Resolving patterns of population genetic and phylogeographic structure to inform control and eradication initiatives for brown rats Rattus norvegicus on South Georgia

Abstract

Summary The control and eradication of invasive species is a common management strategy to protect or restore native biodiversity. On South Georgia in the Southern Ocean, the brown rat Rattus norvegicus was brought onto the island with the onset of whaling and sealing activity in the 1800s and has had a significant detrimental impact on key bird species of conservation concern. Efforts to eradicate rats from South Georgia using poisoned bait are ongoing. Despite the South Georgia rat eradication programme being the geographically largest and most ambitious eradication initiative to date, its success is facilitated by the potential that rat populations are effectively isolated by glacial barriers. This allows for localized eradication effort at manageable scales, leading to sequential eradication of individual populations with minimal risk of incursion from neighbouring areas. Here, we use the levels of population genetic divergence estimated from 299 single nucleotide polymorphism (SNP) loci and DNA sequence variation across 993 base pairs of the mitochondrial DNA cytochrome B locus to examine whether rat populations from nine glacially isolated areas on South Georgia are genetically distinct and so can be treated as independent eradication units. Bayesian clustering of individuals based on SNP similarity identified seven different genetic groups, which were confirmed using analyses based on pairwise genetic distance estimates and ordination of individuals using principal coordinate analysis. From a management perspective, these seven groups represent individual targets in baiting operations. Two mtDNA haplotypes were resolved across South Georgia, with a distinct geographical separation between the north‐western and south‐eastern populations. Approximate Bayesian computation (ABC) was used to identify that this divergence was a consequence of two separate historical colonization events. Synthesis and applications. We illustrate that molecular markers are a valuable tool in species management and pest eradication given that the spatial distribution of genetic diversity can: (i) identify demographically and genetically independent populations on which local eradication effort can be focussed, (ii) distinguish between incomplete eradication and immigration in situations where individuals remain after eradication has been attempted and (iii) identify the source of migrants when dispersal occurs over large spatial scales.

Global population divergence and admixture of the brown rat (Rattus norvegicus)

“…Eradication of invasive Rattus species on islands and in ecosystems with high biodiversity is a priority for conservation of at-risk species, as rats outcompete or kill native fauna. It remains challenging to gauge the success of eradication programs, because it is difficult to distinguish between post-intervention survival and reproduction as opposed to recolonization by new immigrants (Piertney, et al 2016). Understanding fine-scale population genetic structure using dense nuclear marker sets (Robins, et al 2016), as in this study, would allow managers to more clearly assess outcomes and next steps following an eradication campaign.…”

Phylogeography of Rattus norvegicus in the South Atlantic Ocean

“…Samples of Norway rats from the South Atlantic Ocean were collected at the Falkland Islands (n = 49; 18 locations in five areas), South Georgia (n = 8; seven locations across two regions, previously identified as being genetically distinct [8]), Fernando de Noronha (n = 1) and the towns of Recife and Fortaleza on the coast of Brazil (n = 2) ( Figure 1a). Additional samples (n = 4; four locations from two regions) were obtained from mainland Portugal, the origin of the first maritime explorers to chart the South Atlantic Ocean and from Madeira island (n = 4; 3 locations; Figure 1b), the first oceanic island colonized by the Portuguese and an important stop-over in the Atlantic during the European maritime expansion.…”

Genomics detects population structure within and between ocean basins in a circumpolar seabird: The white‐chinned petrel

“…Despite more comprehensive sampling, our cyt b results were very similar to those for cyt b in Techow et al (2009), suggesting that the newly-detected multi-region structure was based on the temporal resolution of markers and not increased sampling. A growing number of studies on Southern Ocean taxa show previously suspected fine-scale population structure using GBS data in species ranging from Durvillaea kelp and brown rats Rattus norvegicus, to emperor and king penguins (Fraser et al, 2016;Fraser et al, 2018;Piertney et al, 2016;Younger et al, 2017Younger et al, , 2015; but see Clucas et al, 2016;Trucchi et al, 2014). In the case of white-chinned petrels, the finer-scale multi-region structure identified using GBS data is consistent with tracking and isotopic work which shows that birds from these regions differ in foraging habitat or at-sea distribution, particularly during the nonbreeding period (Catard et al, 2000;Jaeger et al, 2013;Phillips et al, 2006;Rexer-Huber, 2017;Rollinson et al, 2018).…”