A global analysis of elevational gradients in leaf herbivory and its underlying drivers: Effects of plant growth form, leaf habit and climatic correlates

1. Research on elevational gradients in species interactions holds that herbivore pressure increases towards warmer and more stable climates found at lower elevations. However, the generality of this expectation has been challenged by recent studies reporting no evidence of expected trends or even positive associations between elevation and herbivory, presumably due to uncontrolled biotic and abiotic factors influencing such relationships. 2. Using a world-wide dataset of insect leaf herbivory including 1,027 plant species and spanning an elevation gradient of 2,755m, we tested if elevational gradients in herbivory were contingent on whether species were found at tropical vs. temperate latitudes, plant growth form (non-woody vs. woody species), and leaf habit (deciduous vs. evergreen woody species). In addition, we tested the influence of climatic correlates of elevation presumably underlying such elevational gradients. 3. Although there was no evidence of an overall relationship between elevation and damage, we found that elevational gradients in herbivory were contingent on species growth form whereby herbivory increased towards lower elevations for woody species, but no gradient was observed for non-woody species. We further found that elevational gradients in herbivory were contingent on leaf habit as herbivory increased towards lower elevations for deciduous species, but no gradient was present for evergreen species. Elevational variation in damage was not contingent upon latitudinal region. Finally, analyses of climatic factors indicated that although a significant association between temperature and herbivory for woody species was detected, the elevational gradient in leaf herbivory remained largely unchanged after accounting for temperature and precipitation, suggesting that climate does not fully account for the gradient. 4. Synthesis. This study delivers the first global assessment of elevational gradients in leaf herbivory and emphasizes the role of plant (e.g. life-history or defensive) traits and climatic factors in shaping elevational gradients in herbivory.