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.