Competition is generally understood to refer to the negative effects on plant growth or fitness caused by the presence of neighbors, usually by reducing the availability of resources. Competition can be an important factor controlling plant communities, along with resources, disturbance, herbivory, and mutualisms. Since all plants require a few basic elements, the resource involved is generally light, water, nitrogen, or phosphorus, depending upon the species and the location. The effects of competition are widespread and easily observed in mixtures of crops and managed forests, which is why weeding and thinning are practiced. Competition is also widespread in native habitats, from deserts to wetlands, and is known to have important—indeed crucial—effects upon recruitment, growth, and reproduction. In the late 1800s, Darwin wrote extensively about the importance of competition in nature, particularly its role in driving natural selection. Thereafter, interest in the phenomenon grew. Many experiments with both crops and wild species were conducted. Models of competitive interactions were also constructed, with the number and size of the models increasing rapidly with the advent of computers in the 1970s. Because the word competition has a common usage in English, what it represents in biological systems is frequently assumed, rather than explicitly stated, leading to misunderstanding. Care must be taken in using or interpreting the word without specifying what kind of competition is being investigated, as different forms of competition can have different types of consequences. For example, competition may be looked at from the perspective of an individual, a population, or a species, it may be symmetric or asymmetric, and it can occur among single or multiple species simultaneously. Experimental design carries its own assumptions, which are often not stated in published articles. One of the most difficult tasks in exploring published studies is the need to sift through large numbers of experiments in which investigators have haphazardly selected (a pair of) species and grown them in mixture, without adequately justifying their choice of species or the experimental design. Another difficult task is distinguishing between models that, at least in principle, have measurable inputs or make measurable predictions (or both) and those that do not and cannot be tested. Overall, the very ease of growing plants in mixture, as well as the ease of making new models, may have made some people careless, with the result that basic questions are remaining unaddressed. Ongoing issues of importance include mechanisms of competition, types of competition, effects of competition on plant coexistence, and the intensity of competition under different sets of conditions.
Competitor relatedness, indirect soil effects and plant coexistence