Box 3
Escalation of defences refers to the macroevolutionary increase in host plant
defensive traits. Under this scenario, derived plant lineages should possess
more diverse or escalated (potent) defences than their less derived counterparts
[4, 133]. As such, their trait values are a function of their phylogenetic
distance, and closely related species should possess similar defensive traits.
On the other hand, divergent defences are those which show high disparity
between closely related hosts. Divergent traits are more dissimilar between
close relatives than expected under a conserved model of evolution.
On the global scale, Becerra [117] found a strong correlation between herbivory
by specialized insects and chemical variability in the local communities. Plant
communities exposed predominantly to herbivory by specialized insects were
much more chemically diverse than those exposed predominantly to generalist
mammalian herbivores. The composition of insect communities attacking the host
largely forms its defences – assemblages of specialists should select mainly for
divergent traits, whereas assemblages of generalists, sensitive to specialized
defences, should impose selection for escalating and highly toxic defences
[7, 113]. Therefore, plant lineages harboring diverse insect communities consisting
of herbivores with various levels of specialization are expected to possess both
escalating and divergent defences (Fig. 5).
Both escalation and divergence of defensive traits may contribute to diversity of
host plant defences as suggested by recent evidence (e.g., [7, 106, 112, 115]).
Escalation of host plant defences should promote α-diversity of defences within a
given plant lineage [4]. Divergence in host plant defences promotes β-diversity of
defences between closely related hosts [113, 117]. Divergence in defences seems
to be especially prominent in speciose and dominant tree genera. These genera
often represent a large proportion in the local forest communities and form socalled species swarms [115, 118]. For example, the five most speciose tree genera
represent ca 25% of the local tree diversity in Barro Colorado Island, Panama
[119]. Divergence in their defences can thus significantly increase defensive and
chemical diversity on the community level [117]. This has important reciprocal
effects on diversity of associated insects due to the specificity of their response. In
turn, host plant chemical diversity has been shown to be an important driver of
insect diversity almost invariably in all the systems where this was studied (e.g., [7,
115, 117, 120, 121]) (but see Salazar et al. [122]). For example, there is a strong
positive correlation between the number of caterpillar species associated with
Ficus species and host plant polyphenol and triterpene diversities (Fig. 6) [7].
High diversity of defences can probably lower the dominance of the abundant
insect species, preventing any single herbivore from dominating the community
and opening niches for the less dominant ones. Indeed, many insect herbivores
appear at low densities in tropical forests [123], where both insect and chemical
diversities reach their peak.
4 Differential Response of Herbivores to Plant Defence
91
Escalation of defences refers to the macroevolutionary increase in host plant
defensive traits. Under this scenario, derived plant lineages should possess
more diverse or escalated (potent) defences than their less derived counterparts
[4, 133]. As such, their trait values are a function of their phylogenetic
distance, and closely related species should possess similar defensive traits.
On the other hand, divergent defences are those which show high disparity
between closely related hosts. Divergent traits are more dissimilar between
close relatives than expected under a conserved model of evolution.
On the global scale, Becerra [117] found a strong correlation between herbivory
by specialized insects and chemical variability in the local communities. Plant
communities exposed predominantly to herbivory by specialized insects were
much more chemically diverse than those exposed predominantly to generalist
mammalian herbivores. The composition of insect communities attacking the host
largely forms its defences – assemblages of specialists should select mainly for
divergent traits, whereas assemblages of generalists, sensitive to specialized
defences, should impose selection for escalating and highly toxic defences
[7, 113]. Therefore, plant lineages harboring diverse insect communities consisting
of herbivores with various levels of specialization are expected to possess both
escalating and divergent defences (Fig. 5).
Both escalation and divergence of defensive traits may contribute to diversity of
host plant defences as suggested by recent evidence (e.g., [7, 106, 112, 115]).
Escalation of host plant defences should promote α-diversity of defences within a
given plant lineage [4]. Divergence in host plant defences promotes β-diversity of
defences between closely related hosts [113, 117]. Divergence in defences seems
to be especially prominent in speciose and dominant tree genera. These genera
often represent a large proportion in the local forest communities and form socalled species swarms [115, 118]. For example, the five most speciose tree genera
represent ca 25% of the local tree diversity in Barro Colorado Island, Panama
[119]. Divergence in their defences can thus significantly increase defensive and
chemical diversity on the community level [117]. This has important reciprocal
effects on diversity of associated insects due to the specificity of their response. In
turn, host plant chemical diversity has been shown to be an important driver of
insect diversity almost invariably in all the systems where this was studied (e.g., [7,
115, 117, 120, 121]) (but see Salazar et al. [122]). For example, there is a strong
positive correlation between the number of caterpillar species associated with
Ficus species and host plant polyphenol and triterpene diversities (Fig. 6) [7].
High diversity of defences can probably lower the dominance of the abundant
insect species, preventing any single herbivore from dominating the community
and opening niches for the less dominant ones. Indeed, many insect herbivores
appear at low densities in tropical forests [123], where both insect and chemical
diversities reach their peak.
4 Differential Response of Herbivores to Plant Defence
91