Daphnia as keystone predators: effects on phytoplankton diversity and grazing resistance

Theory predicts that a predator can promote coexistence amongcompeting prey, and so enhance prey diversity (the keystonepredation effect), by fostering dominance of slow-growing, consumption-resistantprey. In contrast, if the predator promotes dominance by fast-growingvulnerable prey, theory predicts that the predator is unlikelyto promote prey diversity. Theory is silent about keystone predationeffects when the predator does not cause a net change in thevulnerability of the prey assemblage. I present experimentalevidence that Daphnia can act as a keystone predator withoutcausing a net change in the grazing resistance of the phytoplanktonassemblage. No change in resistance was observed, despite strongDaphnia effects on the species composition of the phytoplankton.