The loss of biodiversity is thought to have adverse effects on multiple ecosystem functions, including the decline of community stability. Decreased diversity reduces the strength of the portfolio effect, a mechanism stabilizing community temporal fluctuations. Community stability is also expected to decrease with greater variability in individual species populations and with synchrony of their fluctuations. In semi-natural meadows, eutrophication is one of the most important drivers of diversity decline; it is expected to increase species fluctuations and synchrony among them, all effects leading to lower community stability. With a 16-year time series of biomass data from a temperate species-rich meadow with fertilization and removal of the dominant species, we assessed population biomass temporal (co)variation under different management types and competition intensity, and in relation to species functional traits and to species diversity. Whereas the effect of dominant removal was relatively small (with a tendency toward lower stability), fertilization markedly decreased community stability (i.e., increased coefficient of variation in the total biomass) and species diversity. On average, the fluctuations of individual populations were mutually independent, with a slight tendency toward synchrony in unfertilized plots, and a tendency toward compensatory dynamics in fertilized plots and no effects of removal. The marked decrease of synchrony with fertilization, contrary to the majority of the results reported previously, follows the predictions of increased compensatory dynamics with increased asymmetric competition for light in a more productive environment. Synchrony increased also with species functional similarity stressing the importance of shared ecological strategies in driving similar species responses to weather fluctuations. As expected, the decrease of temporal stability of total biomass was mainly related to the decrease of species richness, with its effect remaining significant also after accounting for fertilization. The weakening of the portfolio effect with species richness decline is a crucial driver of community destabilization. However, the positive effect of species richness on temporal stability of total biomass was not due to increased compensatory dynamics, since synchrony increased with species richness. This shows that the negative effect of eutrophication on community stability does not operate through increasing synchrony, but through the reduction of diversity.

Under global change, how biological diversity and ecosystem services are maintained in time is a fundamental question. Ecologists have long argued about multiple mechanisms by which local biodiversity might control the temporal stability of ecosystem properties. Accumulating theories and empirical evidence suggest that, together with different population and community parameters, these mechanisms largely operate through differences in functional traits among organisms. We review potential trait-stability mechanisms together with underlying tests and associated metrics. We identify various trait-based components, each accounting for different stability mechanisms, that contribute to buffering, or propagating, the effect of environmental fluctuations on ecosystem functioning. This comprehensive picture, obtained by combining different puzzle pieces of trait-stability effects, will guide future empirical and modeling investigations. Biotic mechanisms of stability: a jigsaw puzzleAs biodiversity is declining at an unprecedented rate, a particularly urgent scientific challenge is to understand and predict the consequences of biodiversity loss on multiple ecosystem functions [1][2][3]. Temporal stability of the functioning of ecosystems is critical to both intrinsic and human purposes (Box 1, Figure 1). Temporal stability can be defined as the ability of a system to maintain, through time, multiple ecosystem properties (see Glossary) in relation to reference conditions. Key elements of stability (Box 1 and Figure 1) are, for example, inter-annual constancy in ecosystem properties, but also resistance and recovery from environmental change and perturbation. Stability is maintained by populations, communities, and ecosystems that can buffer the effects of environmental variation, thus retaining ecosystem functions such as productivity, carbon sequestration, pollination, etc. The idea that greater biodiversity stabilizes natural communities and ecosystems (i.e., diversity begets stability [4,5]) has led to a longrunning debate on the relationship between species diversity and stability [6,7].

Abstract. Understanding the processes regulating population temporal stability is important to infer species coexistence and ecosystem stability patterns. It has been hypothesized that population temporal stability could be driven by functional trade-offs in resource acquisition and growth rate strategies. We tested this hypothesis by analyzing a 13-year data set from a mown grassland community in a factorial experiment with fertilization and dominant removal as the main treatment effects. Population temporal stability, measured as a coefficient of variation of species' biomass over time, was related to plant traits covering different functional trade-offs. These included plant height, leaf dry matter content (LDMC), specific leaf area, seed mass, leaf d 13 C, and rooting depth. Three of the traits (LDMC, rooting depth, and leaf d 13 C) had significant relationships with population temporal stability, even after accounting for species' phylogenetic relatedness. Higher values of LDMC, the best predictor, were consistently associated with greater population temporal stability across all experimental conditions. This suggests a functional trade-off along the leaf economics spectrum, with more conservative, slow-growing species being more stable over time. Incorporating functional trade-offs into the assessment of population temporal dynamics will allow for a more comprehensive understanding of the processes that sustain the stability of ecosystems and species coexistence.

Linking diversity to biological processes is central for developing informed and effective conservation decisions. Unfortunately, observable patterns provide only a proportion of the information necessary for fully understanding the mechanisms and processes acting on a particular population or community. We suggest conservation managers use the often overlooked information relative to species absences and pay particular attention to dark diversity (i.e., a set of species that are absent from a site but that could disperse to and establish there, in other words, the absent portion of a habitat-specific species pool). Together with existing ecological metrics, concepts, and conservation tools, dark diversity can be used to complement and further develop conservation prioritization and management decisions through an understanding of biodiversity relativized by its potential (i.e., its species pool). Furthermore, through a detailed understanding of the population, community, and functional dark diversity, the restoration potential of degraded habitats can be more rigorously assessed and so to the likelihood of successful species invasions. We suggest the application of the dark diversity concept is currently an underappreciated source of information that is valuable for conservation applications ranging from macroscale conservation prioritization to more locally scaled restoration ecology and the management of invasive species.