The twelve theories of co-existence in plant communities: the doubtful, the important and the unexplored

Background: Twelve distinct explanations have been proposed for the co-existence of species in ecological communities. Types of mechanism: The mechanisms can be divided into those that are stabilizing, i.e. with an increase-when-rare mechanism, and those that are equalizing, the latter on their own only delaying the exclusion of species. However, by evening out fitness, equalizing mechanisms can facilitate the operation of stabilizing mechanisms. Importance: It is suggested that circular interference networks, co-evolution of similar interference ability, cyclic succession, equal chance (neutrality) and initial patch composition are likely to be unimportant, or perhaps not even occur. Equal chance is an equalizing mechanism. Allogenic disturbance, alpha-niche differentiation, environmental fluctuation (relative non-linearity and/or the storage effect) and pest pressure are probably important. All four are stabilizing. More evidence is needed on aggregation, interference/dispersal trade-offs and the spatial mass effect. Aggregation and the spatial mass effect are equalizing. Suggestions are made of the evidence needed to make informed judgements on which contribute the most to co-existence in plant communities.