1. Although experiments overwhelmingly show biodiversity increases ecosystem functioning, relationships in natural communities are more variable. This raises the question of under which conditions negative diversity effects might occur.
2. We argue that plant communities containing more core species that stably coexist should have higher biomass production. However, variation in numbers of transient species, whose abundances fluctuate strongly and which cannot stably coexist, may not consistently affect biomass.
3. Distinguishing changes in core and transient species richness is critical. For instance, recent attempts to use novel causal modelling approaches have implied negative effects of biodiversity on community biomass. However, we find these approaches also result in negative relationships when applied to experiments, where we know there is a positive causal effect of biodiversity. We suggest that transient species contribute disproportionately to the variation in biodiversity isolated in these models.
4. We highlight the need for improved approaches to analysing data from 'real-world' communities and call for increased attempts to compare results with experimental systems.
5. Synthesis. Understanding the functional consequences of biodiversity loss is critical but we need to be clear about what type of biodiversity change we are measuring and to focus on the loss of stably coexisting core species.