural gradients includes studies of competition using water depth (Grace and Wetzel 1981), elevation (
Gaudet, Connie L., and Paul A. Keddy. 1995. Competitive performance and species distribution in shoreline plant communities: a comparative approach. Ecology 76: 280-291.
Grace, James B., and Robert G. Wetzel. 1981. Habitat partitioning and competitive displacement in cattails (Typha): Experimental field studies. The American Naturalist 118: 463-474.
Keddy, Paul A. 1991. “Working with heterogeneity: an operator's guide to environmental gradients.” In Ecological Heterogeneity. Edited by Jurek Kolasa and Steward T.A. Pickett, p. 181-201.
An overview of the many opportunities offered by natural environmental gradients. Read here.
Oksanen, Lauri. 1990. Predation, herbivory, and plant strategies along gradients of primary production. In Perspectives on Plant Competition. Edited by James B. Grace and David Tilman, p. 445-474.
Niu, Shuli and Shiqiang Wan. 2008. Warming changes plant competitive hierarchy in a temperate steppe in northern China. Journal of Plant Ecology 1: 103-110.
Pennings, Steven C., and Ragan M. Callaway. 1992. Salt marsh plant zonation: the relative importance of competition and physical factors. Ecology 73: 681-690.
Tansley, Arthur. G. 1917. On competition between Galium saxatile L. (G. hercynicum Weig.) and Galium sylvestre Poll. (G. asperum Schreb.) on different types of soil. Journal of Ecology 5: 173-179.
Wilson, Scott D. 1993. Competition and resource availability in heath and grassland in the
COMPETITIVE HIERARCHIES AND ASYMMETRIC COMPETITION
Interspecific pairwise interactions fall along a continuum, from symmetric to asymmetric. In the former, each population (or individual) has an equal effect upon the other, whereas in the latter, one population (or individual) is dominant over the other (see Figure 1.6 in Keddy 2001, cited under General Overviews). Many studies look at only a pair of species, often very similar ones, hoping that this will demonstrate something useful about competition and coexistence. Even so, it is usually found that one of the species is dominant over the other, that is, that competition is asymmetric in some way (Weiner 1990; Keddy 2001, cited under General Overviews). Even studies of intraspecific competition—which you might think of as the limiting case for similarity—show that size hierarchies arise (Weiner 1985). It is also possible that natural selection may cause even more similarity (Aarssen 1983)—a situation which we can think of as being the limiting case along a gradient from symmetrical to asymmetrical competition. At the other extreme, deliberate studies of asymmetry and hierarchies, one has to look hard for studies that explicitly examine asymmetrical interactions among species differing in traits, and involving large numbers of species. Studies using only a pair of congeneric species, or small groups of morphologically similar species like grasses, are least likely to provide insight or understanding of this general phenomenon. Studies using very different species, or even functional groups of different species, may be more likely to find asymmetry (Johansson and Keddy 1991). Existing published studies showed that competitive hierarchies are relatively widespread (Keddy and Shipley 1989). One can use large pairwise designs to find hierarchies, although as the number of species increases, these designs become cumbersome. An alternative approach to detecting hierarchies is to screen for competitive performance using one (or preferably more) phytometer species (see Traits). Important questions remain. How does asymmetry change with species traits, or environmental conditions? Does the ranking in a hierarchy depend upon the phytometer chosen or vary across environments? What traits might predict position in the hierarchy, or the degree of asymmetry in an interaction? It seems probable that above ground competition inherently more asymmetric (Shipley and Keddy 1989, Cahill and Casper 2000)? A final caution—the term “intensity” of competition is independent from the asymmetry of competition—a distinction that needs to be borne in mind whether one is reading an older experiment or planning a new one.
Aarssen, Lonnie W. 1983. Ecological combining ability and competitive combining ability in plants: towards a general evolutionary theory of coexistence in systems of competition. The American Naturalist 122: 707-731.
Cahill, James F., Jr. and Brenda B. Casper, B. B. 2000. Investigating the relationship between neighbor root biomass and belowground competition: field evidence for symmetric competition belowground. – Oikos 90: 311–320.
Johansson, Mats E., and Paul A. Keddy. 1991. Intensity and asymmetry of competition between plant pairs of different degrees of similarity: an experimental study on two guilds of wetland plants. Oikos 60: 27–34.
Keddy, Paul A., and Bill Shipley. 1989. Competitive hierarchies in herbaceous plant communities. Oikos 54: 234-241.
