2. Revisiting the importance of variations in inducible defense
Ecologist have long recognize intraspecific variation in inducible defense, here we explore the factors involved in intraspecific variation in the inducible defense of Daphnia and synthesize the findings reported by empirical studies. Phenotypic changes show both qualitative (the presence or absence of spines) and quantitative (body size, spine length, and/or migration behavior) traits. Moreover, Daphniaexpress a combination of several unique, species-specific defensive traits in response to chemical cues (self-induced defense; a primary factor) initiated by predators, such as fish and invertebrates (Boersoma et al. 1998; Boeing et al. 2006a/b). Although predator-induced plasticity in Daphnia includes a broad range of traits and shows complicated expression patterns, studies might underestimate or overestimate the variation based on evaluation of only average values for a single trait. Stoks et al. (2016) used univariate and multivariate analyses of phenotypic plasticity to identify a natural Daphnia magna population capable of rapidly tracking changes in fish predation. This integrated, multi-trait approach improved our understanding of the evolution of phenotypic plasticity. The combined value of all the variation capacities of an individual (growth stage and multiple traits) in phenotypic plasticity would be measured as a potential capacity for adaptation.
Although specific traits change adaptively, others might appear to be maladaptive. This discrepancy is referred to as ”trait compensation” (DeWitt et al. 1999) and suggests that the adaptability of an individual cannot be measured using only one trait. Specific traits complement one another, and inducible defenses can show both progression and regression of multiple traits in an individual (Boersoma et al. 1998; Boeing et al. 2006b). In fact, these can occur simultaneously, which warrants the simultaneous observation of multiple traits. From a cost-benefit perspective, Daphnia might develop only a few inducible defense characteristics (Boersma et al. 1998), indicating that the expression of multiple defensive traits is associated with a certain cost in the forms of maintenance, production, and information acquisition. If a single trait is sufficient as an inducible defense against multiple predators, it could be unnecessary to develop multiple defensive traits. For example, development of only an elongated spine can make it more difficult for Daphnia to be captured by several predators (Caramujo and Boavida 2000), which lowers the cost of acquiring this characteristic (Laforsch and Tollrian 2004). In this situation, the costs remain the same, but the benefits increase if it helps against multiple predators at once.
The primary factor is the most important aspect of variation in inducible defense in Daphnia . The factors of predators can be separated into ”predator species/type”, ”predatory kairomone” and ”kairomone concentration” as main or primary factors. First,Daphnia must contend with predators that are size-selective regarding to their prey (Dodson 1974). The predation type for invertebrates is generally gape-limited predation that shows preference for small zooplankters, whereas vertebrate predators, such as fish, tend to be large zooplankters (Brooks and Dodson 1965). Therefore,Daphnia will know exactly what kinds of predators existing there are and will express a moderate degree of defense accordingly. In a meta-analysis, Riessen (1999) showed that the life history responses ofDaphnia to Chaoborus larvae differ substantially from those to Notonecta and fish. In the presence of small-size-selective predation by Chaoborus larvae,Daphnia mature later and show a larger size at that time. By contrast, under large-size-selective predation by fish, Daphniareproduce early and are small at maturity (Riessen 1999). Daphniasizes vary among species (Gliwicz 1990); body size is an important factor in terms of inducible defense traits.
The essential trigger includes predatory kairomone orkairomone concentration . Several studies report strong evidence for dose dependence where inducible defense is concerned (Parejko and Dodson 1990; Hammill et al. 2018; Dennis et al. 2010), and the degree of defense expression tends to vary directly with predator abundance or kairomone concentration. However, studies show that the degree of dose-specific plasticity does not increase indefinitely as kairomone concentration increases, but reach a saturation point beyond which no additional changes in plasticity occur (Reede 1995; Weetman and Atkinson 2002; Hammill et al. 2008). This suggests that plasticity expression is constrained by what is not predatory kairomone.