“Primary herbivore” for sea urchins' role in kelp forest ecosystems. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. Ecosystem processes are constrained by energy conservation and by the cycling of matter. Energy enters mainly through primary production, is lost as metabolic heat at every transfer, and therefore cannot be recycled in the way that carbon, nitrogen, phosphorus, or water can. The remaining alternatives—“Top predator”, “Primary producer”, “Secondary carnivore”—refer to different state
Ref:
Fundamentals of Ecology, Odum & Barrett, 5th Ed., Ch. 4