Impacts of black ratsRattus rattusacross an urban/bushland interface at Sydney's North Head

Abstract

This paper investigates the impacts of a population of introduced black rats Rattus rattus (Rodentia: Muridae) in native bushland at North Head at the entrance to Sydney Harbour, Australia. The black rat is an invasive pest species known the world over for its economic impacts on agriculture and its environmental impacts on native wildlife (Long 2003). The presence of black rats at North Head is of particular concern because of the conservation value of the area. North Head supports endangered populations of long-nosed bandicoots Perameles nasuta, little penguins Eudyptula minor, and sunshine wattle Acacia terminalis, along with an additional 119 species of native fauna (Skelton et al. 2003a). North Head also supports 159 species of exotic flora, 428 species of native flora and an endangered ecological community of Eastern Suburbs Banksia Scrub (Skelton et al. 2003b). Plans are currently under consideration to further protect the native flora and fauna in a sanctuary that would incorporate an existing National Park and part of an ex-military site. Black rats are probably the most abundant pest mammal species on the headland and, for the sanctuary to succeed, identification and management of the potential threats to the native flora and fauna are necessary. The aim of this study is to explore the potential impacts that black rats may have on species of conservation concern at North Head. Around the world, black rats are responsible for the consumption of food stores and agricultural crops. They also forage on native vegetation. On average, black rats consume 10% of their body weight each day (Long 2003), making them potentially important in terms of their grazing pressure on endemic plants. Although they are omnivorous (Watts and Braithwaite 1978; Clark 1981, 1982; Watts 1995; Long 2003), a large portion of black rat diet may include leaves, stems, seeds and/or fruit (Norman 1970; Cox 1997). Importantly, the grazing of seedlings by black rats can negatively impact the population recruitment in some plants (Crawley 1983; Wilson et al. 2003; Brown 2005), and rodent grazing can substantially increase plant mortality (e.g. Hulme 1996). Moreover, when feeding on plants, rodents generally show preferences for younger seedlings over older plants (Ostfield et al. 1997). At North Head, there is concern over the potential impact of black rats on the recruitment of endangered plants (Skelton et al. 2003a), particularly sunshine wattle. Sunshine wattle is an acacia that occurs patchily within heathland. The major threat to this species is habitat reduction (NSW National Parks and Wildlife Service 2004a), but its seeds and seedlings may be vulnerable to rat predation, particularly in remnant heathland near urban areas. Black rats are known predators of sunshine wattle seedlings in commercial nurseries (C. Miller, pers. comm.). The first aim of this study is to determine the level of black rat foraging on sunshine wattle seedlings relative to other vertebrate grazers. Impacts of black rats Rattus rattus across an urban/ bushland interface at Sydney’s North Head Tania A. Rose1,2 and Peter B. Banks1 1School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia