Species Diversity Across Nutrient Gradients: An Analysis of Resource Competition in Model Ecosystems

The capture and efficient use of limiting resources influence the competitive success of individual plant species as well as species diversity across resource gradients. In simulations, efficient nutrient acquisition or nutrient retention by species were key predictors of success when nutrients were limiting. Increased nutrient supply favored species with characteristics that improved light interception or light use. Ecological theory suggests that low diversity on fertile sites may be a consequence of competitive exclusion by one or a few species with superior light-interception characteristics. On infertile sites, competitive exclusion may be a function of superior nutrient-acquisition characteristics in species. At intermediate fertility, a shift from single-resource specialization to a balanced effort in the acquisition of multiple resources should allow for greater species diversity. Thus, a unimodal relationship between diversity and nutrient supply, vegetation biomass, or productivity is predicted. However, simulations demonstrated alternate relationships depending on the ecosystem characteristic to which diversity was compared. Diversity was greatest at intermediate total biomass but increased monotonically with net primary production and nitrogen (N) supply. The highest diversity occurred midrange on a scale of community-level leaf area to fine-root length ratios, which in the context of the model indicates that the vegetation as a whole was simultaneously limited by both N and light and that effort toward the acquisition of both resources is distributed in such a way that both resources are equally exploited. Diversity was lowered by the presence of species with a superior ability to sequester resources.