Invertebrate herbivores can impact plant performance and plant communities. Conversely, plants can affect the ability of herbivores to find, choose, and consume them through their functional traits. While single plant traits have been related to rates of herbivory, most often involving single herbivore-plant pairs, much less is known about which suite of plant traits is important for determining herbivory for a pool of plant species interacting with a natural herbivore community. In this study we measured aboveground herbivore damage on 51 herbaceous species growing in monocultures of a grassland biodiversity experiment and collected 42 different plant traits representing four trait groups: physiological, morphological, phenological, and herbivore related. Using the method of random forests and multiple regression, we identified seven traits that are important predictors of herbivore damage (leaf nitrogen and lignin concentration, number of coleopteran and hemipteran herbivores potentially feeding on the plants, leaf life span, stem growth form, and root architecture); leaf nitrogen and lignin concentration were the two most important predictors. The final model accounted for 63% of the variation in herbivore damage. Traits from all four trait groups were selected, showing that a variety of plant characteristics can be statistically important when assessing folivory, including root traits. Our results emphasize that it is necessary to use a multivariate approach for identifying traits affecting complex ecological processes such as herbivory.

Abstract. Human-caused declines in biodiversity have stimulated intensive research on the consequences of biodiversity loss for ecosystem services and policy initiatives to preserve the functioning of ecosystems. Short-term biodiversity experiments have documented positive effects of plant species richness on many ecosystem functions, and longer-term studies indicate, for some ecosystem functions, that biodiversity effects can become stronger over time. Theoretically, a biodiversity effect can strengthen over time by an increasing performance of high-diversity communities, by a decreasing performance of low-diversity communities, or a combination of both processes. Which of these two mechanisms prevail, and whether the increase in the biodiversity effect over time is a general property of many functions remains currently unclear. These questions are an important knowledge gap as a continuing decline in the performance of low-diversity communities would indicate an ecosystem-service debt resulting from delayed effects of species loss on ecosystem functioning. Conversely, an increased performance of high-diversity communities over time would indicate that the benefits of biodiversity are generally underestimated in short-term studies. Analyzing 50 ecosystem variables over 11 years in the world's largest grassland biodiversity experiment, we show that overall plant diversity effects strengthened over time. Strengthening biodiversity effects were independent of the considered compartment (above-or belowground), organizational level (ecosystem variables associated with the abiotic habitat, primary producers, or higher trophic levels such as herbivores and pollinators), and variable type (measurements of pools or rates). We found evidence that biodiversity effects strengthened because of both a progressive decrease in functioning in species-poor and a progressive increase in functioning in species-rich communities. Our findings provide evidence that negative feedback effects at low biodiversity are as important for biodiversity effects as complementarity among species at high biodiversity. Finally, our results indicate that a current loss of species will result in a future impairment of ecosystem functioning, potentially decades beyond the moment of species extinction.

Plant diversity is a key driver of ecosystem functioning best documented for its influence on plant productivity. The strength and direction of plant diversity effects on species interactions across trophic levels are less clear. For example, with respect to the interactions between herbivorous invertebrates and plants, a number of competing hypotheses have been proposed that predict either increasing or decreasing community herbivory with increasing plant species richness. We investigated foliar herbivory rates and consumed leaf biomass along an experimental grassland plant diversity gradient in year eight after establishment. The gradient ranged from one to 60 plant species and manipulated also functional group richness (from one to four functional groups-legumes, grasses, small herbs, and tall herbs) and plant community composition. Measurements in monocultures of each plant species showed that functional groups differed in the quantity and quality of herbivory damage they experienced, with legumes being more damaged than grasses or non-legume herbs. In mixed plant communities, herbivory increased with plant diversity and the presence of two key plant functional groups in mixtures had a positive (legumes) and a negative (grasses) effect on levels of herbivory. Further, plant community biomass had a strong positive impact on consumed leaf biomass, but little effect on herbivory rates. Our results contribute detailed data from a well-established biodiversity experiment to a growing body of evidence suggesting that an increase of herbivory with increasing plant diversity is the rule rather than an exception. Considering documented effects of herbivory on other ecosystem functions and the increase of herbivory with plant diversity, levels of herbivory damage might not only be a result, but also a trigger within the diversity-productivity relationship.

Plant functional traits affect the capacity of herbivores to find, choose, and consume plants. However, in a community composed of different plant species, it is unclear what proportion of herbivory on a focal plant is explained by its own traits and which is explained by the characteristics of the surrounding vegetation (i.e., nonadditive effects). Moreover, nonadditive effects could be positive or negative, and it is not known if they are related to community properties such as diversity. To quantify nonadditive effects, we developed four different additive models based on monoculture herbivory rates or plant traits and combined them with measurements of standing invertebrate herbivore damage along an experimental plant diversity gradient ranging from monocultures to 60-species mixtures. In all four models, positive nonadditive effects were detected, i.e., herbivory levels were higher in polycultures than what was expected from monoculture data, and these effects contributed up to 25% of the observed variance in herbivory. Importantly, the nonadditive effects, which were defined as the deviance of the models' predictions from the observed herbivory, were positively correlated with the communities' plant species richness. Consequently, interspecific interactions appear to have an important impact on the levels of herbivory of a community. Identifying those community properties that capture the effects of these interactions is a next important challenge for our understanding of how the environment interacts with plant traits to drive levels of herbivory.