Central and Northern European caterpillar assemblages show strong phylogenetic structure

Phylogenetic diversity metrics have been used to tackle an increasing
number of questions in ecology and evolutionary biology. Here we present a
novel use of phylogenetic diversity metrics namely the investigation of
insect herbivore assemblages on select plant species in order to
investigate possible drivers shaping the phylogenetic structure of those
assemblages. We investigated the phylogenetic structure of lepidopteran
herbivore assemblages on 208 species of central European vascular plants.
A dataset of 2553 species of Lepidoptera from 68 families, comprising 8584
individual herbivore-host associations, was analyzed for patterns of
phylogenetic α- and β-diversity in relation to plant growth form, plant
phylogeny and the accumulation of some characteristic major classes of
secondary plant metabolites. Distinct patterns of phylogenetic β-diversity
were detected when plants were partitioned into three growth form
categories (graminoid, herbaceous, and woody). In contrast, groupings of
plants according to higher taxa or key secondary metabolites did not yield
similarly clear patterns. Notable exceptions were assemblages on graminoid
plants, especially those containing silica, which could be distinguished
by both plant phylogeny and their phytochemical characteristics.
Additionally, plants containing highly toxic glucosinolates (all of the
family Brassicaceae) harbor highly distinct lepidopteran assemblages.
Phylogenetic α-diversity of associated caterpillars differed greatly
between graminoid, herbaceous and woody plants. Assemblages on graminoid
plants were found to be the most phylogenetically clustered, followed by
herbs and woody plants which showed the least degree of phylogenetic
clustering. In conclusion, our results indicate that the phylogenetic
structure of the investigated herbivore assemblages in the nemoral and
boreal zones of the Western Palaearctic region is most heavily influenced
by plant growth form while categories derived from higher plant
systematics or classifications reflecting secondary metabolites leave far
less signal at this macroecological scale. The approach taken in this
study proved to be very successful in identifying patterns in the
phylogenetic structure of herbivore assemblages. Our findings are in line
with the ‘classical’ plant apparency hypothesis. Plant secondary compounds
were found to have only limited explanatory power on our scale of
analysis.