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Abstract:
Human-caused declines in biodiversity have stimulated intensive research on the consequences
of biodiversity loss for ecosystem services and policy initiatives to preserve the functioning of
ecosystems. Short-term biodiversity experiments have documented positive effects of plant species richness
on many ecosystem functions, and longer-term studies indicate, for some ecosystem functions, that biodiversity
effects can become stronger over time. Theoretically, a biodiversity effect can strengthen over time
by an increasing performance of high-diversity communities, by a decreasing performance of low-diversity
communities, or a combination of both processes. Which of these two mechanisms prevail, and whether
the increase in the biodiversity effect over time is a general property of many functions remains currently
unclear. These questions are an important knowledge gap as a continuing decline in the performance of
low-diversity communities would indicate an ecosystem-service debt resulting from delayed effects of species
loss on ecosystem functioning. Conversely, an increased performance of high-diversity communities
over time would indicate that the benefits of biodiversity are generally underestimated in short-term studies.
Analyzing 50 ecosystem variables over 11 years in the world’s largest grassland biodiversity experiment,
we show that overall plant diversity effects strengthened over time. Strengthening biodiversity
effects were independent of the considered compartment (above- or belowground), organizational level
(ecosystem variables associated with the abiotic habitat, primary producers, or higher trophic levels such
as herbivores and pollinators), and variable type (measurements of pools or rates). We found evidence that
biodiversity effects strengthened because of both a progressive decrease in functioning in species-poor and
a progressive increase in functioning in species-rich communities. Our findings provide evidence that negative
feedback effects at low biodiversity are as important for biodiversity effects as complementarity
among species at high biodiversity. Finally, our results indicate that a current loss of species will result in a
future impairment of ecosystem functioning, potentially decades beyond the moment of species extinction.
