What Determines the Outcome of Competition

One of the reasons weeds are so successful is because they adapt rapidly to new environmental conditions, including the 'competitive neighbourhood' of other weeds, crops and plants in general. Weeds do not 'know' how competitive others are - if others are much better competitors, the weed simply dies without reproducing. If a weed is at a competitive disadvantage but still produces offspring, there should be selection for the offspring to develop better competitive abilities (as long as the genes are available). What complicates the situation is that weeds are subject to selection from other types of interactions (herbivory, for example - see Chapter 9) and this makes it difficult to



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a) Effect of ragweed on soil moisture no relationship

20 40 60 80 100 120 140 Ragweed biomass (g)

b) Response of plantain to soil moisture

c) Effect of ragweed on light

20 40 60 80 100 120 140 Ragweed biomass (g)

d) Response of plantain to light no relationship

20 40 60 80 100 Percentage full sunlight

e) Net effect of ragweed on plantain no relationship

0 20 40 60 80 100 120 Ragweed biomass (g)

Fig. 8.4. Effect of ragweed (Ambrosia artimisiifolia) biomass on: (a) percentage soil moisture and (b) percentage full sunlight, the response of plantain (Plantago lanceolata) to (c) soil moisture and (d) sunlight, and (e) the net effect of ragweed on plantain (redrawn and adapted from Goldberg, 1990).

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determine what abilities are a response to the need to compete.

Competitive traits

Despite the complex nature of interactions, the outcome of competition depends on the same processes and structures that influence all aspects of a weed's existence, i.e. selection on the traits that each individual weed has. Weeds have traits that will suppress neighbours or avoid being suppressed by them, but it is rare to find weeds that do both well. For example, Goldberg (1990) found that the presence of common ragweed

Table 8.2. List of characteristics associated with competitive plants (not in rank order) (adapted from Zimdahl, 1999).

Shoot characteristics

Rapid expansion of tall, foliar canopy

Horizontal leaves under overcast conditions and obliquely slanted leaves (plagiotropic) under sunny conditions Large leaves

A C4 photosynthetic pathway and low leaf transmissivity of light Leaves forming a mosaic leaf arrangement for best light interception A climbing habit

A high allocation of dry matter to build a tall stem Rapid extension in response to shading

Root characteristics

Early and fast root penetration of a large soil area

High root density/soil volume

High root-shoot ratio

High root length per root weight

High proportion of actively growing roots

Long and abundant root hairs

High uptake potential for nutrients and water

(Ambrosia artemisiifolia) decreased the percentage sunlight available but did not affect soil moisture. Because its competitor, narrow-leaved plantain (Plantago lanceolata), responded to decreases in moisture, but not to decreases in light, ragweed was not competitively superior to plantain (Fig. 8.4).

Generally, weeds may adapt to a competitive environment and develop traits that allow them to specialize in being relatively superior at competing for one (or a few) resources. Zimdahl (1999) listed traits associated with highly competitive agricultural weeds (Table 8.2). Possessing any one or a few of these traits does not guarantee competitive success as traits vary in their effectiveness in different populations and communities. For example, the Australian native Sydney golden wattle (Acacia longifolia) has a high photo-synthetic rate, but is still outcompeted by weedy tick berry (Chrysanthemoides monilif-era). This is because tick berry has a more efficient leaf arrangement to intercept light and therefore is able to outcompete the native species (Weiss and Nobel, 1984).

