PARENT SESSION
Oral Session #56: Aquatic Ecology: Plankton. Presiding: N. Hairston.
Wednesday, August 8, 2001. 1:00 PM to 5:00 PM. Hall of Ideas I.

Modeling the effects of Chaoborus predation on Daphnia population growth rates.

Young, Joelle¹, Riessen, Howard¹, ¹

ABSTRACT- Predation by larvae of the phantom midge Chaoborus has major effects on Daphnia population dynamics in freshwater zooplankton communities. We develop a stage-classified matrix population model for Daphnia to examine the effect of Chaoborus predation on this prey species. This model analyzes the components of the predator-prey interaction between Daphnia and Chaoborus (encounter, attack, strike efficiency) and uses this to modify the instar-specific survival probabilities in the projection matrix. Encounter rates are a function of Daphnia body size, Chaoborus density, and the vertical migration patterns of both predator and prey. Strike efficiency is a function of Daphnia and Chaoborus body sizes. Here we examine three different Chaoborus species that vary in size and behavior, and their effects on the population growth rate of Daphnia pulex. Since larger Chaoborus species (C. trivittatus > C. americanus > C. punctipennis) have larger mouth gapes, they can ingest larger Daphnia and thus have a greater predation impact on adults. Those Chaoborus species that exhibit diel vertical migration (C. trivittatus and C. punctipennis) will increase spatial overlap with Daphnia in the water column relative to Chaoborus that do not migrate (C. americanus). We simulate differences in predator and prey morphology and behaviour to examine the relative effects of these three Chaoborus species on Daphnia population growth rates.

KEY WORDS: Daphnia, Chaoborus, predation, matrix population model