At eight European field sites, the impact of loss of plant diversity on primary productivity was simulated by synthesizing grassland communities with different numbers of plant species. Results differed in detail at each location, but there was an overall log-linear reduction of average aboveground biomass with loss of species. For a given number of species, communities with fewer functional groups were less productive. These diversity effects occurred along with differences associated with species composition and geographic location. Niche complementarity and positive species interactions appear to play a role in generating diversity-productivity relationships within sites in addition to sampling from the species pool.

Abstract. Insurance effects of biodiversity can stabilize the functioning of multispecies ecosystems against environmental variability when differential species' responses lead to asynchronous population dynamics. When responses are not perfectly positively correlated, declines in some populations are compensated by increases in others, smoothing variability in ecosystem productivity. This variance reduction effect of biodiversity is analogous to the riskspreading benefits of diverse investment portfolios in financial markets.We use data from the BIODEPTH network of grassland biodiversity experiments to perform a general test for stabilizing effects of plant diversity on the temporal variability of individual species, functional groups, and aggregate communities. We tested three potential mechanisms: reduction of temporal variability through population asynchrony; enhancement of long-term average performance through positive selection effects; and increases in the temporal mean due to overyielding.Our results support a stabilizing effect of diversity on the temporal variability of grassland aboveground annual net primary production through two mechanisms. Two-species communities with greater population asynchrony were more stable in their average production over time due to compensatory fluctuations. Overyielding also stabilized productivity by increasing levels of average biomass production relative to temporal variability. However, there was no evidence for a performance-enhancing effect on the temporal mean through positive selection effects. In combination with previous work, our results suggest that stabilizing effects of diversity on community productivity through population asynchrony and overyielding appear to be general in grassland ecosystems.

Ecosystem effects of biodiversity manipulations in European grasslands AbstractWe present a multisite analysis of the relationship between plant diversity and ecosystem functioning within the European BIODEPTH network of plant-diversity manipulation experiments. We report results of the analysis of 11 variables addressing several aspects of key ecosystem processes like biomass production, resource use (space, light, and nitrogen), and decomposition, measured across three years in plots of varying plant species richness at eight different European grassland field sites. Differences among sites explained substantial and significant amounts of the variation of most of the ecosystem processes examined. However, against this background of geographic variation, all the aspects of plant diversity and composition we examined (i.e., both numbers and types of species and functional groups) produced significant, mostly positive impacts on ecosystem processes.Analyses using the additive partitioning method revealed that complementarity effects (greater net yields than predicted from monocultures due to resource partitioning, positive interactions, etc.) were stronger and more consistent than selection effects (the covariance between monoculture yield and change in yield in mixtures) caused by dominance of species with particular traits. In general, communities with a higher diversity of species and functional groups were more productive and utilized resources more completely by intercepting more light, taking up more nitrogen, and occupying more of the available space. Diversity had significant effects through both increased vegetation cover and greater nitrogen retention by plants when this resource was more abundant through N2 fixation by legumes. However, additional positive diversity effects remained even after controlling for differences in vegetation cover and for the presence of legumes in communities. Diversity effects were stronger on above-than belowground processes. In particular, clear diversity effects on decomposition were only observed at one of the eight sites.The ecosystem effects of plant diversity also varied between sites and years. In general, diversity effects were lowest in the first year and stronger later in the experiment, indicating that they were not transitional due to community establishment. These analyses of our complete ecosystem process data set largely reinforce our previous results, and those from comparable biodiversity experiments, and extend the generality of diversity-ecosystem functioning relationships to multiple sites, years, and processes. Abstract. We present a multisite analysis of the relationship between plant diversity and ecosystem functioning within the European BIODEPTH network of plant-diversity manipulation experiments. We report results of the analysis of 11 variables addressing several aspects of key ecosystem processes like biomass production, resource use (space, light, and nitrogen), and decomposition, measured across three years in plots of varying plant species ri...

Geographic variation can lead to the evolution of different local varieties, even in widespread forage plants. We investigated the performance of common forage plants in relation to their genetic diversity and local adaptation at a continental scale using reciprocal transplants at eight field sites across Europe over a 2‐year period. The overall performance of the three test species, Trifolium pratense, Dactylis glomerata, Plantago lanceolata, was generally highest for plants replanted at their home site and declined with increasing transplanting distance. The three species differed in the fitness components responsible for the increased overall performance and selection advantage at home sites. In addition to the effects of local adaptation, the majority of measured traits in all three species also showed ecotypic variation. However, no single ecotype of any species was able to outperform the locally adapted strains and do best at all sites, highlighting the importance of maintaining these plant genetic resources.