Weed ecologists may study 'size' as being the most important trait in competition. 'Size', however, is really not a trait itself, it is more of a general description. We say this because size could mean a weed has adapted (or is phenotypically plastic enough) to grow taller, branch out more or produce more roots to capture resources (Goldberg and Werner, 1983; Schoener, 1983; Goldberg, 1987). Hence, size results from interacting traits like rate of cell division, leaf expansion, seed germination and seedling emergence time and speed. One of the implications of the Bergelson (1996) review we discussed earlier is that larger individuals are often those that germinated or emerged first and captured more resources. In agriculture and forestry, early emerging weeds are the ones that cause crop losses because they compete with the young and vulnerable crops for nutrients and light, depending on the planting conditions used (Forcella, 1993; Van Acker et al., 1993; Knezevic et al., 1994; Chikoye et al., 1995; Weinig 2000). The same principle applies to competition in non-crop ecosystems (Gerry and Wilson, 1995; Tremmel and Bazzaz, 1995). For example, garlic mustard (Alliaria petiolata) is probably competitive because it germinates in the autumn, over-winters (and perhaps photosynthesizes) and quickly grows tall as soon as the temperature, moisture and light allow, usually before native spring plants. In this manner, garlic mustard captures early season light, nutrients, mois ture and space at the expense of individuals of native species.

The actual impact of size on the outcome of competition can be difficult to quantify. You might expect that a weed that is (initially) twice the size of a competitor would imply that the weed is then twice as competitive. Indeed, this may happen if competition is 'size-symmetric'. In other cases, competition is 'size-asymmetric', meaning that an individual that is (initially) twice the size of a competitor may be (for example) four times as competitive. In practice, you might measure the outcome of competition by examining the relative weights of the weed and its competitor: (i) in competition and (ii) not in competition.

Size, however, is not always a determinant of competitive success (Wilson, 1988; Gerry and Wilson, 1995). To test for size advantage, Grace et al. (1992) grew six grasses alone and in pairs. During the first 2 years the initial plant size was correlated to competitive success measured as relative yield (a comparison of yield when grown alone and when grown in competition). In the third year, however, the initial size did not confer an advantage. Individuals with higher relative growth rate were at an advantage rather than those that were initially bigger. Weigelt et al. (2002) suggest that size is more important during the seedling stage, whereas species-specific traits such as biomass allocation patterns are more important during the adult stages of a plant's life cycle. Size is less likely to be advantageous in situations of low nutrients and high light, where size does not improve an individual's chance of obtaining resources.

Below-ground competition (for nutrients or water) is more likely to be size-symmetric, while above-ground competition (for light) is more likely to be size-asymmetric (Casper and Jackson, 1997; Schwinning and Weiner, 1998). This is because a weed that successfully outcompetes others for early-season light often has accelerated growth, leading to faster suppression of competitors, capture of increasingly available light as daylength increases, further suppression of competitors and so on. This type of 'feedback' is what leads to size-asymmetric com petition. Below ground, weeds that have more roots (or more efficient roots) will capture more resources, but the process is much slower as the water and nutrients are less ubiquitous than light and harder to find. A lack of accelerated capture of resources means that the competitive advantage of a weed with a large root system is restricted to being closely equivalent to its size advantage.

Effect of the environment on competition

Selection pressures change such that a trait may be advantageous in some locations at a given time but may be less advantageous under different environmental circumstances. We have already emphasized that the existence of spatial and temporal variability in the environment is the reality that plants must survive. The more unpredictable the environmental variability, the more risk to existence. In terms of competitive traits, a genotype may survive for years with a suite of traits but if the environment changes drastically, then the genotype may be quickly placed at a competitive disadvantage. This is actually a principle of any weed management: how to outcompete the weeds. The problem again is that weeds tend to adapt more quickly to change and produce a wide variety of genotypes that can be fit to a range of environments.

How adaptable are weeds in changing environments? Generally, weeds can only adapt if they have the genes available. For most weeds, this is rarely a problem as they reproduce sexually and recombine genes constantly (see Chapter 4). When the environment changes, some weed genotypes will die or at least be disadvantaged, but other genotypes will survive to ensure the population and species of weeds will survive. For example, genotypes of lambsquarters were variable enough to live in different concentrations of nutrients and outcompeted the less well adapted carrot crop (Daucus carota) (Li and Watkinson, 2000).

