Individual study: Establishing causes of failure to eradicate ship rats Rattus rattus based on genetics, Ste. Anne Archipelago, Martinique
Abdelkrim J., Pascal M. & Samadi S. (2007) Establishing causes of eradication failure based on genetics: Case study of ship rat eradication in Ste. Anne Archipelago. Conservation Biology, 21, 719-730
Determining the causes of a failed eradication of invasive species is important as it may enable adjustment of methods to improvement future eradication attempts. The authors of this study examined how molecular monitoring can help distinguish between eradication failures in terms of survival of some individuals vs. recolonization after eradication. This study investigated genetic variation in seven microsatellite loci in ship (black) rat Rattus rattus populations from four islets off the Martinique coast (French Caribbean) with a view to establishing best management practices in light of the findings to guide further eradication attempts.
Rat samples: During the first eradication attempts undertaken in 1999, 269 ship rats were trapped on the four Sainte Anne islets (Martinique) in the Caribbean. During eradication attempts in 2001, there were no rats on Percé Islet, but rats were trapped on Hardy and Burgeaux islets. There was no data for the fourth islet, Poirier, as 12,000 pairs of nesting sooty terns Sterna fuscata made it inaccessible due to inevitable disturbance that would otherwise occur to the breeding birds. In 2002 only one rat was captured on Burgeaux, and again none were found on Percé. On Hardy and Poirier islets 34 and 182 rats were captured respectively. Thus, before final eradication was complete 490 rats were captured on the islets between 1999 and 2002.
Tissue preservation problems prevented analyses of the four 2001 rats and the one rat trapped on Burgeaux islet in 2002. For each island and each year, all trapped rats were included in the analyses when their number was <30. When more than 30 were trapped, a subsample of about 30 individuals was randomly defined. Twenty ship rats from four coastal localities of the Martinique main island were also analyzed.
DNA extraction, amplification and genotyping: Rat tissues were preserved in 80% alcohol and stored at 4° C before extraction of genomic DNA. Many microsatellite markers have previously been developed for the related brown rat R.norvegicus for genome mapping. Therefore, seven markers previously characterized for this species were used on R.rattus.
In 1999 an eradication attempt was conducted on the four islets. Three years later rats were observed again on two of them. The genetic signatures of the populations were compared before and after the eradication attempts.
On one of the islands (Poirier Islet), the ‘new’ rat population was considered likely to stem from rats that had survived the eradication attempts. A weak genetic differentiation was found between them, with almost no new alleles observed in the new population and moderate FST values (0.15), and assignment procedures clustered the two populations together. In contrast, on the other islet (Hardy Islet), many new alleles were observed after the eradication attempt, resulting in an increase in genetic and strong FST values (0.39). Genetic clustering also clearly separated the two samples (i.e. before and after the eradication attempt) in two different populations. This strongly suggested reinvasion from a different source population (probably from the mainland).
Conclusions: The authors conclude that to achieve long-term eradication on these islets, it seems necessary to redevelop the eradication procedure to avoid individuals surviving and to prevent reinvasion, probably from the mainland, by installing permanent trapping/bait stations and conducting regular monitoring to identify at an early stage any reoccurence of rats. They further recommend that managers conducting eradication campaigns to integrate similar molecular monitoring, which can be done easily for most common invasive species, if it is unsure whether eradication attempts have failed due to repopulation by individuals that have survived control attempts, or due to reinvasion from other populations.
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