Fig. 1.

Conceptual diagram of niche–efficiency (A) and demonstration of diminishing marginal productivity (B). Loss in any type of the plant species (trees, grass, ferns, etc.) in forested ecosystems may reduce individual plant productivity by affecting both resource acquisition (through niche complementarity) and efficiency of resource utilization (through diminishing marginal productivity) (A). Based on this model, we developed the PII to quantify individual species’ inherent value in maintaining current ecosystem productivity. Empirical analysis shows that large woody species in general have greater PII values than any other plant species (Fig. 3). Redrawn from the observation relationship between net photosynthetic rate and light energy in a terrestrial ecosystem (23), diminishing marginal productivity means that marginal resource productivity decreases monotonically as resource abundance moves beyond the compensation point through the presaturation range (indicated by a), the resource saturation range (indicated by b), and the postsaturation range (indicated by c) (B).