Low biodiversity stable state persists two decades after cessation of nutrient enrichment

Human activities are causing many gradual, but chronic, environmental changes worldwide, such as an increase in atmospheric nitrogen (N) deposition rates, which decreases biodiversity. It remains unclear whether such human influences on biodiversity are readily reversible. For example, cessation of nutrient enrichment may not lead to rapid recovery of biodiversity if the ecosystem has already been driven across a critical threshold, and has shifted to an alternative attractor. Here we show that chronic nutrient enrichment can shift grasslands from a diverse native-dominated regime to a depauperate exotic-dominated regime, and that it can be difficult to reverse these biodiversity declines. Using data from a 30-year experiment, we found that grassland plant diversity

• Was decreased well below control levels after one decade of high rates (95 to 270 kg N ha-1 yr-1), or three decades of medium rates (20 to 54 kg N ha-1 yr-1), of N addition
• Remained well below control levels two decades after cessation of high rates of N addition
• Exhibited a discontinuous, hysteretic response to increases and subsequent decreases in soil nitrate concentrations

The exceptionally long duration of our experiment provided a strict empirical test of the stability of these alternative regimes. Our results suggest that the unassisted recovery of biodiversity following cessation of nutrient enrichment in depauperate grasslands that have undergone this regime shift will require several decades at a minimum, and may never occur. These results are consistent with a growing body of evidence suggesting that anthropogenic influences on biophysical processes could shift natural and managed systems to alternative regimes that are undesirable and persistent.