Considering the current rate of extinctions, it is crucial to understand the consequences of these losses of biodiversity for the functioning of ecosystems. Grasslands proved a very suitable ecosystem for biodiversity-ecosystem functioning research.
In earlier experiments, nitrogen-fixing legumes dominated the effects of plant diversity, but in our experiment without legumes we also found that diverse plant communities have higher productivity than species-poor ones. These results could not be explained by sampling effects. Instead, complementarity in nutrient uptake and more efficient nutrient use at high diversity are the most important driving forces behind the positive effects of diversity. We also found that the negative effect of plant diversity on the invasibility of the community may be a sampling effect, as the increased presence of two particular plant species were the most important factors affecting invasion success.
Plant diversity may also affect many other organisms in grasslands. We found that nematodes responded positively to plant diversity. Specific associations between plants and nematodes resulted in increased nematode diversity at high plant diversity.
Of course, higher trophic level organisms do not only respond to plant diversity, they may also regulate plant diversity. We found that arbuscular mycorrhizal fungi (AMF) alter competition between plants and contribute to plant diversity by enhancing the establishment of plants.
Earlier studies also reported strong effects of herbivores, pathogens and mutualists on plant communities, but most of these studies investigated a single group of organisms in isolation from other driving forces. We show that the separate effects of these organisms on plant diversity may change dramatically when studied in combination. A neutral effect of an aboveground insect herbivore on plant diversity and a positive effect of a belowground insect herbivore turned strongly negative when both herbivores were present in the plant community.
It is argued that a combined approach, using interactions between changes in plant diversity and multiple 'drivers of plant diversity' (both biotic and abiotic) is needed to fully understand plant community dynamics. To be able to predict the consequences of biodiversity loss worldwide it is necessary to integrate the results of this approach over different ecosystems and ecosystem processes across large spatial and temporal scales.