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 four 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 biogeographical-ecological 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.

The methods for the seaweed surveys are detailed in the scientific paper.