Invertebrate herbivory damage of lowland plant species decreases after an experimental shift to higher altitudes

Many species of plants and animals shift to higher altitudes in response to the ongoing climate warming. Such shifts of species distributions lead to the co-occurrence of species that have not previously lived in the same environment and allow the emergence of novel plant-animal interactions with potential implications for species diversity and community composition in mountain habitats. According to the enemy release hypothesis, the spread of plants in new geographic regions may be facilitated by the reduction of damage caused by natural enemies, such as herbivores. While the importance of this mechanism for the spread of invasive exotic species has been established, it is unclear whether the movement of plants uphill within their native region in response to increasing temperatures may be also facilitated by the reduction of herbivory at sites above their current upper altitudinal limit. In our study, we experimentally tested this hypothesis. We compared herbivory damage of six species of lowland plants grown in pots exposed to herbivores at their native sites in the lowland and at sites above their current upper altitudinal limit. As a control, we also measured herbivory damage of six plants growing naturally across the entire range of altitude. We found that lowland plants had reduced herbivory damage when they were moved to highland sites, while herbivory damage of species naturally growing at both altitudes did not differ. Changes of herbivory damage were modulated by leaf dry matter content and to a lesser degree also by specific leaf area and plant height. Our results provide support for the enemy release hypothesis in the novel context of altitudinal range shifts. We conclude that the reduction of herbivory damage may facilitate the spread of plants above their current upper altitudinal limit in response to increasing temperature.