Climate change is radically changing the frequency and intensity of severe weather events, such as heatwaves, droughts, floods and fires. As the time interval shrinks between recurrent shocks, the responses of ecosystems to disturbances are likely to be increasingly affected by the history of earlier extreme events. Ecological memory, defined as the ability of the past to influence the present trajectory of ecosystems, is critically important for understanding how ecosystems are responding to changes in disturbance regimes. The present study examined the emergence of ecological memory during unprecedented back-to-back bleaching of corals along the 2,300km length of the Great Barrier Reef in 2016 and 2017, revealing that the impacts and geographic patterns of the second severe heatwave were contingent on the first. The study underscore the imperative of understanding the interactions among sequences of climate-driven events, and highlight the strengthening and cumulative impacts of new disturbance regimes on vulnerable ecosystems.
Results/Conclusions
The level of heat exposure in 2016 and 2017 explained the distinctive geographic patterns of severe bleaching along the Great Barrier Reef in 2016 and 2017. However, the thermal threshold for bleaching shifted between years. Depending on their experience in 2016, corals assemblages were either substantially more, or less, vulnerable to heat stress and bleaching in 2017. Northern reefs bleached severely in 2016, but much less bleaching than expected recurred in 2017 because of the earlier loss of vulnerable species. In the central region, where bleaching was moderate in 2017, more bleaching occurred in 2017 than expected from the degree of heat exposure in the second year. In the south, reefs escaped bleaching in both years, despite 2017 being warmer. Consequently, the geographic pattern of heat exposure in 2016 had a profound impact on the spatial footprint of bleaching during the subsequent heatwave one year later. These results point to the imperative of seeking to understand the combined, interactive effects of sequences of recurrent climate-related disturbances at very large spatial scales, and the critical role of recent history in an era of rapid global change. It is imperative now more than ever to scrutinize sequences of multiple disturbance events to reveal the complex and enduring role of ecological memory, and its geographic extent.