However, most natural systems are more complex, and may include a larger number of focal species and/or competitors, as well as more complex interactions. This system represents what Wootton called an 'interaction chain': one species (the releasing factor) indirectly affects another (the focal species) by influencing the abundance of an intermediate species (the competitor) that interacts with both ( Fig 1A). In this system, a predator snail ( Thais lapillus) is capable of releasing a focal species (the barnacle Balanus balanoides) from the competitive effect of a superior competitor (the mussel Mytilus edulis). A classic example is the rocky intertidal system studied by Menge. The simplest system of competitive release consists of three elements–a focal species, a competitor capable of excluding the focal species from the community, and a releasing factor capable of preventing such competitive exclusion ( Fig 1A). The hypothesis that species diversity reflects some balance between these mechanisms lies at the heart of classical ecological theory, and understanding the extent to which and the mechanisms by which these contrasting forces affect the diversity of ecological communities has been a central question in ecology.
Competitive exclusion and competitive release are two sides of the same coin: competitive exclusion refers to situations in which a species is excluded from a local community by competitive interactions with other species, while competitive release refers to situations in which a certain factor (hereafter, a 'releasing factor') limits the ability of the competitors to exclude a species, thereby allowing it to exist in the community.
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