Correct statement about herbivores and productivity:
Herbivory can reduce plant productivity most clearly in unproductive ecosystems because plants already face strong nutrient or water limitation and replace lost tissue slowly. Removal of photosynthetic area then deepens resource stress. In productive systems, moderate grazing may be compensated by rapid regrowth, altered allocation, nutrient recycling, or reduced self-shading, although heavy grazing can still depress production. The effect therefore depends on grazing intensity, plant traits, and resource availability. Energy is lost as metabolic heat at every trophic transfer, while elements
Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 17-19