Exotic tree species have consistently lower herbivore load in a cross-Atlantic tree biodiversity experiment
Creators
- 1. University of Freiburg
- 2. Université de Sherbrooke
- 3. University of Quebec at Montreal
- 4. University of Alberta
- 5. University of Minnesota
Description
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 change 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 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 likely reflects the effect of generalist herbivores that feed heavily on plant species they have not evolved with. 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 strict 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 non-native trees have generally reduced herbivory, regardless of tree community diversity and species identity, but for different reasons depending on the dominant herbivore guild.