Original paper

Can costs and benefits of Daphnia defense against fish be measured in a field experiment?

Boeing, Wiebke J.; Wissel, Björn; Ramcharan, Charles W.

Abstract

We estimated costs and benefits of antipredator defense in the planktivorous fish - Daphnia system in lake enclosures, taking advantage of two features of the antipredator defense: (1) Daphnia exposed to only the predator-chemical (kairomone) should incur the cost of defense but not the cost of predation, and (2) antipredator response varies among Daphnia clones. 'Controls' (C) had no predators, 'predation' (P) exposed Daphnia to predators and in 'kairomone' (K) treatments, predators were sequestered in a mesh tube apart from Daphnia. The mesh did not hinder exchange of kairomone but prevented predation by fish. Population growth (r) of two Daphnia clones, one responsive (RC) and the other non-responsive (NRC) to fish kairomone was estimated for each treatment. The responsive clone lowered its vertical distribution in response to kairomone during daytime when compared to the control. Cost of defense was calculated as r(C, RC) - r(K, RC). Benefit was calculated as r(P, RC) - r(P, NRC), adjusted for growth rate differences in control enclosures. During the first six days of the experiment, Daphnia relative population growth of the responsive clone was reduced by 18% in the presence of fish chemical probably due to the behavioral (migration) defense. The benefit of the defense was a 35% increase in population growth by the responsive clone in the presence of actual predation threat. The benefit of the antipredator defense did exceed the cost, but initial cost was nevertheless substantial. However, between days 6-14, costs and benefits were both negative, possibly due to compensation of population growth by life-history responses or increased algae quality/quantity caused by nutrient cycling from fish. Chlorophyll-α increased in the predation enclosures, however this did not impact our cost and benefit calculations as predation enclosures were not compared to control or kairomone enclosures. Thus, we conclude that costs and benefits can be measured in field experiments but rapid changes make measurements difficult.

Keywords

fish kairomonevulnerableinvulnerablepredator-preyinduced responses