Figure 18.10 Natural patterns of productivity and species richness. (a) The relationship between productivity and species richness can be described by one of five types of curves. (b) Among six categories of organisms, a hump-shaped curve is the most commonly observed relationship between productivity and species richness for aquatic vertebrates, aquatic and terrestrial invertebrates, and aquatic and terrestrial plants.
Data from G. G. Mittelbach et al., What is the observed relationship between species richness and productivity?, Ecology 82 (2001): 2381–2396.