Evidence suggests that biodiversity supports ecosystem functioning. Yet, the mechanisms driving this relationship remain unclear. Complementarity is one common explanation for these positive biodiversity-ecosystem functioning relationships. Yet, complementarity is often indirectly quantified as overperformance in mixture relative to monoculture (e.g., 'complementarity effect'). This overperformance is then attributed to the intuitive idea of complementarity or, more specifically, to species resource partitioning. Locally, however, several unassociated causes may drive this overperformance. Here, we differentiate complementarity into three types of species differences that may cause enhanced ecosystem functioning in more diverse ecosystems: (i) resource partitioning, (ii) abiotic facilitation, and (iii) biotic feedbacks. We argue that disentangling these three causes is crucial for predicting the response of ecosystems to future biodiversity loss.

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The natural world is increasingly defined by change. Within the next 100 years, rising atmospheric CO 2 concentrations will continue to increase the frequency and magnitude of extreme weather events. Simultaneously, human activities are reducing global biodiversity, with current extinction rates at B1,000 Â what they were before human domination of Earth's ecosystems. The co-occurrence of these trends may be of particular concern, as greater biological diversity could help ecosystems resist change during large perturbations. We use data from a 200-year flood event to show that when a disturbance is associated with an increase in resource availability, the opposite may occur. Flooding was associated with increases in productivity and decreases in stability, particularly in the highest diversity communities. Our results undermine the utility of the biodiversity-stability hypothesis during a large number of disturbances where resource availability increases. We propose a conceptual framework that can be widely applied during natural disturbances.

Abstract. Recent work has demonstrated that competition and facilitation likely operate jointly in plant communities, but teasing out the relative role of each has proven difficult. Here we address how competition and facilitation vary with seasonal fluctuations in environmental conditions, and how the effects of these fluctuations change with plant ontogeny. We planted three sizes of pine seedlings (Pinus strobus) into an herbaceous diversity experiment and measured pine growth every two weeks for two growing seasons. Both competition and facilitation occurred at different times of year between pines and their neighbors. Facilitation was important for the smallest pines when environmental conditions were severe. This effect decreased as pines got larger. Competition was stronger than facilitation overall and outweighed facilitative effects at annual time scales. Our data suggest that both competition and the counter-directional effects of facilitation may be more common and more intense than previously considered.