• Interactions can have positive, negative or neutral effects on the individuals involved.

• Competition is a negative interaction where individuals make simultaneous demands that exceed limited resources and, while both suffer, one individual suffers less.

• Competition may be less important in habitats where resources are so poor and scattered that populations never get dense or large enough to cause simultaneous demands that exceed limiting resources.

• Competition can be difficult to test because weeds can compete for more than one resource at a time (above and below ground). Competition is influenced by other interactions like herbivory and parasitism (see Chapter 9), and environmental and genetic variation.

• Individuals also may deny access of resources to others (interference competition); it is difficult to tell whether this occurs in weeds. Allelopathy, possibly a form of interference competition, involves the production of toxins that can make tissues or habitats unsuitable for use.

• The outcome of competition is often related to the size of individuals and density of populations.

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