Metabolizable energy and biomass of plants consumed by caribou (Rangifer tarandus) in tundra communities of northern Alaska and deer (Odocoileus spp.) in forests and grasslands of Washington, United States of America

A ubiquitous interaction operates at the base of food webs in many terrestrial ecosystems of the world, creating the foundation for bottom-up regulation of consumers. This interaction plays out as follows. Populations of herbivores deplete plant biomass by foraging. Increasing herbivore population size intensifies this depletion, which in turn, creates a negative feedback regulating herbivore population growth. Large herbivores and the plants they consume offer a useful system for studying this interaction because populations of large herbivores are often regulated by density dependence, defined as the reduction in the per-capita growth rate that occurs as populations grow. Diminished body mass of individuals has been repeatedly observed in high-density populations, implicating plant-mediated, diminished nutrition as the primary cause of density dependence. However, there is no general explanation for why these nutritional deficiencies occur.  The data deposited here were used to demonstrate fit new model of the feedbacks from plant biomass to herbivores. The model shows how reduced nutrition of herbivores can result from increased dilution of metabolizable energy in the plant tissue they consume as populations grow even when a large fraction of the consumable plant biomass remains uneaten. This result provides a tidy, mechanistic explanation for bottom-up control of population dynamics of primary consumers in a "green world."