Stronger diversity effects with increased environmental stress: A study of multitrophic interactions between oak, powdery mildew and ladybirds

Abstract

Recent research has suggested that increasing neighbourhood tree species diversity may mitigate the impact of pests or pathogens by supporting the activities of their natural enemies and/or reducing the density of available hosts. In this study, we attempted to assess these mechanisms in a multitrophic study system of young oak (Quercus), oak powdery mildew (PM, caused by Erysiphe spp.) and a mycophagous ladybird (Psyllobora vigintiduopunctata). We assessed ladybird mycophagy on oak PM in function of different neighbourhood tree species compositions. We also evaluated whether these species interactions were modulated by environmental conditions as suggested by the Stress Gradient Hypothesis. We adopted a complementary approach of a field experiment where we monitored oak saplings subjected to a reduced rainfall gradient in a young planted forest consisting of different tree species mixtures, as well as a lab experiment where we independently evaluated the effect of different watering treatments on PM infections and ladybird mycophagy. In the field experiment, we found effects of neighbourhood tree species richness on ladybird mycophagy becoming more positive as the target trees received less water. This effect was only found as weather conditions grew drier. In the lab experiment, we found a preference of ladybirds to graze on infected leaves from trees that received less water. We discuss potential mechanisms that might explain this preference, such as emissions of volatile leaf chemicals. Our results are in line with the expectations of the Natural Enemies Hypothesis and support the hypothesis that biodiversity effects become stronger with increased environmental stress.

Neighborhood diversity simultaneously increased and decreased susceptibility to contrasting herbivores in an early stage forest diversity experiment

“…Ultimately, further assessments of the degree to which biodiversity protects plants from specialist herbivores and pathogens can consolidate our understanding of the generality and mechanistic basis of these relationships (e.g. Damien et al, ; Dillen et al, ; Muiruri, Milligan, Morath, & Koricheva, ).…”

Enhancing Tree Performance Through Species Mixing: Review of a Quarter-Century of TreeDivNet Experiments Reveals Research Gaps and Practical Insights

“…Several studies used model caterpillars made from plasticine to measure predation rates of arthropods and/or birds [e.g. [94][95][96], but also counts of spider webs [97], and assessment of mycophagy [98] were used to assess predation. In addition, survival of specific leaf herbivores was classified here [99,100].…”

Linking Anthropogenic Landscape Perturbation to Herbivory and Pathogen Leaf Damage in Tropical Tree Communities

“…Species richness and phylogenetic distance between species affected the incidence of fungus spots, which was greater in sites with lower tree species richness and phylogenetic diversity. With a greater phylogenetic distance between tree communities, fewer common pathogens are shared between them [86], leading to a dilution of susceptible hosts (dilution effect) [87], as has been observed in other pathogens such as the oak powdery mildew species [88,89] and the fungal root pathogen (Heterobasidion annosum) [90]. Likewise, local lesions (i.e., a localised hypersensitive response where host plants present damages containing viral infection) [91] were susceptible to variations in vegetation composition and species richness but showed higher incidence in more diverse tree communities.…”