Abstract
A fundamental notion in community ecology is that local species diversity reflects some balance between the contrasting forces of competitive exclusion and competitive release. Quantifying this balance is not trivial, and requires data on the magnitude of both processes in the same system, as well as appropriate methodology to integrate and interpret such data. Here we present a novel framework for empirical studies of the balance between competitive exclusion and competitive release and demonstrate its applicability using data from a Mediterranean annual grassland where grazing is a major mechanism of competitive release. Empirical data on the balance between competitive exclusion and competitive release are crucial for understanding observed patterns of variation in local species diversity and the proposed approach provides a simple framework for the collection, interpretation, and synthesis of such data.
Highlights
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 limits the ability of the competitors to exclude a species, thereby allowing it to exist in the community
It is important to note that competitive release differs from 'trophic cascades': in both cases the releasing factor might be a predator, competitive release involves a maximum of two trophic levels and the 'release' is from competition, while a trophic cascade involves at least three trophic levels and the release is from predation
The proposed framework is based on the concept of effectiveness, which we define as the ratio between the number of species released from competitive exclusion by the releasing factor, and the number of species excluded by the relevant competitor(s) in the absence of the releasing factor
Summary
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. In spite of a considerable progress in our understanding of the mechanisms by which potentially releasing factors such as predation, grazing, and disturbance, affect the diversity of ecological communities[5, 7, 11,12,13,14,15,16] little is known about the actual effectiveness of such factors in offsetting species loss caused by competitive exclusion In this contribution we attempt to reduce this gap by providing a conceptual framework for empirical studies of the balance between the effects of competitive exclusion and competitive release on species diversity. As far as we are aware, the case study described here is the first attempt to explicitly quantify the balance between these two fundamental forces
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