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

Many plant and animal species move to higher altitudes in response to climate warming, leading to new species co-occurrences and novel plant-animal interactions. These shifts can impact species diversity and community composition in mountain habitats. According to the enemy release hypothesis, plant expansion into new areas may be facilitated by reduced damage from natural enemies, like herbivores. While this mechanism is known to facilitate the spread of invasive species, it is unclear whether the uphill movement of native plants, in response to rising temperatures, is also aided by reduced herbivory at sites above their current altitudinal range. In our study, we experimentally tested this hypothesis. We compared herbivore 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 herbivore damage of six plants growing naturally across the entire range of altitude. We found that lowland plants had reduced herbivore damage when they were moved to highland sites, while herbivore damage of species naturally growing at both altitudes did not differ. Changes of herbivore damage were modulated by leaf dry matter content, specific leaf area and to a lesser degree also by plant height. Our results support the enemy release hypothesis in the context of altitudinal range shifts. We conclude that reduced herbivore damage may help plants spread above their current upper altitudinal limit in response to rising temperatures.