Bottom-Up Effects and Feedbacks in Simple and Diverse Experimental Grassland Communities

Summary

Finding a consistent pattern in the effects of plant diversity on higher trophic levels is a major challenge as populations at all trophic levels of an ecosystem may be regulated by a mixture of top-down and bottom-up forces. A starting point to experimentally approach the problem is to measure the effects of changing plant-species diversity on primary productivity, the basis of each food web, and to explore the potential underlying mechanisms. This was done within the European BIODEPTH project (Biodiversity and Ecological Processes in Terrestrial Herbaceous ecosystems). In this project, a common methodology was used at each of eight sites across Europe to experimentally assemble grassland communities of defined plant-species numbers (e.g. 1, 2, 4, 8, 32) from the local species pools. Ecosystem processes were then monitored in these different herbaceous ecosystems. Here, we report findings gathered from the sites in the UK, Switzerland, Germany and Sweden. Our data suggest that trophic levels cannot be treated as homogeneous units since the response to changes in plant diversity of individual groups within trophic levels was correlated with group-specific attributes such as host specificity, mobility or different size classes of consumers. There was also no evidence for a resource concentration effect, i.e. for a disproportionately higher specialist insect density in plots with high host density and biomass such as monocultures. Part of the diversity effects observed at higher trophic levels was indirectly driven by changes in primary productivity with changing plant diversity. However, experimental additions of a generalist insect herbivore and a plant hemiparasitic species showed that some polyphagous groups within higher trophic levels can benefit from increased diversity not only by the higher quantity, but also by the higher variety of resources.