Factors affecting the expression of inducible defences in Euplotes:: genotype, predator density and experience

1. Inducible defences alter the strength of interaction in food webs. Their effectiveness depends both on the maximum level of induction and the speed at which induction happens. Maximum level and speed of induction should therefore evolve in concert. 2. We examined the effect of genotype, number of predators and previous exposure to predators on speed and maximum level of induction in a morphological defence expressed by eight clones in three species of the ciliate Euplotes. 3. Both speed and maximum level of induction, and the reaction to predator density varied among genotypes. These results show that there is genetic variance for all aspects of this inducible defence and the potential for complex evolutionary change under selection. 4. Higher predator densities led to higher maximum levels of defence and, for one measure of induction speed, more rapid induction. Previous exposure to predators had no detectable effect on either speed or maximum level of induction. 5. Our results demonstrate that Euplotes can precisely and rapidly adjust their morphological defence to the magnitude of predation risk. The speed of induction and the maximum level of defence varied among genotypes and this will lead to variation in defence level and vulnerability under natural conditions. Variation in prey vulnerability is a key factor promoting stability in food webs.