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.

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Abstract:Plant invasions in tropical agricultural landscapes have been poorly studied so far. Here, we use plot data collected in 2015 in the La Gamba valley in southern Costa Rica to analyse the spread of two invasive alien species (Hydrilla verticillata, Hedychium coronarium) on the landscape level. In total, we recorded the aquatic H. verticillata, which was locally introduced into the La Gamba valley in 2005, in 26% of all plots (size: 15 m2), while H. coronarium, which occurs along water courses, was recorded in 80% of all plots (size: 25 m2). Generalized Linear Models (GLMs) showed that the habitat type invaded and shading had no significant effect on H. verticillata presence and abundance. However, proximity to the point of local introduction and habitat diversity in the adjacent landscape did positively affect its presence. For H. coronarium, GLMs showed a significant correlation with the invaded habitat type. The probability of H. coronarium presence was highest in herbaceous vegetation and cover was higher compared with other habitats. There was no effect on native plant species richness and cover by H. verticillata, while H. coronarium exerted a negative impact on herbaceous plant species number and, in particular, cover. Hydrilla verticillata is a rather recent addition to the local alien flora, and thus dispersal limitation still restricts its local range. Compared with observed rates of spread in other regions, spread velocity within the first decade of its local presence was low. In contrast, H. coronarium has been locally present for a substantially longer time period. This species encounters suitable habitat conditions in herbaceous vegetation along water courses, where it builds up dense clonal stands which negatively affect herbaceous vegetation. Our results show that while both study species are invasive in Central American lowlands, their local abundance and impacts on native vegetation may differ profoundly.