Pickett and Bazzazz (1978) examined how a resource gradient (soil moisture) affected competition among six weed

Giant foxtail

Giant foxtail

Pennsylvania smartweed

Pennsylvania smartweed

Common ragweed

Common ragweed low high

Redroot pigweed




Velvetleaf low high

Soil moisture

Fig. 8.5. Proportional response of six weeds to a soil moisture gradient when grown alone (pure) and in competition (mixed) (redrawn from data in Pickett and Bazzaz, 1978).

species by growing them alone and in competition with each other (six species together). When grown alone, all species had a broad tolerance to a water gradient (Fig. 8.5). When grown in competition, however, peak biomass tended to shift in four species while the most competitive species, redroot pigweed (Amaranthus retroflexus) and giant foxtail (Setaria faberii) did not shift. Species primarily responded through phenotypic plasticity rather than higher mortality.

Both of these examples illustrate the effects of spatial heterogeneity on weed competition. Spatial distributions of weeds are the patterns of locations of weed species that we see, for example, in a maizefield or a meadow. Weed distributions are often scattered or 'patchy', in part, because in some areas the weed can outcompete other plants but in other areas the weed is excluded or suppressed. As discussed in Chapter 6, such patchiness also relates to weed seed dispersal. In non-crop habitat, patchiness occurs since variation in topography and substrate will create local micro-environments favouring some weeds and not others. The patchi-ness may be less obvious in these non-crop habitats since placement of nutrients and water is more precise in crop habitats. In crop fields, the patchiness is exacerbated because weeds will colonize areas where farmers add nitrogen (for example) and where machinery disperses the seeds (Casper and Cahill, 1998; Dieleman and Mortensen, 1999).

One of the implications of clumped population of weeds is that there will be a lot of crowding; in other words, the local density of weeds is often high. This has implications for competition involving weeds. In theory, the more weeds that exist in a given area, the greater the demands placed on local patches of limited resources. Each weed competes for its own benefit. Therefore, when a weed competes it is as likely to harm other weeds (conspecific or otherwise) as it is to harm crops or native plants. Conspecific weeds may be more likely to compete because they should have similar resource demands and probably have similar traits.

Because of the potential importance of plant densities, ecologists and crop scientists often study how density quantitatively affects the outcome of intra- and interspecific competition (e.g. Cousens et al., 1987; Lonsdale, 1990; Kropff and Lotz, 1992; Kropff and Spitters, 1992; Kropff et al., 1992a,b; Cousens and O'Neill, 1993; Frantik, 1994; Knezevic et al., 1994; Chikoye et al., 1995; Ives, 1995; Lindquist and Kropff, 1996; Lutman et al., 1996). It appears that competition is often 'density dependent'. This means exactly what it says - as density changes, so too does the outcome of competition. The change may be direct for a while: every time another weed germinates and starts to grow, there is one more demand on the limited common resource pool and competition increases by an amount directly related to the extra demand.

Most studies, again, tend to examine competition between two species only or, perhaps, between a crop and weed populations comprised of a few weed species. In these studies, density-dependent effects can be illustrated as in Fig. 8.6. Notice that as pigweed density increases beyond a relatively low value of 0.5 pigweed per m2, competition, as measured by yield loss in maize, slows. This is because the pigweed individuals start to compete intraspecifically whereas before this they compete mainly with maize (interspecifically). While this example does not show it, and it is difficult to demonstrate this experimentally, there also can be a period when the weed density is not yet high enough to cause significant impacts.

Density-dependent competition is typically important but density is neither the only important factor nor is it independent of other factors. The time of emergence of weeds, their morphology (e.g. big leaves or small leaves, tall plants or short plants), and other density-dependent interactions like allelopathy, herbivory (Chapter 9) and parasitism (Chapter 9), can influence competition separately, synergistically or antagonistically (Weidenhamer et al., 1989; Bergelson, 1990; Molofsky, 1999). They are all influenced by genetic and environmental variation, and all of these will not only vary spatially, but temporally. The competitive outcome of a changing environment will be dependent on the timing of the change and on its interaction with the plant's phenology, especially seed germination and seedling emergence.

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