4. Conclusion
The variation of degree in inducible defense of Daphnia among conspecific individuals has long been recognized in experimental and field work. Despite a fast-growing study on the variation in inducible defense, we lack a general framework for understanding the variation by which factors influences to express. Then we classified seven secondary factors related evolutionary and ecology in predator-induced plasticity. The secondary factors can be distinguished by their relative relationship to primary factors, i.e., presence of predator and/or predatory kairomone. Abiotic factors, food, clone/genotype, and instars are promoted or inhibited the degree of expression in inducible defense by working with primary factors. And the others, abiotic factors, ecological traps and alarm substance, and maternal effect work alone, but the degree of expression may be equivalent, smaller or larger compared with the degree of variation from the primary factors. Variation of inducible defense is associated with vulnerability of predator. Therefore, it will be important to clarify the factors and the degree of variation in the future.