A behavioral cascade: Top-predator induced behavioral shifts in planktivorous fish and zooplankton

It is well known that the effects of direct (lethal) predator-prey interactions propagate through food webs (the trophic cascade). However, in the present study we show for the first time that, parallel to the trophic cascade, there exists a behavioral cascade in the sense that behavioral responses, induced by the nonlethal presence of a top predator, are transmitted down the food chain over more than one trophic link. By using a new method, horizontal echo sounding, in an enclosure study in a shallow lake in southern Sweden, we recorded the swimming activity and refuge use of young-of-the-year (0+) roach (Rutilus rutilus) in the presence and the absence of a caged predatory fish. By connecting the predator avoidance behavior of 0+ roach with that of zooplankton throughout summer, we show that species interactions are more dynamic than had been predicted earlier by food web theory. In our study, 0+ roach changed their behavior by becoming less active in early summer and by hiding in a refuge in late summer in the presence of a piscivore, and this caused Daphnia to spend more time feeding in the open water than when piscivores were absent. Thus, we conclude that, to explain patterns of habitat use in natural systems, it is of crucial importance that we recognize the existence of behavioral cascades.