The legacy of Big South Cape: rat irruption to rat eradication

Abstract

Big South Cape Island (Taukihepa) is a 1040 ha island, 1.5 km from the southwest coast of Stewart Island/Rakiura, New Zealand. This island was rat-free until the incursion of ship rats (Rattus rattus) in, or shortly before, 1963, suspected to have been accidentally introduced via local fishing boats that moored at the island with ropes to the shore, and were used to transport the mutton birders to the island. This incursion was reported by the muttonbirders -local Iwi who harvest the young of titi (sooty shearwater, Puffinus griseus) -to the then New Zealand Wildlife Service (via the New Zealand Department of Lands and Survey). Investigation into the reports found ship rats had reached the island and had decimated the local land bird populations. Brian Bell and Don Merton attempted some of the first translocations of South Island saddleback (Philesturnus c. carunculatus), Stewart Island snipe (Coenocorypha aucklandica iredalei) and Stead's bush wren (Xenicus longipes variabilis) with only the saddleback being successful. Extinctions of the snipe, wren and greater short-tailed bat (Mystacina robusta) were recorded. This was the first time rats were definitively recognised as the cause of extinction of native land birds and directed further debate into the impacts of rats and how to deal with them.

Home range and population density of black rats (Rattus rattus) on a seabird island: a case for a marine subsidised effect?

“…Losses of biodiversity have been most severe on islands, where endemic species are often predatornaïve (Courchamp et al 2003). The invasion and subsequent irruption of ship rats on Big South Cape Island/ Taukihepa in 1963 led to the extinction of two endemic birds, a bat species, a large weevil and the local extinction of an additional four bird species (Bell 1978;Ramsay 1978;Bell et al 2016).…”

Where did the rats of Big South Cape Island come from?

“…The island is visited seasonally by small groups of people collecting mutton birds and was rat-free until the 1960s (Bell 1978). By 1964 however, ship rats (R. rattus) were well established throughout the island, and damage to the mutton-birders' cabins, bedding and stored food supplies was evident (Bell et al 2016). There was subsequently a marked decline in birdlife, invertebrates, and some plants including both local and global extinctions (Bell 1978).…”

“…Prevailing knowledge at the time of the invasion also suggested that the introduction must have been a human-assisted dispersal. Fishing boats working in the area, possibly even those that transported the mutton-birders, were generally suspected as the source of the rats (Bell et al 2016), although where the rats might have boarded such boats was an open question. Once again in hindsight, at only 1.5 kilometres offshore, Big South Cape Island is within the known swimming range of Norway rats (Russell & Clout 2005), and while it is outside the known swimming range of ship rats (e.g.…”

“…The invasion of Big South Cape Island by ship rats also marked a changing point in the culture and management towards introduced rats in New Zealand (Bell et al 2016). The impact of rats on the island was clear and led managers to understand the urgency with which rats should be eradicated from other islands (Moors 1985;Taylor & Thomas 1993).…”

“…The island was continually inhabited by ship rats from the time of the 1964 irruption to the eradication in 2006 (Bell et al 2016;Harper & Rutherford 2016). Our analysis is possible due to the availability of rat samples from the island which are both contemporary (just prior to the eradication; Harper & Rutherford 2016) and historical (just after the invasion; Te Papa Museum specimens).…”