'Competition' refers to a negative interaction between organisms. When one organism physically restricts another organism's access to resources, it is referred to as 'interference competition'. Interference competition is common in animals such as songbirds, which maintain exclusive spatial territories with the aid of vocalizations. Although it is more difficult to envision, interference competition also occurs between plants. Many plants secrete chemicals into the soil via their root systems in a process called 'allelopathy'. These chemicals negatively impact the root systems of neighboring plants, which helps promote a plant's exclusive access to soil nutrients and water. A second, distinctly different type of competition is called 'exploitative competition'. Exploitative competition occurs between organisms that consume the same resources, when resource consumption by one organism lowers its availability for other organisms. Examples of exploitative competition include shading by neighboring plants, or when nectar consumption by one pollinator lowers nectar availability for other pollinators. Both interference and exploitative competition can occur within (intraspecific) and between (interspecific) species. Both forms of competition can operate simultaneously in natural populations (Figure 1a). When one competitor is more effective than another, competition is said to be 'asymmetric'. For example, taller plant species are often better competitors for light than shorter species. Conversely, 'symmetric' competition occurs between similarly matched competitors (Figure 1b).
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