Competition requires that individuals make demands on a common pool of resources (e.g. water, light, nutrients and space) that are limited. Because this situation occurs frequently, competition is a key interaction that determines population and community structures and dynamics. It would be better for organisms to avoid competition because it harms all involved. Being a better competitor just means you suffer less and have a greater relative fitness; if you could avoid allocating resources to characteristics good only for competition and, for example, allocate resources instead to produce more seeds, then your absolute fitness would increase. Because most plants require the same types of resources, avoiding competition may not be feasible during a plant's life cycle. Thus, individuals are often selected for characteristics that allow them to outcompete others in order to increase their relative fitness (by surviving and reproducing). Outcompeting others requires being better at acquiring more of the common resource pool. It requires being able to tolerate a lack of resources when others acquire more. Hence, the competitive ability of a plant can be measured in two ways: its ability to suppress other individuals (competitive effect), and its ability to avoid being suppressed (competitive response) (Aarssen, 1989; Goldberg, 1990; Goldberg and Landa, 1991).
Competition involving plants has been dissected by many authors (Zimdahl, 1980; Keddy, 1989; Grace and Tilman, 1990; Bengtsson et al., 1994; Casper and Jackson, 1997). We focus on the main issues related to competition since these are especially relevant to weeds:
• What is the importance of competition in different environments?
• What general mechanisms of competition for different resources exist?
• What determines the outcome of competition?
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