Keddy, Paul, Lisa Twolan-Strutt, and Bill Shipley. 1997. Experimental evidence that interspecific competitive asymmetry increases with soil productivity. Oikos 80: 253-256.
Weiner, Jacob. 1985. Size hierarchies in experimental populations of annual plants. Ecology 66: 743–752.
Weiner, Jacob. 1990. Asymmetric competition in plant populations. Trends in Ecology and Evolution 5: 360-364.
ROOTS AND SHOOTS
Plants must forage simultaneously in two completely different kinds of environments— above and below ground. The obvious differences in morphology of roots and shoots illustrate the different selective pressures and ecological forces between these two environments. Below-ground interactions tend to be less studied, partly because they are “out of sight, out of mind”, although data from many published experiments suggests that root competition is often greater than shoot competition (Wilson 1988, Casper and Jackson 1997). But below-ground effects are also less studied simply because they are harder to investigate—experiments are hard to do, unintended effects multiply, and traits may be nearly impossible to measure. As a further complication, many studies use crop plants, which provide limited insight into natural communities, since crop soils tend to be fertile and regularly tilled. A clarifying and frequently overlooked exmple is the use of guy wires and trenches to manipulate above- and below-ground competition from trees on pine seedlings (Putz 1992). In this situation, below-ground competition clearly matters a great deal. Another approach is to manipulate resource availability to see, if, say, adding water or nutrients shifts species composition, or, better still, the intensity of interactions between pairs of species (Tilman 1986). Another approach is to use naturally occurring gradients to measure above- and below-ground competition in a variety of conditions (Twolan-Strutt and Keddy 1996). With enough studies, it is possible to draw generalizations, such as the relative importance of below-ground competition in forests as opposed to prairies (
Capser, Brenda B. and Robert. B. Jackson. 1997. Plant competition underground. Annual Review of Ecology and Systematics 28: 545-570.
Putz, Francis E. 1992. Reduction of root competition increases growth of slash pine seedlings on a cutover site in
Tilman, David. 1986. “Evolution and differentiation in terrestrial plant communities: The importance of the soil resource: light gradient.” In Community Ecology. Edited by Jared Diamond and Ted J. Case, p. 359–380.
Twolan-Strutt, Lisa and Paul A. Keddy. 1996. Above- and below-ground competition intensity in two contrasting wetland plant communities. Ecology 77: 259-270.
Watkinson, Andrew. R. and Rob P. Freckleton. 1997. Quantifying the impact of arbuscular mycorrhizae on plant competition. Journal of Ecology 85: 541–545.
Wilson, J. Bastow. 1988. Shoot competition and root competition. Journal of Applied Ecology 25: 279-296.
Wilson, Scott D. 1993. Belowground competition in forest and prairie. Oikos 68: 146-150.
Woodward, F. Ian, and Colleen K. Kelly. 1997. “Plant functional types: towards a definition by environmental constraints.” In Plant Functional Types. Edited by Thomas M. Smith, Herman H. Shugart, and F. Ian Woodward, p. 47-65.
There are many, many published models of competition, and they continue to proliferate. Models may be constructed to (1) define problems; (2) organize thoughts; (3) understand data; (4) communicate principles; (5) test understanding; (6) make predictions. Too often writers and readers get these confused. Models constructed for one purpose may be quite unsuitable for another. The last category is likely the most important. Predictive models are designed to predict the future states of systems based on relationships specified within the model between measurable predictor (independent) variables and measurable predicted (dependent) variables. Keddy 2001, cited under General Overviews, explores many different kinds of competition models, and summarizes some twenty of them, from which this account is largely drawn. These provide a foundation to newer models, which often do not even acknowledge the presence and value of the previous models. New is not necessarily better. The following selection tends to reflect models with demonstrable general application to entire plant communities. It begins with Skellam’s 1951 patch dynamics model and Pielou’s 1975 gradient model, both of which often have been frequently overlooked. One could argue that by addressing patches (Skellam) and gradients (Pielou), these two models are foundational. An early attempt to model interactions among much larger numbers of plants based on competition coefficients is found in Yodzis 1978. A resource consumption model is found in Tilman 1982, cited under Traits, while another model of root/shoot allocation can be found in Tilman 1988. There is ongoing debate over whether these latter models are testable, much less predictive. Large forest models such as JABOWA (Botkin 1993) include competition among trees for access to units of land.. These pragmatic models tend to be general and powerful, but are often not even cited, apparently in order to direct attention to studies using few species of grasses or agricultural weeds rather than entire plant communities. Single species stands (monocultures) are rare in nature, but exhibit strong quantitative relationships between plant size (or reproductive output) and density (Watkinson 1985). Another distinctive class of models considers the biomechanical aspects of photosynthetic structures (Givnish 1982, cited under Traits); clearly there are costs to growing taller. Another class of descriptive models explores the effects of competition upon plant diversity (the humped back model, Grime 1973,) and at larger scales, the centrifugal model (Wisheu and Keddy 1992).
