Legacy effects of extreme events

Abstract

Global change is increasing the intensity of environmental extremes. Abiotic extremes include hotter summers and more intense heatwaves1-2. Biotic extremes include disproportionate increases in herbivory3. Both have well-understood immediate impacts on the plant communities that support life across the globe. However, their longterm effects are difficult to predict, as it is unknown how heat and herbivory differ in the way they create historical legacies. These are changes in a community that still affect dynamics and recovery long after a disturbance has passed. Here we show differences in legacy construction by mild and extreme heatwaves and small- and large-bodied herbivores. We analysed patterns in the population dynamics of a consumer added to replicated primary producer communities after disturbance had ended. We find that the legacy induced by extreme heat drives post-disturbance consumer declines in all replicates. This results in stochastic extinctions as the loss of populations becomes more likely when their density gets close to zero. In contrast, the legacy created by the large-bodied herbivore does not drive extinctions. It diminishes population growth by orders of magnitude across six generations. The observed effects appear after a time-lag that is shorter for extreme heat than for herbivory. Our results show that legacy effects drive complex causal sequences that may involve deterministic and chance components, time-lags, stressor amplification and regime shifts. Insight in how these combine to shape the future is essential for our capacity to accurately forecast and repair the consequences of global change.