The Maintenance of Species‐richness in Plant Communities: The Importance of the Regeneration Niche

SUMMARY According to ‘Gause's hypothesis’ a corollary of the process of evolution by natural selection is that in a community at equilibrium every species must occupy a different niche. Many botanists have found this idea improbable because they have ignored the processes of regeneration in plant communities. Most plant communities are longer‐lived than their constituent individual plants. When an individual dies, it may or may not be replaced by an individual of the same species. It is this replacement stage which is all‐important to the argument presented. Several mechanisms not involving regeneration also contribute to the maintenance of species‐richness: differences in life‐form coupled with the inability of larger plants to exhaust or cut off all resources, also the development of dependence‐relationships, differences in phenology coupled with tolerance of suppression, fluctuations in the environment coupled with relatively small differences in competitive ability between many species, the ability of certain species‐pairs to form stable mixtures because of a balance of intraspecific competition against interspecific competition, the production of substances more toxic to the producer‐species than to the other species, differences in the primary limiting mineral nutrients or pore‐sizes in the soil for neighbouring plants of different soecies, and differences in the competitive abilities of species dependent on their physiological age coupled with the uneven‐age structure of many populations. The mechanisms listed above do not go far to explain the indefinite persistence in mixture of the many species in the most species‐rich communities known. In contrast there seem to be almost limitless possibilities for differences between species in their requirements for regeneration, i.e. the replacement of the individual plants of one generation by those of the next. This idea is illustrated for tree species and it is emphasized that foresters were the first by a wide margin to appreciate its importance. The processes involved in the successful invasion of a gap by a given plant species and some characters of the gap that may be important are summarized in Table 2. The definition of a plant's niche requires recognition of four components: the habitat niche, the life‐form niche, the phenological niche, and the regeneration niche. A brief account is given of the patterns of regeneration in different kinds of plant community to provide a background for studies of differentiation in the regeneration niche. All stages in the regeneration‐cycle are potentially important and examples of differentiation between species are given for each of the following stages: Production of viable seed (including the sub‐stages of flowering, pollination and seed‐set), dispersal, in space and time, germination, establishment, and further development of the immature plant. In the concluding discussion emphasis is placed on the following themes: the kinds of work needed in future to prove or disprove that differentiation in the regeneration niche is the major explanation of the maintenance of species‐richness in plant communities, the relation of the present thesis to published ideas on the origin of phenological spread, the relevance of the present thesis to the discussion on the presence of continua in vegetation, the co‐incidence of the present thesis and the emerging ideas of evolutionists about differentiation of angiosperm taxa, and the importance of regeneration‐studies for conservation.