Abstract
Summary The diversity of the surrounding plant community can directly affect the abundance of insects on a focal plant as well as the size and quality of that focal plant. However, to what extent the effects of plant diversity on the arthropod community on a focal plant are mediated by host plant quality or by the diversity of the surrounding plants remains unresolved. In the field, we sampled arthropod communities on focal Jacobaea vulgaris plants growing in experimental plant communities that were maintained at different levels of diversity (one, two, four or nine species) for 3 years. Focal plants were also planted in plots without surrounding vegetation. We recorded the structural characteristics of each of the surrounding plant communities as well as the growth, and primary and secondary chemistry (pyrrolizidine alkaloids, PAs) of the focal plants to disentangle the potential mechanisms causing the diversity effects. Two years after planting, the abundance of arthropods on focal plants that were still in the vegetative stage decreased with increasing plant diversity, while the abundance of arthropods on reproductive focal plants was not significantly affected by the diversity of the neighbouring community. The size of both vegetative and reproductive focal plants was not significantly affected by the diversity of the neighbouring community, but the levels of PAs and the foliar N concentration of vegetative focal plants decreased with increasing plant diversity. Structural equation modelling revealed that the effects of plant diversity on the arthropod communities on focal plants were not mediated by changes in plant quality. Synthesis. Plant quality can greatly influence insect preference and performance. However, under natural conditions, the effects of the neighbouring plant community can overrule the plant quality effects of individual plants growing in those communities on the abundance of insects associated to this plant.
Highlights
In plant communities, the presence and identity of neighbouring plants can greatly influence host plant location and host selection of insect herbivores
We sampled arthropod communities on focal Jacobaea vulgaris plants growing in experimental plant communities that were maintained at different levels of diversity for 3 years
To examine the effect of plant diversity on the arthropod abundance, focal plant growth and chemistry, we used mixed-effects models with plant diversity (0–9 species and 1–9 species) as continuous fixed factor to incorporate the continuity of plant diversity in the analysis
Summary
The presence and identity of neighbouring plants can greatly influence host plant location and host selection of insect herbivores These effects are called associational effects (reviewed in Barbosa et al 2009). Neighbouring plants can influence characteristics of a focal plant, such as plant size and quality (primary and secondary chemistry; Barton & Bowers 2006; Temperton et al 2007; Broz et al 2010). These effects may result from competition between the focal plant and its neighbours that alters the availability of nutrients, light and space (Crawley 1997). Whether the effects of neighbouring plants on the insect communities associated to a focal plant are mediated by the changes in focal plant quality or by the neighbouring community itself remains unresolved
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