Lymantria dispar, in which the oxidative stress itself cannot reduce growth rates on
its own [59, 105].
7
Implications for Evolution of Host Plant Defences and
Insect Diversity
The differential response of herbivores has important implications for evolution of
host plant defences. Mainly, it restricts plants from developing a universal antiherbivore defence [12, 13]. Specialists have been shown to prefer or tolerate hosts
with high levels of specific defensive compounds in the case of multiple plant genera
(e.g., [7, 13, 106, 107]). This might have been one reason for the decline in specific
defences in Asclepias [107]. Similarly, specialized insects were able to adapt to
salicylates and reach high densities on salicylate-rich willow hosts as outlined above
[19]. Although salicylates play a significant role in structuring insect communities,
their protective value against specialized herbivores appears to be low. Maintaining
an efficient defence thus probably requires several defensive mechanisms, such as
chemical defence and trichomes, which affect both generalists and specialists on
willows [13]. As a result, defensive traits are often mutually independent or positively correlated, forming suites of complementary defences or so-called defensive
syndromes [7, 108, 109]. Trade-offs between individual defensive traits may be
expected only under specific conditions, such as low nutrients or in the case of
negative dependence in metabolic pathways (e.g., a competition for a specific
precursor) [110, 111]. Furthermore, some recent results suggest that defensive
syndromes can consist of traits following different evolutionary trajectories, possibly
making adaptation even harder for herbivores [7]. This seems to shape the evolution
of plant defensive traits into a dynamic system, with traits undergoing periods of
diversification, divergence, and sometimes decline [5].
Indeed, the differential response of insect herbivores can shape evolutionary
trajectories in individual defensive traits. Ehrlich and Raven [4] proposed escalation
of host plant defences over evolutionary time, allowing plants to escape herbivory by
unadapted generalist herbivores. An escalation of host plant defences has been found
in several plant genera, with Asclepias and Bursera being the most iconic examples
[107, 112]. Divergent, rather than escalating, defences (Box 3) have been found in
sympatric communities of closely related hosts. Such a divergence in defences
between sympatric congeners appears to lower the risk of sharing specialized
herbivores [113, 114]. As such, the ability to employ divergent defensive traits,
which are harder to follow for specialized herbivores, may be beneficial and facilitate
coexistence of closely related hosts [5, 115]. For example, divergence and a character displacement in leaf shapes help closely related Passiflora hosts to avoid herbivory by impairing host recognition by ovipositing butterfly females [116]. Similarly, a
divergence in chemistry among closely related species growing in sympatry have
been recently found in many plant genera such as Bursera, Eugenia, Ficus, Inga,
Ocotea, and Psychotria [7, 113–115].
90
M. Volf
its own [59, 105].
7
Implications for Evolution of Host Plant Defences and
Insect Diversity
The differential response of herbivores has important implications for evolution of
host plant defences. Mainly, it restricts plants from developing a universal antiherbivore defence [12, 13]. Specialists have been shown to prefer or tolerate hosts
with high levels of specific defensive compounds in the case of multiple plant genera
(e.g., [7, 13, 106, 107]). This might have been one reason for the decline in specific
defences in Asclepias [107]. Similarly, specialized insects were able to adapt to
salicylates and reach high densities on salicylate-rich willow hosts as outlined above
[19]. Although salicylates play a significant role in structuring insect communities,
their protective value against specialized herbivores appears to be low. Maintaining
an efficient defence thus probably requires several defensive mechanisms, such as
chemical defence and trichomes, which affect both generalists and specialists on
willows [13]. As a result, defensive traits are often mutually independent or positively correlated, forming suites of complementary defences or so-called defensive
syndromes [7, 108, 109]. Trade-offs between individual defensive traits may be
expected only under specific conditions, such as low nutrients or in the case of
negative dependence in metabolic pathways (e.g., a competition for a specific
precursor) [110, 111]. Furthermore, some recent results suggest that defensive
syndromes can consist of traits following different evolutionary trajectories, possibly
making adaptation even harder for herbivores [7]. This seems to shape the evolution
of plant defensive traits into a dynamic system, with traits undergoing periods of
diversification, divergence, and sometimes decline [5].
Indeed, the differential response of insect herbivores can shape evolutionary
trajectories in individual defensive traits. Ehrlich and Raven [4] proposed escalation
of host plant defences over evolutionary time, allowing plants to escape herbivory by
unadapted generalist herbivores. An escalation of host plant defences has been found
in several plant genera, with Asclepias and Bursera being the most iconic examples
[107, 112]. Divergent, rather than escalating, defences (Box 3) have been found in
sympatric communities of closely related hosts. Such a divergence in defences
between sympatric congeners appears to lower the risk of sharing specialized
herbivores [113, 114]. As such, the ability to employ divergent defensive traits,
which are harder to follow for specialized herbivores, may be beneficial and facilitate
coexistence of closely related hosts [5, 115]. For example, divergence and a character displacement in leaf shapes help closely related Passiflora hosts to avoid herbivory by impairing host recognition by ovipositing butterfly females [116]. Similarly, a
divergence in chemistry among closely related species growing in sympatry have
been recently found in many plant genera such as Bursera, Eugenia, Ficus, Inga,
Ocotea, and Psychotria [7, 113–115].
90
M. Volf