Ecosystem-level patterns of primary productivity and herbivory in terrestrial habitats

Abstract

ECOSYSTEMS are structurally organized as food webs within which energy is transmitted between trophic levels and dissipated into the environment. Energy flow between two trophic levels is given by the amount of production at the lower level and by the proportion of production that is consumed, assimilated and res-pired at the higher level. Considerable evidence indicates that food-web structure varies predictably in different habitats1–5, but much less is known about quantitative relationships among food web fluxes. Many of the energetic properties of herbivores in African game parks are associated with rainfall and, by inference, with net primary productivity6,7. Respiratory costs per unit produc-tion at the consumer trophic level are higher for homeotherms than for heterotherms8. Plant secondary chemicals affect herbivore dietary choices9,10 and the allocation of plant resources to those chemicals varies with resource availability11. How these phenomena are translated into ecosystem fluxes is unknown. We present evidence that herbivore biomass, consumption and produc-tivity are closely correlated with plant productivity, suggesting that the latter is a principal integrator and indicator of functional processes in food webs.