Energy availability or environmental productivity is supposed to be the main determinant of species richness. Despite strong evidence for the role of energy and numerous hypotheses, we still don’t understand the processes behind this pattern. The more individuals hypothesis (MIH) explains increase of diversity with available energy by the increase of total abundances. According to MIH, more productive sites support more individuals, which leads to lower extinction rates of species’ populations, leading to higher diversity. However, there is lack of direct tests caused by lack of abundance data. Gentry’s dataset, consisting from 370 0.1ha samples across 6 continents of tropical and temperate forests, represents one of the best possibilities for testing the MIH. We have used only tree data including all tree counts with stem diameters < 2.5 cm d.b.h. and reanalyzed the dataset using the rarefaction method. As trees differ in their sizes, we expressed abundances also as the total metabolic rate per site calculated using a scaling relationship from the stem diameter.
Results/Conclusions
There is strong positive relationship between raw species richness and AET (the best measure of productivity). When using rarefied species richness this relationship becomes even stronger, indicating that the increase in species richness is not caused by the pure increase of the number of individuals. This is in contrast with MIH and in accord with Currie’s (2004) results, with an addition that there is a sampling effect in the data but acts in opposite way: more individuals produce less species than predicted by the random distribution from rarefaction. Surprisingly, there is a very weak relationship between climatic variables and total metabolic rate per site, indicating more complex relationships between climate, abundance, energy consumption and species richness than assumed in species-energy theory.
