The hierarchical continuum concept

Two general models have been proposed to explain the structure of the plant community: the community-unit model of Clements and the continuum model of Whittaker and Curtis, the latter based on Gleason's individualistic distribu- tion of species. It is generally assumed that most ecologists now accept the continuum model. Empirical evidence sug- gests, however, that the continuum in its current form does not fully describe the observed patterns of vegetation along envi- ronmental gradients. In this paper, we introduce the hierarchi- cal continuum as a general concept to represent dynamic community structure along regional spatial gradients. The hierarchical continuum is derived from a combination of the individualistic distribution of species, hierarchical assemblage structure, and the core-satellite species hypothesis. The hierar- chical continuum concept predicts that the distribution of species across sites in a region will be polymodal, which reflects hierarchical structure, and that the distribution and abundance of species within and between sites will be spa- tially and temporally dynamic. Regional distribution of plant species in North American tallgrass prairie, southeastern flood- plain hardwood forests, northern upland hardwood forests, and boreal forests were either bimodal or polymodal as pre- dicted by the hierarchical continuum concept. Species in tallgrass prairie were spatially and temporally dynamic with an average turnover of 8-9 species per 50 m2 yr-1. In addition, the hierarchical continuum concept predicts the potential for fractal (self-similar) patterns of community structure, and provides a framework for testable hypotheses concerning spe- cies distributions along environmental gradients.