The functional role of biodiversity in ecosystems: incorporating trophic complexity

Understanding how biodiversity affects functioning of ecosystems requires integrating diversity within trophic levels (horizontal diversity) and across trophic levels (vertical diversity, including food chain length and omnivory). We review theoretical and experimental progress toward this goal. Generally, experiments show that biomass and resource use increase similarly with horizontal diversity of either producers or consumers. Among prey, higher diversity often increases resistance to predation, due to increased probability of including inedible species and reduced efficiency of specialist predators confronted with diverse prey. Among predators, changing diversity can cascade to affect plant biomass, but the strength and sign of this effect depend on the degree of omnivory and prey behaviour. Horizontal and vertical diversity also interact: adding a trophic level can qualitatively change diversity effects at adjacent levels. Multitrophic interactions produce a richer variety of diversity‐functioning relationships than the monotonic changes predicted for single trophic levels. This complexity depends on the degree of consumer dietary generalism, trade‐offs between competitive ability and resistance to predation, intraguild predation and openness to migration. Although complementarity and selection effects occur in both animals and plants, few studies have conclusively documented the mechanisms mediating diversity effects. Understanding how biodiversity affects functioning of complex ecosystems will benefit from integrating theory and experiments with simulations and network‐based approaches

Non-random species extinction and plant production: implications for ecosystem functioning

1. Understanding ecosystem responses to plant species loss is essential for the optimal management of grasslands. Recent studies have examined the effects of simulated random species loss in experimental plant communities but not those of realistic non-random species loss resulting from transient extinction pressures in semi-natural grasslands. 2. To investigate the potential effects of non-random species loss on grassland productivity, we established mesocosms with mixed communities comprising 15 plant species, and exposed them to 2 years of high-intensity management (an extinction phase) followed by 2 years of low-intensity management (a restoration phase) allowing recolonization from differentially managed neighbouring plots. In addition, monocultures of each component species were subject to the same extinction–restoration phases. 3. During the extinction phase, species with high monoculture biomass had lower extinction probabilities in the mixed community than species with low monoculture biomass, but there was also species-specific variation. The species that were most productive or most persistent during the extinction phase were not the same as those performing best in the restoration phase. 4. No consistent effects of spontaneous recolonization from neighbouring communities on species richness or productivity of the focal communities were observed during the restoration phase. 5. We estimated that extinction of all but the species with the lowest extinction risk reduced biomass productivity by 42–49%; loss of all but the four species with the lowest extinction risk reduced it by 2–35%. Identical calculations for a random extinction scenario yielded reductions of 52% and 26–54%, respectively. 6. Synthesis and applications. Prediction of the effects of species loss on plant production and on other aspects of ecosystem functioning in semi-natural grasslands must account for both specific non-random extinction processes and post-extinction conditions. For European mesic grasslands experiencing a shift from high-intensity to low-intensity management, our results suggest that recolonization by 'missing' species must be actively assisted if high production is a management objective

Resource dilution effects on specialist insect herbivores in a grassland biodiversity experiment

1. The resource concentration hypothesis predicts that specialist insect herbivores attain higher loads (density per unit mass of the host-plant species) when their food plants grow in high-density patches in pure stands. 2. We tested the resource concentration hypothesis for nine specialist insect herbivore species sampled from a field experiment where plant diversity had been manipulated experimentally, generating gradients of host-plant abundance. 3. The specialist insects responded to varying host-plant abundance in two contrasting ways: as expected, specialist herbivore species were more likely to be present when their host-plant species were abundant; however, counter to predictions, in plots where specialists were present we found strong negative linear relationships between herbivore loads and host-plant abundances - a 'resource dilution' rather than concentration effect. 4. Increased plant species-richness had an additional, but weak, negative influence on loads beyond that due to host-plant abundance. 5. We discuss the implications of resource dilution effects for biodiversity manipulation experiments and for the study of plant–herbivore interactions more generally

Overyielding in experimental grassland communities – irrespective of species pool or spatial scale

In a large integrated biodiversity project ('The Jena Experiment' in Germany) we established two experiments, one with a pool of 60 plant species that ranged broadly from dominant to subordinate competitors on large 20 × 20 m and small 3.5 × 3.5 m plots (= main experiment), and one with a pool of nine potentially dominant species on small 3.5 × 3.5 m plots (= dominance experiment). We found identical positive species richness–aboveground productivity relationships in the main experiment at both scales. This result suggests that scaling up, at least over the short term, is appropriate in interpreting the implications of such experiments for larger-scale patterns. The species richness–productivity relationship was more pronounced in the experiment with dominant species (46.7 and 82.6% yield increase compared to mean monoculture, respectively). Additionally, transgressive overyielding occurred more frequently in the dominance experiment (67.7% of cases) than in the main experiment (23.4% of cases). Additive partitioning and relative yield total analyses showed that both complementarity and selection effects contributed to the positive net biodiversity effect