The Distribution and Abundance of Tallgrass Prairie Plants: A Test of the Core-Satellite Hypothesis

Abstract

Levins (1969) and Hanski (1982a,b) presented dynamic models that describe the fraction of population sites occupied by species. Levins' model predicts a unimodal distribution of species at sites, whereas Hanski's model yields a bimodal distribution: a mode of widespread "core" species, and a mode of rare "satellite" species. The existence of this dichotomy is the core-satellite hypothesis. Hanski's model also predicts a positive correlation between the fraction of sites occupied by a species (distribution) and the average population size within occupied sites (abundance). Levins' model generates no such correlation between distribution and abundance. We find that a large set of quadrat data on tallgrass prairie plants matches some of the predictions of Hanski's model: for plants of seven different soil series, distribution and abundance are positively correlated, and the distribution of species at sites is bimodal. The bimodality is obscured if distributions from the seven soil series are aggregated. Two earlier hypotheses have been proposed to explain the core-satellite dichotomy. Williams (1964) showed that the degree of bimodality in species distributions depends on the number of quadrats examined: specifically, the smaller the sample, the larger the size of the core mode, and this is true even for randomly assembled communities of species. For our data, the size of the core mode correlates negatively with sample size, as Williams predicted. However, a computer simulation showed that the observed core modes were significantly larger than would be expected by chance. Thus, Williams' sampling hypothesis does not entirely account for the bimodal distribution of prairie plant species. Raunkiaer (1934, see "law of distribution of frequencies") argued that the core mode represented those species best adapted to a particular habitat. He also noted that when quadrats of dissimilar habitat were combined, the bimodality disappeared. The distributions of tallgrass prairie plants, however, do not conform to Raunkiaer's explanation: core species tend to be widespread across all soil series. If Raunkiaer's hypothesis were correct, each species would be widespread on only a few soil series, and each soil series would support a different assemblage of core species. Although our analyses confirm the existence of the core-satellite dichotomy, they do not provide an explicit test of Hanski's model. The essence of the models by Hanski and Levins is that the distribution of each species is dynamic and fluctuates owing to stochastic variation in immigration and local extinction. The strongest test of Hanski's model would be to demonstrate that the distribution of each species varied through time and that the composition of the core and satellite modes was not constant. Long-term data about the distribution of species in a community are needed for such comparisons.