Doctor of Philosophy (PhD)


Biological Sciences

Document Type



Predator-prey interactions are an important factor in energy transfer through food webs. To estimate effects of altered energy flow caused by costs and benefits of antipredator defenses and to study their implications for community stability, I used the planktivorous fish - Chaoborus - Daphnia system. In the laboratory, I found that clonal variance of Daphnia antipredator defenses was high, and that behavioral, morphological, and life-history defenses were uncoupled. To estimate the costs and benefits of antipredator defenses in the presence of predators, I conducted lake-enclosure (1-m diameter x 7-m deep) experiments in the field, taking advantage of two features of the antipredator defense: (1) Daphnia exposed to only the predator-chemical should incur the cost of the defense, but not the cost of predation, and (2) antipredator response varies among Daphnia clones. Controls had no predators, 'real' predation exposed the clones to predators and in 'ghost' predation, predators were sequestered in a mesh tube away from Daphnia. I exposed a Daphnia clone responsive and a clone non-responsive to each predator, and I measured population growth (r) of Daphnia populations. Cost of predation was calculated as [responsive clone (r) in control treatments - responsive clone (r) in the ghost predation treatments]. Benefit was calculated as [responsive clone (r) - non-responsive clone (r) in the real predation treatments]. I found that, due to the antipredator defenses, Daphnia population growth was reduced by 32 % in the presence of Chaoborus predators. The benefit of the defense, however, was 68 % increased population growth. Defenses to fish predators caused a cost of 16 % and benefits of 35 %. In both cases, benefit of antipredator defense did exceed cost, but cost was nevertheless substantial. I also found the fish predator to benefit from the antipredator behavior of its prey. Planktivorous fish lost weight (-0.375 g) when preying on the non-responsive clone and gained weight (+0.225 g) in presence of the responsive clone, which was able to sustain a higher population density. Prey stability was highest in predation treatments with the responsive clone. In control treatments, Daphnia showed typical boom and bust cycles. A population model yielded similar results.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Charles W. Ramcharan