Pugnaire F.I. Valladares F. Functional Plant Ecology
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15
Plant Interactions: Competition
Heather L. Reynolds and Tara K. Rajaniemi
CONTENTS
Introduction ....................................................................................................................... 457
C–S–R and Resource Competition Models........................................................................ 458
C–S–R............................................................................................................................. 459
Resource Competition .................................................................................................... 459
Competition Intensity......................................................................................................... 461
Soil Fertility or Productivity Gradients.......................................................................... 462
Disturbance Gradients.................................................................................................... 463
Traits .................................................................................................................................. 464
Soil Heterogeneity .............................................................................................................. 465
Conclusions and Future Directions.................................................................................... 468
Acknowledgments .............................................................................................................. 469
Appendix 15.1: Experimental Design and Competition Indices.........................................469
References .......................................................................................................................... 473
Not until we reach the extreme confines of life, in the arctic regions or on the borders of an
udder desert, will competition cease. The land may be extremely cold or dry, yet there will
be competition between some few species, or between the individuals of the same species,
for the warmest or dampest spots.
—Darwin (1859)
INTRODUCTION
Organisms are said to compete when shared resource needs have mutual negative effects (– –) on
survival, growth, or reproduction. This happens when resources are limiting, that is, when
organismal demand for resources exceeds the supply of resources from the environment.
Resources are quantities of matter or energy whose consumption promotes positive per capita
growth or reproduction of an organism over at least some range of resource availability (Tilman
1982). As autotrophs, plants require the inorganic resources carbon dioxide (CO
2
), water, light, and
mineral nutrients. The major mineral nutrients are nitrogen, phosphorus, potassium, iron,
calcium, magnesium, and sulfur; the minor ones are molybdenum, copper, zinc, manganese,
boron, chlorine, sodium (some Chenopodiaceae), aluminum (ferns), cobalt (legumes that sym-
biotically fix nitrogen), and silicon (diatoms) (Banister 1976). Because plants are sessile and do
not exhibit behavior, the main mechanism of plant competition is exploitation; competitors
interact solely by consuming (i.e., depleting or preempting) resources (Figure 15.1; see Weiner
and Thomas 1986 and Thomas and Weiner 1989 for discussion of resource depletion vs. resource
preemption). Conversely, interference competition involves direct interactions between competi-
tors that prevent access to resources, such as fighting between animals. In plants, interference
competition can occur by one individual overgrowing another. Allelopathy, or production of
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457
chemicals that inhibit plant function, may also be (but is not necessarily) an evolved mechanism
of interference competition (Williamson 1990). Allelopathy has received renewed attention of
late, particularly for its relevance to understanding exotic plant invasions (e.g., Wardle et al. 1998,
Mallik and Pellissier 2000, Bais et al. 2003, Callaway and Ridenour 2004).
The relative importance of competition versus other biotic interactions (e.g., mutualism or
facilitation, predation, herbivory) and abiotic factors (e.g., fire, pH, soil fertility) to the
ecology and evolution of species and the structure of communities is an old debate in ecology
(Roughgarden and Diamond 1986). Related issues concern whether the intensity of compe-
tition varies with environmental conditions and what traits are important to competitive
ability. During about a 20 year period from the late 1970s to the mid-1990s, debate about
these issues among plant ecologists crystallized in the contrasting perspectives of the compe-
tition–stress–ruderal (C–S–R) model of Grime (1977, 1979, 1988) and the resource competi-
tion model of Tilman (1982, 1988, 1990). After a number of stimulating exchanges and a
substantial associated literature, the Grime–Tilman debate largely faded away, as attention
turned to a number of new pressing issues, including the relationship between biodiversity
and ecosystem functioning (Naeem et al. 2002, and see Chapter 10) and the causes and
consequences of exotic species invasions (Levine et al. 2003, Hierro et al. 2005). Yet C–S–R
and resource competition theory continue to be used as guiding frameworks in these and
other areas of plant ecology (e.g., Wardle et al. 1997, Shea and Chesson 2002, MacDougall
and Turkington 2005). This chapter reviews C–S–R and resource competition theory and the
associated literature on competition intensity (CI) and competitive traits. We then turn to a
relatively new area of focus for plant ecologists who study competition—soil heterogeneity,
asking how current work in this area is informed by, and informs, C–S–R and resource
competition theory. We conclude with a brief summary and suggestions for future research.
