Competition

Our ability to understand competition is dependent on our ability to detect it experimentally. When designing competition experiments (or any experiment), it is important that the experiments you design address the question(s) you are asking (Connolly et al., 2001). The simplest type of competition experiment is to have three treatments: species A grown alone, species B grown alone, and species A and B grown in competition (Fig. 10.10). If individuals grow better alone than they do when in competition, then it is possible to conclude that competition has an effect on their growth rate. This type of experiment is of little practical value because it does not tell us anything about the type of competition (i.e. root vs. shoot) or how plants will react outside of the pot, in the wild.

Competition can be affected by the abiotic environment such as nutrient status. In Chapter 8 we discussed a study by Wilson and Tilman (1991). This is an example of an experiment designed to look at the effect of nutrient status (three levels of nitrogen) and type of competition (above vs. below ground) on three grasses (little bluestem,

Fig. 10.10. Basic experimental design to look at competition between two individuals of different species. Each species is grown separately (treatments 1 and 3) as well as together (treatment 2), and plant growth, biomass or other measurement is taken to compare them.

Treatment 1 Treatment 2 Treatment 3

Species A alone Species A and B together Species B alone

Fig. 10.10. Basic experimental design to look at competition between two individuals of different species. Each species is grown separately (treatments 1 and 3) as well as together (treatment 2), and plant growth, biomass or other measurement is taken to compare them.

Schizachyrium scoparium, Kentucky blue-grass, P. pratensis, and quack grass, Elytrigia repens). This experiment has a much more complex design than the above experiment. While we do not know the exact set-up of the experiment, by reading the paper we can illustrate the general design. On an existing long-term study on the effect of nitrogen, the authors randomly selected eight 4 x 4 m plots each of low, medium and high nitrogen treatments (Fig. 10.11). Each plot was then subdivided into 45 subplots, and then 15 of each of three grass species were transplanted into the existing vegetation in the subplots. The 15 individuals of each species were then divided into three competition treatments: no competition (all neighbours removed), root competition (shoots of neighbours held back using nets), and root and shoot competition. Throughout the season, the researchers measured growth characteristics (tiller number, height) and then harvested the plants and measured biomass at the end of the season.

The experimental design allowed the authors to conclude that competition was important at all levels of nitrogen. In addi tion, below-ground competition dominated at low nitrogen, whereas both above- and below-ground competition were important at higher levels of nitrogen. The results of the Wilson and Tilman (1991) study have broad ecological application because they gives us information on the underlying mechanisms of competition. Only by using a complex experimental design were the authors able to look at the effect of an abiotic factor (nitrogen) on above- and below-ground competition of three grass species.

Density effects on competition

Plant density has a major influence on the outcome of competition. As the density of one species increases so will its effect on the other species. Low densities of a species may have no competitive effect on a target species, but as density increases, so does its competitive effect. For example, an agricultural weed may only influence crop yield if it is above a specific density. There are four general types of experiments that look at the effect of density on competition.

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