4m3, mesocosm, freshwater, California, USA
Theory predicts that a predator can promote coexistence among competing prey, and so enhance prey diversity (the keystone predation effect), by fostering dominance of slow-growing, consumption-resistant prey. In contrast, if the predator promotes dominance by fast-growing vulnerable prey, theory predicts that the predator is unlikely to promote prey diversity. Theory is silent about keystone predation effects when the predator does not cause a net change in the vulnerability of the prey assemblage. I present experimental evidence that Daphnia can act as a keystone predator without causing a net change in the grazing resistance of the phytoplankton assemblage. No change in resistance was observed, despite strong Daphnia effects on the species composition of the phytoplankton.