Genetic evaluation of an attempted Rattus rattus eradication on Congo Cay, U.S. Virgin Islands, identifies importance of eradication units

Abstract

Congo Cay, U.S. Virgin Islands, has high value for breeding seabirds and is a potential reintroduction site for the endangered Virgin Islands tree boa (Epicrates monensis granti). However, introduced ship rats (Rattus rattus) undermine its conservation value. Three unsuccessful eradication attempts have been conducted since 1990, with the latest in 2006; rats were trapped 1.5 years later. We examined microsatellite DNA and mitochondrial DNA sequences of ship rats from Congo Cay and three other nearby cays to determine if rats found after the most recent eradication effort were surviving individuals or reinvaders from neighboring cays; we had no pre-eradication samples. Only one mitochondrial haplotype was present, implying that historically there was a single invasion or if multiple invasions, rats came from a single source with limited haplotype diversity. Low genetic variation on Congo Cay suggested either a population bottleneck resulting from survivors or a founder event resulting from invaders. F ST estimates, cluster distances, migrant detections, and factorial correspondence analysis indicated low but meaningful levels of gene flow between Congo and Lovango Cays and between Mingo and Grass Cays. Except for two alleles, all other alleles found on Congo were also present on Lovango. Without pre-eradication samples we could not eliminate the possibility of survivors from a failed eradication. However, our data suggest reinvasion from Lovango Cay was likely and that future eradication efforts should consider both pairs of cays as eradication units. Cay juxtaposition and orientation along with ocean currents may explain rat movement, or lack thereof, among these cays.

Experimental island invasion of house mice

“…For large mammal invasions, typically more than five propagules must be introduced to ensure successful establishment (Forsyth and Duncan 2001). Retrospective genetic analyses of rodent invasions suggest populations routinely and consistently establish from a similar small number of propagules (Russell et al , 2010Savidge et al 2012). In contrast, competition with Norway rats (R. norvegicus) was the likely reason that house mice experimentally released on Oxna Island (in the Shetland Islands) failed to produce a viable breeding population, despite hundreds of individuals being released over multiple occasions (Berry et al 1982).…”

Significant Genetic Impacts Accompany an Urban Rat Control Campaign in Salvador, Brazil

“…These campaigns are designed to reduce rat numbers, control their geographic spread, or completely remove all rats from an area. Efforts to completely eradicate rats have been limited mostly to islands separated geographically from other populations that may serve as sources of recolonizing rats (Howald et al, 2007;Russell et al, 2010;Savidge et al, 2012). However, cities across the world invest heavily to control or reduce rat populations in close proximity to dense human settlements, even if complete eradication is unlikely (Bonnefoy et al, 2008;de Masi et al, 2009;Parsons et al, 2017).…”

Cryptic diversity in Black rats Rattus rattus of the Galápagos Islands, Ecuador

“…We demonstrate high levels of genetic variability at microsatellite loci in populations of the invasive rodent R. rattus present on the islands. The levels of heterozygos- Savidge et al 2012). Low levels of genetic diversity have been observed in some invasive species, as founder populations often represent a subset of the species-wide gene pool (Frankham 2005).…”