PARENT SESSION
Oral Session # 51: Limnology II: Plankton; Carbon.
Presiding: K Forshay
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 101.

Keystone-predation revisited: Herbivore effects on phytoplankton diversity and grazing-resistance.

Sarnelle, Orlando^*,1, ¹ Department of Fisheries and Wildlife, East Lansing, MI, USA

ABSTRACT- Keystone predation models predict that a selective predator can promote coexistence among competing prey, and so enhance prey diversity, when the predator selectively kills the competitively-superior prey. This implies that there is a tradeoff between resistance to predation and competitive ability at low resource levels among prey species. In this case, keystone predators promote both prey diversity and dominance by more resistant prey. It is also possible for a selective predator to promote dominance by prey species that are less resistant to predation, if there is a tradeoff between resistance to predation and competitive ability at high resource levels among prey species. However in this case, keystone predation theory predicts that the predator is not likely to promote prey diversity. I present experimental evidence demonstrating that Daphnia, a selective grazer, can act as a keystone predator (i. e., promote phytoplankton diversity) without shifting the phytoplankton community to dominance by more resistant species. There appear to be multiple strategies for dealing with high predation pressure among the phytoplankton, such that a selective predator can increase prey diversity by simultaneously promoting dominance by species that are resistant to predation and species with high maximum growth rates that are vulnerable to predation.

Key words: diversity, keystone, herbivory, phytoplankton