Aim: Ecologists traditionally study how contemporary local processes, such as biological interactions and physical stressors, affect the distribution and abundance of organisms. By comparison, biogeographers study the distribution of the same organisms, but focus on historic, larger-scale processes that can cause mass mortalities, such as earthquakes. Here we document cascading effects of rare biogeographical (seismic) and more common ecological (temperature-related) processes on the distribution and abundances of coastal foundation species.Location: Intertidal wave-exposed rocky reefs around Kaikōura, New Zealand, dominated by large, long-lived, and iconic southern bull kelps (Durvillaea antarctica and
Durvillaea willana).
Methods:In November 2016, a 7.8 Mw earthquake uplifted the coastline around Kaikōura by up to 2 m, and a year later the region experienced the hottest summer on record. Extensive sampling of intertidal communities over 15 km coastline were done shortly after the earthquake and heatwaves and 4 years after the earthquake.Results: Durvillaea lost 75% of its canopy to uplift and the heatwaves reduced canopies that had survived the uplift by an additional 35%. The survey done 4 years after the earthquake showed that Durvillaea had not recovered and that the intertidal zone in many places now was dominated by small turfs and foliose seaweed.
Main conclusions:Cascading impacts from seismic uplift and heatwaves have destroyed populations of Durvillaea around Kaikōura. Surviving smaller and sparser Durvillaea patches will likely compromise capacity for self-replacement and lower resilience to future stressors. These results are discussed in a global biogeographicalecological context of seismic activity and extreme heatwaves and highlight that these events, which are not particularly rare in a geological context, may have common long-lasting ecological legacies.
Marine heatwaves (MHWs) can cause dramatic changes to ecologically, culturally, and economically important coastal ecosystems. To date, MHW studies have focused on geographically isolated regions or broad-scale global oceanic analyses, without considering coastal biogeographical regions and seasons. However, to understand impacts from MHWs on diverse coastal communities, a combined biogeographical-seasonal approach is necessary, because (1) bioregions reflect community-wide temperature tolerances and (2) summer or winter heatwaves likely affect communities differently. We therefore carried out season-specific Theil–Sen robust linear regressions and Pettitt change point analyses from 1982 to 2021 on the number of events, number of MHW days, mean intensity, maximum intensity, and cumulative intensity of MHWs, for each of the world’s 12 major coastal biogeographical realms. We found that 70% of 240 trend analyses increased significantly, 5% decreased and 25% were unaffected. There were clear differences between trends in metrics within biogeographical regions, and among seasons. For the significant increases, most change points occurred between 1998 and 2006. Regression slopes were generally positive across MHW metrics, seasons, and biogeographical realms as well as being highest after change point detection. Trends were highest for the Arctic, Northern Pacific, and Northern Atlantic realms in summer, and lowest for the Southern Ocean and several equatorial realms in other seasons. Our analysis highlights that future case studies should incorporate break point changes and seasonality in MHW analysis, to increase our understanding of how future, more frequent, and stronger MHWs will affect coastal ecosystems.
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