Insect herbivory in alpine grasslands is constrained by community and host traits

Abstract

Question: The consumption of plant biomass by herbivores is an important biotic interaction with possible cascading effects from the level of individual plant performance up to ecosystem functions. However, little is known about the degree and patterns of insect herbivory in alpine ecosystems. We searched for altitudinal differences in herbivory and tested for effects of traits of both the host plants and the surrounding plant communities on herbivory incidence to evaluate predictions derived from the resource availability and the resource concentration hypothesis. Location: High-altitude grasslands of the eastern European Alps. Methods: We recorded insect herbivory on leaves of five plant taxa differing in traits related to nutritional value and anti-herbivore resistance to provide the first large-scale assessment of herbivory in high-altitude environments. Regression analyses were used to test for significant effects of plant community diversity, host species abundance and soil fertility at grasslands in the lower and upper alpine belt. Results: We found a positive correlation of the incidence of folivory with local plant species richness, soil fertility and local host species abundance. The effects of community traits were stronger at higher altitudes, although the overall level of herbivory was lower at higher altitudes. Taxa lacking strong chemical defences, but possessing either high nutrient or high silica content, were particularly strongly and weakly affected by herbivory, respectively. Conclusions: These results are in line with predictions derived from the resource availability hypothesis, but remain ambiguous concerning the resource concentration hypothesis. The degree of herbivory not only depends on traits of hosts but also on features of the surrounding plant community. The generally high incidence of herbivory with low levels of damage suggests that insect folivores are omnipresent in high-altitude grasslands.