To investigate how plant diversity loss affects nitrogen accumulation in above‐ground plant biomass and how consistent patterns are across sites of different climatic and soil conditions, we varied the number of plant species and functional groups (grasses, herbs and legumes) in experimental grassland communities across seven European experimental sites (Switzerland, Germany, Ireland, United Kingdom (Silwood Park), Portugal, Sweden and Greece). Nitrogen pools were significantly affected by both plant diversity and community composition. Two years after sowing, nitrogen pools in Germany and Switzerland strongly increased in the presence of legumes. Legume effects on nitrogen pools were less pronounced at the Swedish, Irish and Portuguese site. In Greece and UK there were no legume effects. Nitrogen concentration in total above‐ground biomass was quite invariable at 1.66±0.03% across all sites and diversity treatments. Thus, the presence of legumes had a positive effect on nitrogen pools by significantly increasing above‐ground biomass, i.e. by increases in vegetation quantity rather than quality. At the German site with the strongest legume effect on nitrogen pools and biomass, nitrogen that was fixed symbiotically by legumes was transferred to the other plant functional groups (grasses and herbs) but varied depending on the particular legume species fixing N and the non‐legume species taking it up. Nitrogen‐fixation by legumes therefore appeared to be one of the major functional traits of species that influenced nitrogen accumulation and biomass production, although effects varied among sites and legume species. This study demonstrates that the consequences of species loss on the nitrogen budget of plant communities may be more severe if legume species are lost. However, our data indicate that legume species differ in their N2 fixation. Therefore, loss of an efficient N2‐fixer (Trifolium in our study) may have a greater influence on the ecosystem function than loss of a less efficient species (Lotus in our study). Furthermore, there is indication that P availability in the soil facilitates the legume effect on biomass production and biomass nitrogen accumulation.

Aims Although biological invasions occur throughout the world, and some invaders are widespread in many habitats, few studies on the ecological impact of invaders have examined multiple sites. We tested how the impact of three widespread plant invaders changed depending on the identity of the species and the invaded island. We also tested whether relative species loss was lower in species-rich communities than in species-poor ones.Location We conducted floristic surveys and soil analyses in eight Mediterranean Basin islands: Crete and Lesbos (Greece), Sardinia (Italy), Corsica, Bagaud and Porquerolles (France), and Mallorca and Menorca (Spain).Methods We compared native species richness and diversity, proportion of life forms, soil percentage nitrogen, percentage organic carbon, C/N, and soil pH in nearby paired plots of 2 · 2 m: one control and one invaded by either the deciduous tree Ailanthus altissima, the succulent subshrubs Carpobrotus spp. or the annual geophyte Oxalis pes-caprae, across eight Mediterranean Basin islands.Results On average, the presence of invaders reduced species diversity, Carpobrotus spp. exhibiting the largest impact and Oxalis the least. However, the relative impact was island-dependent, and was positively but weakly associated with the species richness of the recipient community. Therophytes were the life form that experienced the largest decrease across islands. The effects of invasion on soil properties were very variable. Total N changed (increased) only in plots invaded by Ailanthus, significantly decreasing the C/N ratio. The presence of this tree increased soil pH, whereas the opposite was found in plots invaded by the other two species. Organic C increased in plots invaded by Ailanthus and Carpobrotus species.Main conclusions By conducting an analysis at multiple sites, we found that the three plant invaders had an impact on plant community structure not entirely concordant with changes in soil properties. The impacts depended on the identity of the species and of the invaded island, suggesting that impact of invaders is context-specific. The impact in terms of species loss was not lower in species-rich than in species-poor communities.

Understanding the relative importance of the abiotic environment and species interactions in determining the distribution and abundance of organisms has been a challenge in ecological research. Serpentine substrata are stressful environments for plant growth due to multiple limitations, collectively called the "serpentine syndrome". In the present review, our aim is not only to describe recent work in serpentine ecology, but also to highlight specific mechanisms of species tolerance and adaptation to serpentine soils and their effects on community structure and ecosystem functioning. We present hypotheses of the development of serpentine endemism and a description of functional traits of serpentine plants together with a synthesis of species interactions in serpentine soils and their effects on community structure and ecosystem productivity. In addition, we propose hypotheses about the effects of the 'serpentine syndrome' on ecosystem processes including productivity and decomposition.

Summary 1.We examined the species richness of theoretical communities in relation to interaction strength between species. 2. To do so, we used randomly constructed interaction matrices for competitive systems. To determine co-existence, we tested for local stability and equilibrium feasibility of these theoretical assemblages. 3. As expected, we found that a low mean species interaction strength could allow for many species to co-exist. However, variance in the interaction strengths may alter previous results; two systems with the same mean interaction strength show markedly different diversity depending critically on the magnitude of the variance. If species are similar enough then many can co-exist, even if they compete strongly. 4. In addition we found that the species richness of a competitive community can greatly depend on the correlation between interaction strengths, an issue that so far has gone unreported. This correlation, a result of trade-offs between species' characteristics, may profoundly increase the potential for stable co-existence of a highly species-rich community. 5. Competition may not be an anathema to diversity. Statistical properties of species' interactions may be critical factors that contribute to the explanation of species diversity in natural communities.