Evolution of diversity, efficiency, and community stability.

SYNOPSIS. The response in species diversity associated with successional change in vegetation, or in a more general sense, species diversity as a function of time in any system of primary producers, has been the subject of much speculation but little direct study. All evidence available shows that pioneer communities are low in diversity, that in mesic environments the peak in diversity in forest communities can be expected 100-200 years after the initiation of a secondary successional sequence (when elements o£ both the pioneer and the stable communities are present), and that a downturn in both diversity and primary production takes place when the entire community is made up of the shade-tolerant climax species. The natural tendency in forest systems toward periodic perturbation (at intervals of 50-200 years) recycles the system and maintains a periodic wave of peak diversity. This wave is associated with a corresponding wave in peak primary production. Specialization for the habitats in the early, middle, and later phases of the cycle has figured prominently in species-isolating mechanisms, giving rise to the diversity in each stage of the forest succession. It is concluded that any modifications of the system that preclude periodic, random perturbation and recycling would be detrimental to the system in the long run.