Why are there so many small plants? Implications for species coexistence

Summary The prominently right‐skewed distribution of species sizes has been the subject of a large literature in animal ecology, but has received comparatively little attention from plant ecologists. It is evident that not all explanations that have been offered for animals are directly applicable to plants. We suggest three hypotheses that require further study in the interpretation of size‐dependent species richness in plants. These hypotheses are all based on mechanisms that have generated, for smaller plants, a greater historical opportunity for speciation: (i) large adult plant size confers significant adaptation primarily in habitat types that have been relatively uncommon in space, across evolutionary time; (ii) relatively small species are more widely differentiated from each other in the environmental qualities defining their niches, many of which are made possible by the mere presence of larger species residing in the same habitat; and (iii) compared with large species, smaller species generally have higher fecundity allocation, i.e. they can produce a greater number of offspring per unit plant size per unit time, which generally confers a higher premium under most circumstances of natural selection, thus generating a potentially greater number of descendant individuals, and derived species. We discuss the implications of these hypotheses in addressing an underlying paradox in plant competition/coexistence theory, i.e. that large adult size is assumed to be the principal trait that confers competitive ability yet, even in those habitat types where competition is assumed to reach the highest levels of intensity within vegetation, the vast majority of the resident species are, nevertheless, relatively small.