Basically, competition is defined as a reciprocal negative interaction between biological organisms living close together in the same community at the same trophic level of an ecosystem. Competition mainly occurs when those organisms utilize common resources that are in short supply in the habitat; this indirect interaction is called 'resource competition' (also 'exploitative competition' or 'scramble competition'). Even if resources are not in short supply, competition also occurs when the organisms seeking those resources nevertheless harm one another in the process. This direct interaction by which one competitor actively and negatively influences another organism is called 'interference competition' (or 'contest competition'). Allelopathy is a typical example of asymmetric interference competition in plant communities: the growth of neighbors belonging to other species are inhibited by toxic compounds produced by the competitor and diffusing in the soil or in the atmosphere. Direct competition for space as a limiting, renewable, resource is sometimes called 'preemptive competition'.
By contrast to mobile organisms such as most animals, plants interact locally so that competition usually occurs only among neighboring plants rooted in position in a spatially structured community. The spacing of organisms is thus important in plant competition. Moreover, photosynthetic plants compete essentially for space and for abiotic resources, such as light, water, and a limited number of mineral elements (nitrogen, phosphorous, potassium, etc.), which are also locally consumed and supplied from the environment. Plants may also compete for pollinators and for soil organic compounds, such as vitamins or hormones produced by microorganisms. Nevertheless, contrary to herbivores or carnivores, all plants require the same few essential resources and consequently they can usually partition resources only in space.
Competition among organisms affects population processes within communities: two species sharing the same habitat negatively influence each other's population growth rates and depress each other's population sizes, performance, or dominance. Intraspecific competition occurs between organisms of the same population, that is, belonging to the same species. Interspecific competition occurs when two or more species experience depressed growth rate attributed to their mutual presence in an area.
The importance of interspecific competition in explaining species abundances in plant communities has received increasing attention in the field of community ecology and more specifically in the ecology of vegetation change. Among many other assembly rules, such as facilitation, herbivory, or other indirect interactions within and between trophic levels, competitive interactions between plants may influence compositional and structural changes in plant communities together with habitat selection and stochastic processes affecting the regional species pool.
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