The invasive pathogenic fungus Hymenoscyphus fraxineus alters predator-herbivore-ash food webs

Plant pathogens have potential to change the relative importance of bottom-up and top-down regulation in forest ecosystems and may determine whether trophic cascades are present in local food chains. While evidence for such effects from field studies in forest ecosystems remains sparse, this would be valuable for the management of invasive plant pathogens. Hymenoscyphus fraxineus is an invasive pathogenic fungus that causes massive dieback across Europe, mainly of the common ash Fraxinus excelsior. Here, we investigated by a correlative approach how infestation by H. fraxineus affects the association between herbivory of ash leaves and densities of herbivores (herbivorous Hemiptera, Psocoptera, herbivorous Coleoptera, caterpillars and Symphyta) and predators (Araneae, Formicidae, and Dermaptera) in young ash plantations. Our results indicate that infestation with the pathogenic fungus changed community-wide regulatory mechanisms in arthropod food webs of young ash plantations. Bottom-up and top-down regulation was relatively balanced in plantations with low infestation levels, but with pronounced top-down control of herbivory by spiders in autumn. In highly infested plantations, by contrast, spiders were bottom-up limited as they aggregated in herbivore-rich patches and had no significant effect on herbivory. There was a top-down effect of herbivores on ash leaves in high infested plantations, but not in low infested plantations. These results suggest that the invasive fungus affects ecosystem functioning through a reduction of functional complementarity and intensification of negative intraguild interactions among predators. This consequently resulted in strong bottom-up limitation of predators and their reduced ability to suppress herbivores and herbivory.

Automated summary

This study found that infestation by the invasive pathogenic fungus changed the regulatory mechanisms in arthropod food webs of young ash plantations. In low infestation levels, bottom-up and top-down regulation were relatively balanced, but in highly infested plantations, spiders were bottom-up limited and there was a top-down effect of herbivores on ash leaves. As a result, the invasive fungus affected ecosystem functioning through a reduction of functional complementarity and intensification of negative intraguild interactions among predators.