Plant density determines species richness along an experimental fertility gradient

A number of authors have suggested that, within areas a few square meters t o many square kilometers in size, species diversity appears to peak at mode rate levels of productivity, and this pattern is currently unexplained. Amo ng the best examples of this pattern have been description of vegetation in which species richness declines as soil fertility increases. We tested two hypotheses that have been proposed to explain this pattern. The interspeci fic competitive exclusion hypothesis proposes that dominant species suppres s the growth of competitively subordinate species and exclude subordinate s pecies as fertility rises. In contrast, the assemblage-level thinning hypot hesis proposes that individuals of all species tend to become larger as fer tility rises, and individuals of all species tend to exclude subordinate in dividuals of each species. Because total density declines, samples of finit e numbers of individuals will result in fewer species by chance alone. To test these hypotheses, we established an experimental productivity gradi ent in a first-year old field using four levels of slow-release NPK fertili zer (0, 8, 16, and 32 g N/m(2)). At the end of the growing season, we sampl ed aboveground biomass and numbers of stems for each species in 72 20 x 20 cm subplots (18 reps x 4 levels), with an average sample size of 260 indivi dual stems per plot. We observed an 80% decline in stem density with increa sing fertility, and a 50% decline in species richness along this fertility gradient. A simulation of random thinning along a fertility gradient showed a nearly identical decline in species richness, supporting the assemblage- level thinning hypothesis. We also found that responses of individual speci es to the soil fertility gradient showed virtually no support for interspec ific competitive exclusion. The overwhelming influence of density found in this study suggests that plant species richness along many productivity gra dients may be strongly influenced by total stem density, and that differenc es in competitive ability among species, although generally important, are not necessary to create dramatic changes in species richness along fertilit y gradients.