The appendix to this chapter provides an overview of the various approaches to designing
competition experiments and calculating competition indices.
C–S–R AND RESOURCE COMPETITION MODELS
It has sometimes been asserted that different definitions of competition are at the root of any
disagreement between the C–S–R and resource competition models (Thompson 1987,
Grace 1991a). However, both Grime and Tilman define competition in terms of exploitation
(Tilman 1987): ‘‘Here, competition is defined as the tendency of neighboring plants to utilize
the same quantum of light, ion of a mineral nutrient, molecule of water, or volume of space’’
(Grime 1973) and ‘‘The mechanism of competitive displacement is resource consumption’’
(Tilman 1988).
Competition can only occur for resources when they are limiting. Some resources required
by plants (e.g., CO
2
) are generally superabundant, and use by one organism has no effect on
Organism 1 Organism 2
Resource
Organism 1 Organism 2
Resource
FIGURE 15.1 Mechanisms of competition. (Left) In exploitation competition, organisms interact only
indirectly, via shared resource use. (Right) In interference competition, organisms interact directly for
the shared resource. Interference competition is common in animals (e.g., fighting over mates or food),
although plants can sometimes interfere by growing over one another, or via chemical production
(allelopathy).
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458 Functional Plant Ecology
other organisms requiring the same resource. Resources become limiting when potential
demands on resources by organisms exceed resource supply. The major source of disagree-
ment between the C–S–R and resource competition models is when resource demand has the
potential to exceed resource supply.
C–S–R
The C–S–R model is a conceptual model that categorizes vegetation into general strategies
based on adaptation to combinations of two types of selection pressures: stress and disturb-
ance (Figure 15.2). Stress is defined as any environmental condition that limits plant produc-
tion (e.g., extremes of pH or temperature), and disturbance as any environmental condition
that removes plant biomass (e.g., herbivory, pathogens, wind, fire, human activities) (Grime
1977). Low availability of mineral nutrients (low soil fertility) is most often associated with
stress in this model. Stress and disturbance are thought to reduce competition by preventing
development of a dense vegetation and thereby reducing demand for resources. Competition
is hypothesized to be of highest intensity and greatest importance in highly productive
vegetation, such as that which develops on fertile soil under relatively undisturbed conditions.
Proponents of the C–S–R model (Grime 1977, Campbell et al. 1991) reason that dense, highly
productive vegetation result in increased demand for both light and belowground resources
(water, mineral nutrients). Therefore, traits important to the competitive strategy are con-
sidered to be those that maximize acquisition of all resources. These traits include high growth
rates, large size and height, and high plasticity in roots and shoots to allow active foraging for
undepleted resource pockets. Investment in resource acquisition comes at the expense of
investment in traits important for adapting to stress and disturbance. Such trade-offs in
allocation to competition, stress, and disturbance mean that each species cannot be adapted
to the extremes of more than one strategy, although adaptation to intermediate intensities of
competition, stress, and disturbance may occur (Grime 1977, 1988).
RESOURCE COMPETITION
The resource competition model is a mechanistic, mathematical model of plant population
dynamics that considers essentially similar selection pressures (or environmental constraints):
C–S C–R
C–S–R
SS–R
R
C
0
25
50
75
100
0
25
50
75
100
100
75
50
25
0
I
s
(%)
I
c
(%)
I
d
(%)
FIGURE 15.2 Model showing the various equilibria between competition (C), stress (S), and disturb-
ance (d) in vegetation and the location of primary and secondary strategies. I
c
, relative importance
of competition (——); I
s
, relative importance of stress (– ––––);I
d
, relative importance of disturbance
(—– – —–). (Source: Grime, J.P., Am. Nat., 111, 1169, 1977. With permission.)
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Plant Interactions: Competition 459