Abstract
High biodiversity is known to increase many ecosystem functions, but studies investigating biodiversity effects have more rarely looked at multi‐trophic interactions. We studied a tri‐trophic system composed of Centaurea jacea (brown knapweed), its flower head‐infesting tephritid fruit flies and their hymenopteran parasitoids, in a grassland biodiversity experiment. We aimed to disentangle the importance of direct effects of plant diversity (through changes in apparency and resource availability) from indirect effects (mediated by host plant quality and performance). To do this, we compared insect communities in C. jacea transplants, whose growth was influenced by the surrounding plant communities (and where direct and indirect effects can occur), with potted C. jacea plants, which do not compete with the surrounding plant community (and where only direct effects are possible). Tephritid infestation rate and insect load, mainly of the dominant species Chaetorellia jaceae, decreased with increasing plant species and functional group richness. These effects were not seen in the potted plants and are therefore likely to be mediated by changes in host plant performance and quality. Parasitism rates, mainly of the abundant chalcid wasps Eurytoma compressa and Pteromalus albipennis, increased with plant species or functional group richness in both transplants and potted plants, suggesting that direct effects of plant diversity are most important. The differential effects in transplants and potted plants emphasize the importance of plant‐mediated direct and indirect effects for trophic interactions at the community level. The findings also show how plant–plant interactions critically affect results obtained using transplants. More generally, our results indicate that plant biodiversity affects the abundance of higher trophic levels through a variety of different mechanisms.
Highlights
Several studies have shown that plant diversity affects the diversity and abundance of other trophic levels (e.g., Scherber et al, 2010)
Transplants were regularly mown, along with the rest of the plant community. We found that their survival strongly decreased with increasing plant species richness, to the extent that none of the transplants survived in 60-s pecies plots and only about 30% survived in the 16-species plots
Tephritid infestation rate increased with increasing resource density in both years (Table 1, Figure S2). These results suggest that plant species and functional group richness effects are stronger when both direct and indirect effects are operating
Summary
Several studies have shown that plant diversity affects the diversity and abundance of other trophic levels (e.g., Scherber et al, 2010). Effects on the third trophic level are likely to be weaker as they are even more indirect (Kagata, Nakamura, & Ohgushi, 2005; Scherber et al, 2010) To separate these direct and indirect effects, we used transplanted and potted host plants (the common knapweed, Centaurea jacea) placed into experimental plant communities differing in species richness. | 3 there may have been limited interaction (short period of potential light competition) with other plants in the community In this case, plant diversity can have only direct effects on higher trophic levels because indirect effects mediated by changes in plant performance or quality are excluded. Parasitoids are likely to respond strongly to structural complexity (strong effects of LAI would be hypothesized in this case) and to resource density (quantity of tephritid hosts) We test for these effects using a large grassland diversity experiment, the Jena Experiment (Roscher et al, 2004). We analyzed data from experimental plant communities containing 1–8 species and 1–4 plant functional groups (i.e., grasses, legumes, small herbs, tall herbs)
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