Restorative Rafting - A case study from an earthquake-uplifted intertidal community
Creators: name: Peters, Johnette
Publisher: name: The Australian National University
Year: 2018

description: Dispersal is generally considered a crucial ecological process, assisting in the movement of individuals and possible genetic connectivity of populations. Recent research into processes that shape genetic diversity has revealed that dispersal among populations, even when frequent, can fail to result in gene flow, leading to distinct geographic structure both within and among species. This structure is thought to be maintained by ‘density-blocking’ or ‘Founder Takes All’ processes, whereby early immigrants densely colonise an area and impede the establishment of later arrivals. The release of density-blocking should be achievable, however, through large-scale disturbance events, where original lineages are completely removed and new, immigrant lineages can gain a foothold. The aim of this research is to contribute to an understanding of the extent of normally-ineffective dispersal events, and thereby to help understand how disturbance events can release density-blocking and allow for genetic turnover. The 2016 earthquakes in Kaikoura, New Zealand, resulted in massive coastal uplift that partially destroyed intertidal bull-kelp and invertebrate populations at some sites, and completely extirpated populations at other sites. By analysing genomic data from beach-cast rafts of southern-bull kelp (Durvillea antarctica and D. poha), keystone intertidal taxa, across 2017-2018 in north Canterbury, New Zealand, I tested the pool of potential colonisers reaching the uplifted coast. Raft sources were determined via genome-wide SNP data generated by genotyping-by-sequencing (GBS), which reveal fine-scale phylogenetic structure of populations from across New Zealand and the sub-Antarctic, including pre-quake populations from the within the uplift zone. Genomic analyses revealed that a number of rafts from both species came from distant lineages, including the Auckland Islands and Fiordland, over 1200 km away. Despite this evidence of long-distance dispersal from southern New Zealand and sub-Antarctic populations, the majority of beach-cast rafts were closely related to local populations, north of Banks Peninsula. Of these local rafts, 60% were close matches to Durvillaea populations growing in the area prior to the earthquake. While evidence of frequent rafting from distant source populations may confirm that density-blocking processes played a role in regulating the genetic structure of the area pre-earthquake, the continued rafting of local populations suggests incomplete macroalgal mortality from the uplift. The existence of these relictual populations could mean that the pre-earthquake genetic structure will be maintained, as the area will be rapidly recolonised by offspring of surviving original populations. Broadly, this research also provided detailed information on the fine-scale structure within Durvillaea, and raised some taxonomic questions. This research has important implications for the genetic (intraspecific) consequences of disturbance. With increases in the frequency, severity and extent of disturbances with climate change, these predictions have important applications for conservation management and invasion biology.