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.