Stable coexistence of members of an ecological guild by definition requires negatively frequency-dependent dynamics among guild members, but negative frequency dependence should not be construed too narrowly. First, it does not mean that vital rates depend only on species relative abundances. They depend on absolute abundances too. Second, frequency-dependence is often scale dependent, emerging on long temporal or spatial scales. Third, numerous ecological phenomena lead to negative frequency dependence among guild members, including resource partitioning, predator partitioning, frequency-dependent behavior of natural enemies, and species-specific responses to varying environmental factors. Finally, these various processes interact to create novel forms of frequency dependence that are not immediately evident from individual ecological processes considered in isolation. Recent theory is developing a synthetic understanding of frequency-dependent phenomena affecting species coexistence. Methods of quantifying species coexistence mechanisms provide the means for a high level understanding of the origin of frequency-dependence, its dependence on spatial and temporal scale, and the interactions between different coexistence mechanisms.
Results/Conclusions
Natural enemies affecting the members of any ecological guild are diverse, and may be relatively specialized on different guild members. This is natural enemy partitioning and leads to negative frequency dependence in a similar way to resource partitioning. However, negative frequency dependence can be generated between guild members by a single predator. Although such negative frequency dependence has been observed experimentally, its cause is not well understood. Learning constraints are one hypothetical cause. In this case, low abundance of a species is a defense against a predator because it undermines learning to capture that prey species. As a species becomes more abundant, this defense breaks down. Negative frequency dependence is the result. Negative frequency dependence can also result from spatial and temporal variation. Assuming that predators are only active at times or places of significant prey activity, frequency dependence emerges when guild members differ in the times or places of their own activity. A species at low abundance would not support predators when it is active, if other species are not active then, and so it would experience a reduced predation rate. As a species becomes more abundant, these times or places of its activity do support predator activity, leading to higher morality. At any given time or place, negative frequency dependence is not evident, but it emerges on the larger scale of space or time.