Results
Phylogenetic distance between non-native host trees and their native
congeners was the main explanatory of resident insect abundance (i.e.
number of individuals) whereas the effects of geographic as well as
climatic distances were not significant (Figure 1). Insect abundance
declined with increasing phylogenetic distance for all four tree genera.
In contrast, insect abundance responded differently for the different
tree genera with respect to climatic and geographic distance (Fig. 1a &
b). Number of insects tended to increase with climatic distance forAcer and Betula and with geographical distance forQuercus and Fraxinus , but both not significantly. A
significant negative effect of phylogentic distance was also observable
for Heteroptera (true bugs) as well as Coleoptera (beetles) abundance,
diversity and species richness (Figs. 2 & 3). Climatic distance showed
significant effects on Heteroptera abundance and diversity whereas
geographic distance significantly affected Heteroptera abundance (Fig.
3). For the beetles, neither climatic nor geographic distances did
significantly affect insect abundance, diversity or species richness
(Fig. 2). Effects were generally stronger for Heteroptera than for
Coleoptera.
The effect of phylogenetic distance on abundance, Shannon diversity and
species richness of Coleoptera and Heteroptera was visibly stronger for
phytophagous species in comparison to non-phytophagous species (Fig. 4).
Similar patterns were observable for the effects of climatic and
geographic distance (see Fig. 4a & b). We identified a total of 144
different Coleoptera species, with the majority (61.8%) being
phytophageous, followed by zoophageous (17.4%), mycophagous (9.7%),
detritivor (4.2%) and omnivor (1.4%), with one xylomycovor species
(all classified as non-phytophagous). The predominance of phytophageous
beetles holds true at a tree genera level for Acer (70 total
species, 52.9% phytophageous), Betula (36, 69.4%),Fraxinus (42, 61.9%) and Quercus (54, 64.8%). For
Heteroptera, with a total of 51 identified species, the prominent
feeding guild was zoophag (43.1%), followed by phytophag (27.5%),
zoophytophag (23.5%), and omnivor (5.9%). Similar, the generaBetula (24 total Heteroptera species, 50% of dominant feeding
guild), Fraxinus (20, 60%), Quercus (27, 48.1%) were
predominantely inhabited by zoophagous Heteroptera, only Acer(11, 45.5%) was dominated by phytophagous species.