Discussion
Our results make two key
conceptual contributions to understanding allocation of plant defenses.
First, we showed that non-native species, which presumably have escaped
from specialist natural enemies, had lower constitutive defenses and
higher induced defenses, which correlated with higher growth rates.
Second, non-native plant species, ostensibly exposed primarily to
generalists, showed a strong positive relationship between herbivore
pressure and constitutive defense, and a strong negative relationship
between herbivore pressure experienced by a species and the intensity of
induced defenses exhibited by that species. These findings expand our
understanding of constitutive vs. induced defense allocation associated
with herbivore pressure on non-native plants.
Integrating these results suggests that reduced herbivore attack (Figure
1C and 1F; i.e., escape from enemies) favored allocation to induced
defenses (Figure 2B) instead of constitutive defenses (Figure 2A). Our
results also suggest that induced defenses are a “cheaper” overall
strategy which favors higher growth rates (Figure 5). It appears that
the generalist-dominated herbivore community that attacked non-native
plant species drove a strong positive relationship between the intensity
of herbivore pressure and allocation to constitutive defense among our
target species (Figure 2A and 2C). This supports long-held theory that
has had minimal empirical support - intense or consistent herbivory
should select for more expensive
constitutive defenses over cheaper induced defenses (Ito & Sakai,
2009).
Generalist herbivore
pressure
Native generalist herbivores often avoid non-native plant species or do
not suppress them (Schaffner et al., 2011; Sedio et al., 2020). For
example, Cappuccino & Carpenter found that non-native invasive plants
experienced 96% less leaf damage than non-native species that were not
classified as invasive (Cappuccino & Carpenter, 2005). One hypothesis
for this pattern is the novelty of biochemicals, in their new ranges,
produced by non-native plants
(Schaffner et al., 2011; Callaway & Aschehoug, 2000). Cappuccino &
Arnason (2006) found that a large suite of invasive species had more
unique secondary chemicals than exotic plants, relative to native
species. These, and other studies (see review by Inderjit et al. 2021),
indicate that unusual or novel biochemicals of some non-native species
may be why native herbivores avoid them (Inderjit et al., 2021).
Defense strategies
Intense and predictable herbivory is thought to select for constitutive
plant defenses (Kalske & Kessler, 2020). Induced defenses appear to be
adaptive to less intense and infrequent herbivory
(Agrawal & Karban, 1999;
Ito & Sakai, 2009) as a
cost-saving alternative to constitutive defense (Detto & Xu, 2020).
Relevant literature on invasives is minimal, but the invasive genotypeLespedeza cuneata exhibited greater induced defense but lower
constitutive defense than ancestral native genotypes (Beaton et al.,
2011) as might be predicted by reduced or more sporadic herbivory in the
non-native range. In contrast, non-native genotypes ofAlternanthera philoxeroides have higher constitutive defenses and
lower induced defenses (Liu et al., 2020). Clearly, the literature on
evolutionary changes in constitutive or induced defenses by exotic
plants is limited.
Other recent results indicate that
secondary metabolism plays key roles in the interactions between
non-native plants and herbivores (Tian et al., 2021; Yu et al., 2022).
Phenolics inhibit the digestion of protein (Rehman et al., 2012) and
have been central in many studies of plant-defense mechanisms (e.g.,
Kumar et al., 2020). We showed that weight gain of a generalist
herbivore was negatively correlated with phenolic concentration across
all species and herbivory
treatments, suggesting that defense might derive from variation in
constitutive and induced phenolic concentrations. Triterpenes are
terpenoids, also abundant in plants (Kumar et al., 2020), and larval
herbivore weight gain was negatively correlated with triterpenoid
content across our native species. However, triterpenoid content did not
correspond with herbivore pressure. Triterpenes have many other
functions (González-Coloma et al., 2011), and the functional
diversification of triterpenes may have masked simple responses to
herbivores.