In grasslands, aboveground herbivory not only affects individual plant performance and community composition, but also interactions between plants and their surrounding soil ecosystems. Recent research suggests that the outcome of these plant-soil feedback effects (PSFs) is influenced by aboveground herbivorous insects under natural conditions. However, until now it remains unclear to what extent the intensity of herbivory affects the outcome and hence the importance of PSFs under natural conditions. Therefore, we performed a standardized comparative PSF experiment in nine grasslands along a gradient of land-use related intensity of aboveground herbivory within the German Biodiversity Exploratories. Within this field experiment we tested PSF effects for four common grass species, both with and without aboveground herbivores.
Results/Conclusions
Without aboveground herbivores, PSFs were similar in direction and magnitude for each of the four grass species tested along the land-use related herbivory gradient. However, in the presence of aboveground herbivores, the PSFs differed from those measured under herbivory exclusion, depending on the intensity of herbivory. Increasing intensity of aboveground herbivory increased the absolute difference between the PSFs measured with and without herbivores, for all four grass species. Our study provides empirical evidence that the intensity of aboveground herbivory affects the outcome of PSFs under natural conditions. Our results suggest that PSFs might be important for plant performance in ecosystems where the influence of aboveground herbivores on plants is low. However, in grasslands with larger impacts of herbivores, PSF effects can be changed or overridden by herbivory suggesting multiple drivers of density dependent control in grassland plant communities resulting in lower dominance structures and potentially higher diversity.