Impacts of plant diversity on biomass production increase through time because of species complementarity

TLDR

Accelerating species extinction rates have led researchers to manipulate plant richness to study its impact on ecosystem functioning, and previous interpretations of a positive net diversity effect as selection effects have been questioned. We summarize 44 experiments manipulating plant richness to assess how plant diversity influences biomass production. The meta‑analysis shows that plant mixtures yield on average 1.7 times more biomass than monocultures, are more productive than the average monoculture in 79 % of experiments, yet only surpass the most productive species in 12 % of cases; species complementarity, which grows over time, largely drives this effect and suggests that past studies may have underestimated the productivity impact of species extinction.

Abstract

Accelerating rates of species extinction have prompted a growing number of researchers to manipulate the richness of various groups of organisms and examine how this aspect of diversity impacts ecological processes that control the functioning of ecosystems. We summarize the results of 44 experiments that have manipulated the richness of plants to examine how plant diversity affects the production of biomass. We show that mixtures of species produce an average of 1.7 times more biomass than species monocultures and are more productive than the average monoculture in 79% of all experiments. However, in only 12% of all experiments do diverse polycultures achieve greater biomass than their single most productive species. Previously, a positive net effect of diversity that is no greater than the most productive species has been interpreted as evidence for selection effects, which occur when diversity maximizes the chance that highly productive species will be included in and ultimately dominate the biomass of polycultures. Contrary to this, we show that although productive species do indeed contribute to diversity effects, these contributions are equaled or exceeded by species complementarity, where biomass is augmented by biological processes that involve multiple species. Importantly, both the net effect of diversity and the probability of polycultures being more productive than their most productive species increases through time, because the magnitude of complementarity increases as experiments are run longer. Our results suggest that experiments to date have, if anything, underestimated the impacts of species extinction on the productivity of ecosystems.