Insect herbivory increases from forest to alpine tundra in Arctic mountains

Abstract

Current theory holds that the intensity of biotic interactions decreases with increases in latitude and elevation; however, empirical data demonstrate great variation in the direction, strength, and shape of elevational changes in herbivory. The latitudinal position of mountains may be one important source of this variation, but the acute shortage of data from polar mountains hampers exploration of latitude effects on elevational changes in herbivory. Here, we reduce this knowledge gap by exploring six elevation gradients located in three Arctic mountain ranges to test the prediction that a decrease in herbivory occurs with increasing elevation from forest to alpine tundra. Across the 10 most abundant evergreen and deciduous woody plant species, relative losses of foliage to insect herbivores were 2.2‐fold greater at the highest elevations (alpine tundra) than in mid‐elevation birch woodlands or low‐elevation coniferous forests. Plant quality for herbivores (quantified by specific leaf area) significantly decreased with elevation across all studied species, indicating that bottom‐up factors were unlikely to shape the observed pattern in herbivory. An experiment with open‐top chambers established at different elevations showed that even a slight increase in ambient temperature enhances herbivory in Arctic mountains. Therefore, we suggest that the discovered increase in herbivory with elevation is explained by higher temperatures at the soil surface in open habitats above the tree line compared with forests at lower elevations. This explanation is supported by the significant difference in elevational changes in herbivory between low and tall plants: herbivory on low shrubs increased fourfold from forest to alpine sites, while herbivory on trees and tall shrubs did not change with elevation. We suggest that an increase in herbivory with an increase in elevation is typical for high‐latitude mountains, where inverse temperature gradients, especially at the soil surface, are common. Verification of this hypothesis requires further studies of elevational patterns in herbivory at high latitudes.

Meta‐analysis of elevational changes in the intensity of trophic interactions: Similarities and dissimilarities with latitudinal patterns

“…By contrast, our meta‐analysis revealed that elevational changes in herbivory are small or even absent in high‐latitude mountains, in line with the hypothesis based on a study conducted above the Polar Circle (Zvereva et al, 2022). This phenomenon was also confirmed for all trophic interactions combined, which did not decrease with increasing elevation across boreal and polar localities (Figure 2).…”

“…Plants that differ in functional and life‐history traits may exhibit inconsistent elevational patterns in herbivory (Galmán et al, 2018; Kozlov, Zverev, & Zvereva, 2022; Zvereva et al, 2022). In their global analysis, Galmán et al (2018) discovered a decrease in leaf herbivory with increasing elevation in woody plants but not in herbaceous plants.…”

“…Elevational patterns in herbivory may be different on woody and herbaceous plants (Galmán et al, 2018) and on evergreen and deciduous woody plants (Galmán et al, 2018; Zvereva et al, 2020, 2022). A stronger elevational gradient in herbivory on woody plants than on herbaceous plants has been associated with their functional or life‐history traits, whereas the steeper decrease in herbivory in deciduous compared with evergreen woody plants may be expected due to differences in their resource‐use strategies and associated growth rates and antiherbivore defences (Galmán et al, 2018).…”

Effects of abiotic environment on invertebrate herbivory depend on plant community context in a montane grassland

“…However, we cannot rule out the possibility that the effects observed along this elevation gradient were the result of unmeasured processes associated with elevation, including unmeasured abiotic variables, feedbacks between top-down and bottomup regulation of herbivores, and adaptation of plants and herbivores to their local biotic and abiotic environment. Future studies could address these research gaps using combinations of reciprocal transplant, common garden, and warming experiments along elevational gradients (Buckley et al 2019;Zvereva et al 2022).…”

“…Future studies could address these research gaps using combinations of reciprocal transplant, common garden, and warming experiments along elevational gradients (Buckley et al . 2019; Zvereva et al . 2022).…”