Functional equivalence versus functional diversity: Is there a minimum dimensionality for characterizing forests to a reasonable level of realism?

Background/Question/Methods

The neutral theory of biodiversity emphasizes ecological drift but selection as the driver of community dynamics. The fundamental assumption of neutrality has been criticized for its unreality of assuming all species (individuals) are functionally equivalent on a per capita basis with regard to their identical fitness. Based on the argument of unreality, neutral theory has been often reduced to a NULL model appropriate to filter more suitable models and theories. However, the important fact of the neutral theory has not received respect properly: species are not necessarily to be identical in every aspect because differences in life-history traits (functional diversity) do not necessarily lead to differences in their fitness. Since birth and mortality rates largely govern the structure and dynamics of communities, the demographic equivalence (identical fitness) as birth-death trade-offs between different species is also consistent with the functional equivalence assumption allowing differences in functional traits. Based on metabolic theory of ecology, we deduced an allometric scaling rule for plant life-history traits which could also explain the birth-death trade-offs. We test the scaling rule across a broad array of plant species.

Results/Conclusions

Empirical data support the theoretical prediction that species compensate high mortality with high reproduction rates according to a quarter law with the per capita metabolic rate. The same relation accounts for both the number and size of species’ offspring. Dispersal ability is allometrically related to plant body size which indicates a negative correlation with their per capita metabolic rate. The demographic trade-offs between birth and mortality rate are statistically significant for plant populations ranging from seagrasses, ferns, land and salt marsh herbs, succulent plants to trees. It is worth noting that based on metabolic theory of ecology, demographic trade-off and therefore ecological equivalence, at least to a first approximation (similar as the nearly neutral theory of molecular evolution), emerges automatically from metabolic properties of plants. Parts of these results have been observed empirically and validated statistically in previous research of metabolic theory, but were not link to neutral theory so far. We will discuss the potential of an association of neutral theory with metabolic theory for strengthen future ecological research.