Abstract
AbstractHuman‐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.
Highlights
Biodiversity is decreasing at an unprecedented rate (Butchart et al 2010) but is essential for sustaining ecosystem services (Cardinale et al 2012, Naeem et al 2012) and human livelihoods (Millennium Ecosystem Assessment 2005)
This means that the difference between the achieved levels of functioning at low and high biodiversity increased as the experiment progressed (Zr-value for correlation between slope of biodiversity effect and year over all variables measured: mean 0.35 À0.29 + 0.24)
To test whether increasing biodiversity effects over time were caused by decreasing levels of functioning at low biodiversity, or by improving performance at high biodiversity, we investigated whether performance at high and/or low biodiversity changed over time
Summary
Biodiversity is decreasing at an unprecedented rate (Butchart et al 2010) but is essential for sustaining ecosystem services (Cardinale et al 2012, Naeem et al 2012) and human livelihoods (Millennium Ecosystem Assessment 2005). Examples include aboveground plant biomass (Cardinale et al 2007, Marquard et al 2009, Reich et al 2012), N pools (Oelmann et al 2011a), belowground plant biomass (Ravenek et al 2014), soil organisms (Eisenhauer et al 2012), and resistance to biological invasions (Roscher et al 2009a) Such strengthening relationships between biodiversity and ecosystem functioning have been explained by increased complementarity in high-diversity communities (Cardinale et al 2007, Fargione et al 2007, Reich et al 2012), which provides a mechanistic explanation for positive effects of biodiversity on ecosystem functioning in general. Complementarity includes facilitative effects; for example, legumes increase nutrient availability for neighboring plants via fixing atmospheric nitrogen (Fargione et al 2007)
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