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 N with the number of species S . The simplest assumption is that the mean energy input per individual plant is independent of the number of individuals, making N , and thus S 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 ( NPP ). Thus, species energy theory draws a direct correspondence from NPP to S . Although investigations have verified a strong connection between S and NPP , 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 NPP expressed in terms of the principal climate variables, precipitation P, and potential evapotranspiration, PET , 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 NPP ( P , PET ). Consistent with assuming S is proportional to NPP , we show here that the new expression for NPP ( P , PET ) predicts the number of plant species S in an ecosystem as a function of P and PET . 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 NPP ( P , PET ).