Exotic tree species have consistently lower herbivore load in a cross‐Atlantic tree biodiversity experiment

Abstract

It is commonly expected that exotic plants experience reduced herbivory, but experimental evidence for such enemy release is still controversial. One reason for conflicting results might be that community context has rarely been accounted for, although the surrounding plant diversity may moderate enemy release. Here, we tested the effects of focal tree origin and surrounding tree diversity on herbivore abundance and leaf damage in a cross‐Atlantic tree‐diversity experiment in Canada and Germany. We evaluated six European tree species paired with six North American congeners in both their native and exotic range, expecting lower herbivory for the exotic tree species in each pair at each site. Such reciprocal experiments have long been called for, but have not been realized thus far. In addition to a thorough evaluation of overall enemy release effects, we tested whether enemy release effects changed with the surrounding tree diversity. Herbivore abundance was indeed consistently lower on exotics across all six tree genera (12 comparisons). This effect of exotic status was independent of the continent, phylogenetic relatedness, and surrounding tree diversity. In contrast, leaf damage associated with generalist leaf chewers was consistently higher on North American tree species. Interestingly, several species of European weevils were the most abundant leaf chewers on both continents and the dominant herbivores at the Canadian site. Thus, most observed leaf damage is likely to reflect the effect of generalist herbivores that feed heavily on plant species with which they have not evolved. At the German site, sap suckers were the dominant herbivores and showed a pattern consistent with enemy release. Taken together, the consistently lower herbivory on exotics on both continents is not purely a pattern of enemy release in the strictest sense, but to some degree additionally reflects the susceptibility of native plants to invasive herbivores. In conclusion, our cross‐Atlantic study is consistent with the idea that nonnative trees have generally reduced herbivory, regardless of tree community diversity and species identity, but for different reasons depending on the dominant herbivore guild.

Tree species identity, canopy structure and prey availability differentially affect canopy spider diversity and trophic composition

“…Another possible driver of the lack of positive conifer effects in Douglas fir might be the lack of shared evolutionary history with native prey and spider communities (Tallamy et al 2021 ). Douglas fir was shown to only host a fraction of its specialized associated arthropods when planted in Europe (Roques et al 2006 ), and herbivorous arthropods are generally profoundly reduced and altered in their community composition by non-native plants compared to native plants (Tallamy et al 2021 ; Berthelot et al 2023 ). While we acknowledge that generalist predators should not be strongly impacted by tree species-specific community composition of their prey, we emphasize that tree-specific spider communities in our and previous studies cannot simply be explained by structure and prey availability (Mupepele et al 2014 ).…”

“…Previously, broadleaf–conifer mixtures were often shown to mitigate rather than promote arthropod diversity (Barsoum et al 2014 ; Oxbrough et al 2016 ; Matevski and Schuldt 2023 ). Non-native trees were reported to have negative effects mostly on diversity and abundance of herbivorous arthropods (Tallamy et al 2021 ; Berthelot et al 2023 ). It is expected that non-native trees host no or few specialist arthropods in their new range (Roques et al 2006 ), and as generalists are less efficient in their use of resources compared to specialists, the same resources may sustain less individuals (García et al 2018 ).…”

Detection of the invasive tree species Hovenia dulcis Thumb. (Rhamnaceae) in an urban remnant of Atlantic Forest through Remote Sensing techniques combined with RGB images obtained by an unmanned aerial vehicle (UAV)

“…This makes them capable of invading disturbed areas and forests in more advanced regeneration stages (Buechling et al, 2017). Moreover, invasive exotic species substantially experience reduced herbivory regardless of species identity and surrounding tree community diversity (Berthelot et al, 2023). Hovenia dulcis Thumb.…”

How strong is enemy release? A systematic compilation across taxa and approaches

“…a native congener/confamillial). Interestingly, while the few studies that use both methods do not consistently find that one produces stronger support for enemy release than the other (Norghauer et al 2011, Meijer et al 2016, Berthelot et al 2023), reviews of enemy release literature find that biogeographic studies support enemy release more often than do community studies (Colautti et al 2004, Heger and Jeschke 2014). Despite the considerable effect that experimental approach may have on results, individual studies often ignore the distinction when drawing conclusions about invasive potential (Stutz et al 2016, Najberek et al 2020a), or acknowledge the difference while being unable to address it (Davidson et al 2023).…”