A cross-system synthesis of herbivore and nutrient resource control on producer biomass
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Nutrient availability and consumption by herbivores control the biomass of primary producer communities to varying degrees across ecosystems. Ecological theory, individual experiments in many different systems, and system-specific quantitative reviews have suggested that 1) bottom-up control is pervasive but top-down control is more influential in aquatic habitats relative to terrestrial systems, and 2) bottom-up and top-down forces often interact to synergize or dampen relative influences on producer biomass. We use a simple set of dynamic models to review mechanistic hypotheses for these questions, and compare model predictions to empirical data from a comprehensive meta-analysis of 191 factorial manipulations of herbivores and nutrients from freshwater, marine and terrestrial ecosystems. Parameterized model equilibria suggest that interactive outcomes should be weak and less common than strict additivity. Producer community biomass responded positively to fertilization across all systems, although effects were most pronounced in freshwater. Herbivores suppressed producer biomass in both freshwater and marine systems, but effects were inconsistent on land. Importantly, we observed a striking absence of either synergistic or dampening interactive effects of nutrients and herbivores across ecosystem types and within most habitats. Marine temperate rocky reef systems, which showed superadditive synergism of nutrient and herbivore controls, represented an exception to this pattern. Experimental studies showed limited support for emergent interactive effects on producer community-level biomass. We suggest that compensation by multiple herbivore guilds, top-down control of herbivores, spatial and temporal heterogeneity, and herbivore-mediated nutrient recycling tend to reduce the expectation for consistent interactive effects on producer biomass. Continuing studies should expand the temporal and spatial scales of experiments, particularly in understudied terrestrial systems; broaden factorial designs to manipulate independently both multiple producer resources (e.g. nitrogen, phosphorus, light) and multiple herbivore taxa or guilds (e.g. vertebrates and invertebrates); and simultaneously assess the effects on not only producer biomass but also species diversity, community composition and structure, and nutrient status.