Experimental assessment of interaction costs of inducible defenses in plankton

The expression of phenotypically plastic traits in prey organisms, triggered by changes in the abundance of their predators, is customarily assumed to involve costs in some fitness components such as fecundity, growth or survival. However, these plastic responses may also have an interaction cost, which is assessed by the strength of interspecific interactions such as increased vulnerability to other preda-tors or a reduction in competitive ability. This study assesses experimentally the effects of induced morphological defenses triggered by the carnivorous rotifer Asplanchna brightwelli, on the competitive ability of two herbivorous populations, and the reciprocal dominance between the rotifers Brachionus calyciflorus and Brachionus havanaensis. Our results support the existence of a trade-off between the expression of induced morphological defenses and competitive ability in aquatic animals. Specifically, we show that: (i) a consumer which reduces its vulnerability to predation via induced defenses increases its resource requirement to maintain zero population growth; and (ii) the expression of induced defenses promotes a qualitative shift in the relative competitive performance of the two herbivores over a single resource gradient. These results reinforce the general idea of functional community consequences of phenotypic plasticity, in particular the expression of inducible defenses of prey