Botkin, Daniel B. 1993.
Grime, J. Philip. 1973. Control of species density in herbaceous vegetation. Journalof Environmental Management 1: 151–167.
Pielou, E. Christine. 1975. Ecological Diversity,
Skellam, John G. 1951. Random dispersal in theoretical populations. Biometrika 38: 196–218.
Tilman, David. 1988. Plant Strategies and the Dynamics and Structure of Plant Communities.
Wisheu, Irene C., and Paul A. Keddy. 1992. Competition and centrifugal organization of ecological communities: theory and tests. Journal of Vegetation Science 3: 147-156.
Yodzis, Peter. 1978. Competition for Space and the Structure of Ecological Communities.
Watkinson, Andrew R. 1985. “Plant responses to crowding.” In Studies in Plant Demography: A Festschrift for John L. Harper. Edited by James White, p. 275–289.
CURRENT RESEARCH TRENDS
The diversity of definitions, views, and methods present in Grace and Tilman (1990 cited under General Overviews) continues to manifest. Overall, the rate at which important ideas about competition can be tested is slowed inherently by the nature of competition experiments, which need to run for several years, if not more. Moreover, plant communities contain so many species, and there are so many potential mechanisms of competition, and so many other interactions such as mycorrhizal linking. How, then, is one to intelligently simplify or seek generalizations? One simplistic approach that avoids the question is to pick a couple of common species on a field station, and automatically apply reductionism. But such habitual behaviour by ecologists provides no guarantee of progress in understanding how competition structures ecological communities and affects ecosystem functions. The range of current work illustrates work at many scales. At the smallest scales, there are models of nutrient movement in soil (Craine et al. 2005) and experiments on infra-specific variation in competitive ability (Fridley et al. 2007). At the larger scale, there are studies of competitive interactions along stress gradients (Pennings et al. 2005, Maestre et al. 2009). An unexpected consequence of work along gradients has been the detection of mutualism as well as competition. There is growing evidence that as stress increases, the importance of mutualism increases (Maestre et al. 2009). At an even large scale in terms of time and space and number of species, newer phylogenetic methods provide tools for asking about longer term trends in plant competition, offering real data to test Darwin’s proposition that more similar species have stronger competitive interactions (Cahill et al. 2008). Since competition appears to be so prevalent in so many plant communities, the question remains how weaker species can survive at all. Studies of disturbance continue to explore the factors that create gaps in plant communities, the nature of these gaps, and the processes by which species disperse to fill these gaps. The rate of disturbance, and the rate of competitive exclusion, can achieve some sort of balance, determining, in turn, the relate abundance of competitive dominants and weaker competitors, that is, patch-dependent species (Brewer 1998, Cahill and Casper 2000, Pennings et al 2005).
Brewer, J. S. 1998. Effects of competition and litter on a carnivorous plant, Drosera capillaris (Droseraceae). American Journal of Botany 85: 1592–1596.
Cahill, James F. Jr. 1999. Fertilization effects on interactions between above- and belowground competition in an old field. Ecology 80: 466–480
Cahill, James F. Jr., Steven W. Kembel, Eric G. Lamb and Paul A. Keddy. 2008. Does phylogenetic relatedness influence the strength of competition among vascular plants? Perspectives in Plant Ecology, Evolution and Systematics 10: 41-50.
Craine, Joseph M., Joseph Fargione and Shinya Sugita. 2005. Supply pre-emption, not concentration reduction, is the mechanism of competition for nutrients. New Phytologist 166: 933-940.
Fridley, Jason D., J. Philip Grime and Mark Bilton. 2007. Genetic identity of interspecific neighbours mediates plant responses to competition and environmental variation in a species-rich grassland. Journal of Ecology 2007.
Maestre, Fernando T., Ragan M. Callaway, Fernando Valladares and Christopher J. Lortie. 2009. Refining the stress-gradient hypothesis for competition and facilitation in plant communities. Journal of Ecology 97: 199–205.
Pennings, Steven C., Mary-Bestor Grant and Mark D. Bertness. 2005. Plant zonation in low-latitude salt marshes: disentangling the roles of flooding, salinity and competition. Journal of Ecology 93: 159–167.