Plant species natural history
In order to assess natural variation in constitutive plant chemical defences along elevational gradients, we sampled 14 species ofCardamine , out of the 19 currently growing in Switzerland (Aeschimann et al. 2004). Together, all species encompass almost a 3000 m elevational gradient of the Alpine ecosystem. During the radiation of the group, species have colonized a wide range of habitats, including dry and wet alpine meadows, forests and riverbanks, and growing between 300 m above sea level (m a.s.l.) (e.g. C. bulbifera ) and more than 3000 m a.s.l. (e.g. C. alpina ). In the field Cardamine plants are predominantly attacked by leaf chewers such as Pieridae butterflies, flea beetles, aphids, and slugs (Rasmann S., personal observations), and previous work has highlighted a steady decline in herbivore damage with elevation (Pellissier et al.2016; Defossez et al. 2018). The phylogenetic relationships between plants was pruned from a well-resolved and dated phylogeny of European plant species (Durka & Michalski 2012) using the apepackage (Paradis et al. 2004).