Species Richness Net Primary Productivity and the Water Balance Problem.

Species energy theory suggests that, because of limitations on reproduction efficiency, a minimum density of plant individuals per viable species exists and that this minimum correlates the total number of plant individuals with the number of species . The simplest assumption is that the mean energy input per individual plant is independent of the number of individuals, making , and thus as well, proportional to the total energy input into the system. The primary energy input to a plant-dominated ecosystem is estimated as its Net Primary Productivity (). Thus, species energy theory draws a direct correspondence from to . Although investigations have verified a strong connection between and , strong influences of other factors, such as topography, ecological processes such as competition, and historical contingencies, are also at play. The lack of a simple model of expressed in terms of the principal climate variables, precipitation and potential evapotranspiration, , introduces unnecessary uncertainty to the understanding of species richness across scales. Recent research combines percolation theory with the principle of ecological optimality to derive an expression for (, ). Consistent with assuming S is proportional to , we show here that the new expression for (, ) predicts the number of plant species in an ecosystem as a function of and . As already demonstrated elsewhere, the results are consistent with some additional variation due to non-climatic inputs. We suggest that it may be easier to infer specific deviations from species energy predictions with increased accuracy and generality of the prediction of (, ).