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

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.

Fig. 1.

Effects of plant species richness on the production of plant biomass. (A) Net effects of diversity on biomass, LR_net, expressed as the log ratio of biomass in the most diverse polyculture to the average of all species grown in monoculture. (B) A test for transgressive overyielding, LR_trans, which is the log ratio comparing the mean biomass of polycultures with the mean biomass of the single most productive species. Main plots give the mean ± 95% confidence interval for individual estimates of LR_net (n = 104) and LR_trans (n = 83) ranked from largest to smallest effect size for all dates in all experiments. (A and B Insets) Frequency distributions for LR_net and LR_trans (x axis) for 1,019 individual polyculture plots (counts on y axis) where biomass could be matched to the same species in monoculture. (C and D) Shown are how LR_net and LR_trans change through time. Each data point is a single estimate from one experiment, but note that some experiments contribute multiple data points [numbers correspond to experiments listed in supporting information (SI) Datasets 1 and 2].

Fig. 2.

Mechanisms underlying the effects of plant species richness on the production of biomass. (A) Magnitude of complementarity effects (CE) and selection effects (SE) for each of 47 estimates available from 24 experiments. The grand mean ± 95% C.I. values for CE and SE across all experiments is displayed as an open diamond, estimated from mixed model ANOVAs with experiment included as a random effect. (B and C) Shown are how complementarity and selection effects vary with the duration of an experiment. Each data point is a single estimate from one experiment, but note that some experiments contribute multiple data points (numbers correspond to experiments listed in SI Datasets 1 and 2) (see SI Text).