Bottom-up effects of plant diversity on multitrophic interactions in a biodiversity experiment

Biodiversity is rapidly declining1, and this may negatively affect ecosystem processes, including economically important ecosystem services. Previous studies have shown that biodiversity has positive effects on organisms and processes4 across trophic levels. However, only a few studies have so far incorporated an explicit food-web perspective. In an eight-year biodiversity experiment, we studied an unprecedented range of above- and below-ground organisms and multitrophic interactions. A multitrophic data set originating from a single long-term experiment allows mechanistic insights that would not be gained from meta-analysis of different experiments. Here we show that plant diversity effects dampen with increasing trophic level and degree of omnivory. This was true both for abundance and species richness of organisms. Furthermore, we present comprehensive above-ground/below-ground biodiversity food webs. Both above ground and below ground, herbivores responded more strongly to changes in plant diversity than did carnivores or omnivores. Density and richness of carnivorous taxa was independent of vegetation structure. Below-ground responses to plant diversity were consistently weaker than above-ground responses. Responses to increasing plant diversity were generally positive, but were negative for biological invasion, pathogen infestation and hyperparasitism. Our results suggest that plant diversity has strong bottom-up effects on multitrophic interaction networks, with particularly strong effects on lower trophic levels. Effects on higher trophic levels are indirectly mediated through bottom-up trophic cascades

Time course of plant diversity effects on Centaurea jacea establishment and the role of competition and herbivory

Aims Invasion resistance in experimental plant communities is known to increase with increasing diversity and further to depend on the presence of particular functional groups. To test whether these effects also hold true for the invader establishment phase beyond the seedling stage, we studied survival and performance of Centaurea jacea L. (brown knapweecl) planted into experimental grassland communities of varying plant biodiversity over three consecutive years. Moreover, we analysed the role of insect herbivory and biomass of the recipient community for mediating diversity effects. Methods In 2005, seedlings of Centaurea were transplanted into experimental grassland communities (the Jena Experiment) covering a species richness (1-60) and functional group richness (1-4) gradient. Half of these transplants and the community surrounding them in each plot were sprayed with insecticide while the other half served as control. In 2006 and 2007 (during the second and third year after transplantation), we recorded survival, growth-related (e.g. transplant biomass, height) and reproduction-related traits (e.g. number of flower heads). Annual data on community aboveground biomass served as covariate to investigate mediating effects of aboveground competition with the recipient community. Important Findings Species richness was the most important factor responsible for Centaurea limitation. Higher levels of diversity decreased survival and all performance traits in both years. These diversity effects were partly driven by community biomass, but not fully explained by that covariate, suggesting the importance also of further processes. The influence of functional group richness was strong in the second year after transplantation and weaker in the third year. Among the particular functional groups, only the presence of legumes showed strong negative effects on Centaurea survival and weak negative effects on growth and reproduction, the latter two being mediated by biomass. Insect herbivore reduction considerably benefited Centaurea in sprayed monocultures, where it grew significantly larger than in all other diversity levels and than in the control subplots. We conclude that effects of plant community properties on invading individuals change in the course of establishment, that plant species richness effects are also important during later stages of establishment, and that biomass (especially at high diversity) and herbivory (especially at low diversity) of the recipient community are important in mediating community effects on invaders

Effects of plant biodiversity on above-belowground interactions

<p>This graph comes from a manuscript published in <strong>Nature</strong> (http://dx.doi.org/10.1038/nature09492) and shows how<strong> plant species richness</strong> affects <strong>organisms living in the soil and in the vegetation layer</strong>. It was originally published as part of the Supplementary Material and is reproduced here to show it to a wider audience.</p

How biodiversity affects ecosystem processes

<p>This graph shows the effects of <strong>plant species richness</strong> on <strong>ecosystem processes</strong>, e.g.:</p> <p>-effects of plant biodiversity on <strong>decomposition</strong></p> <p>-effects of plant biodiversity on <strong>pathogen infection</strong></p> <p>-effects of plant biodiversity on <strong>herbivory</strong></p> <p>-effects of plant biodiversity on <strong>flower visitation</strong> (pollination)</p> <p>-effects of plant biodiversity on <strong>bioturbation</strong></p> <p>-effects of plant biodiversity on<strong> ant activity</strong></p> <p>-effects of plant biodiversity on <strong>parasitism</strong> and <strong>hyperparasitism</strong>,</p> <p>-effects of plant biodiversity on <strong>biological invasion</strong> and</p> <p>-effects of plant biodiversity on <strong>soil microbial respiration</strong>.</p> <p>The graph comes from the supplementary material of a paper published by Scherber et al. in Nature (http://dx.doi.org/10.1038/nature09492).</p> <p>Biodiversity effects are mostly non-linear, and species richness effects are negative in some cases. For more details, see the Supplementary Material at http://www.nature.com/nature/journal/v468/n7323/extref/nature09492-s1.pdf</p