Why Are Parts of the World Green? Multiple Factors Control Productivity and the Distribution of Biomass

This paper evaluates the multiple factors that determine the production of plant biomass and its distribution among producers and various trophic groups of con_ ?^)^ Osumers. In rough order of their importance, water and nutrient availability, factors 0*S that deter herbivores (plant defenses, environmental heterogeneity and disturbance, nutrient stoichiometry), and consumption by herbivores appear to be the most universal determinants of the production and distribution of plant biomass. In some times and places, indirect effects from enemies of herbivores (predators, parasites, parasitoids and pathogens) propagate through the food web to influence plant biomass, in a manner somewhat consistent with green world and exploitation ecosystem mechanisms. I discuss why such food web dynamics appear to be much more important in water than on land. The only demonstrated cases of communitylevel trophic cascades occur in water. Although species-level cascades are moderately frequent on land, community-level cascades rarely or never occur.