Abstract
Large-scale disturbance events provide ideal opportunities to directly study recolonisation processes in natural environments, via the removal of competitors and the formation of newly vacant habitat. A high magnitude earthquake in central New Zealand in 2016 created major ecological disturbance, with coastal tectonic uplift of up to similar to 6 m extirpating vast swathes of intertidal organisms. One of the affected species was Durvillaea antarctica (rimurapa or southern bull kelp), which is an important habitat-forming intertidal macroalga capable of long-distance dispersal. Across the complex fault system with varying amounts of uplift, the species was either locally extirpated or heavily reduced in abundance. We hypothesised that neutral priority effects and chance dispersal from other populations would influence which lineages would establish. We sampled individuals of D. antarctica across the uplift zone immediately after the earthquake in 2016 and then repeatedly sampled new recruits in the same areas between 2017 and 2020, using genotyping-by-sequencing to provide 'before' and 'after' genomic comparisons. Our results revealed strong geographic clustering but little evidence of new lineages establishing at disturbed sites, although populations at uplifted sites remain at remarkably low densities. We infer that recolonisation has thus far primarily originated from refugial, remnant patches within the uplift zone. To complement the phylogeographic analysis, we estimated oceanographic connectivity among the uplift zone sample locations. The connectivity modelling estimated that northbound dispersal of D. antarctica was more likely, but we have not yet detected southern genotypes in the recolonised populations. As the ongoing recolonisation process transitions from an ecological to an evolutionary timescale, change remains possible. This study provides the first genomic 'snapshots' of a natural recolonisation process following a large-scale ecological disturbance event, and ongoing research has the potential to reveal important insight into both micro- and macroevolutionary processes.