Testing enemy release of non-native plants across time and space using herbarium specimens in Norway

The enemy release hypothesis predicts that invasive plant success is in part due to the absence of natural enemies in the invaded range. However, few studies have assessed how enemy release may vary over time or space. Norway has seen a large increase in non-native plant species over the past few decades. We used historical herbarium records to test whether 10 non-native plant species in Norway have suffered less from natural enemies (foliar herbivores) at different latitudes and over the past 195 years, compared to closely related (congeneric) native species. We analysed over 2200 specimens over 26 species. Chewing herbivory was lower at higher latitudes for both non-native and native species. However, there was no evidence of change over time in overall chewing herbivory for either native or non-native species on average. Chewing herbivory of native and non-native species differed within the genera Centaurea, Epilobium and Salix across latitudes, and in the genera Acer, Barbarea, Campanula and Epilobium across time. Synthesis: Our results suggest that enemy release is unlikely to facilitate these non-native plants in Norway since herbivory levels are similar between both non-native and native plant species. Herbivory of these non-native plants did not change over time, suggesting that they were quickly recognized as food sources by native herbivores. Lower levels of herbivory at higher latitudes suggest that herbivory is unlikely to limit non-native plants further north and that herbivory is likely to increase at higher latitudes as the climate warms.

Automated summary

The study found that in Norway, non-native plant species and native plant species have similar levels of herbivory, suggesting that enemy release does not facilitate the growth of non-native plants. However, lower levels of herbivory at higher latitudes indicate that herbivory may increase as the climate warms.