1 Clapperton et al.: Ship rat diet and breeding in beech forest New Zealand Journal of Ecology (2019) 43(2): 3370 © 2019 New Zealand Ecological Society. DOI: https://dx.doi.org/10.20417/nzjecol.43.22 RESEARCH Diet, population structure and breeding of Rattus rattus L. in South Island beech forest B. Kay Clapperton 1* , Fraser Maddigan 2 , Warren Chinn 3 and Elaine C. Murphy 3 1 56 Margaret Avenue, Havelock North 4130, New Zealand 2 Pest Control Solutions, Christchurch, New Zealand 3 Department of Conservation, Private Bag 4715, Christchurch 8140, New Zealand *Author for correspondence (Email: clapperton.lo@xtra.co.nz) Published online: 10 May 2019 Abstract: The diet, population structure and breeding of ship rats (Rattus rattus L.) from Fiordland National Park were assessed from measurements and gut sample analysis of 248 rats trapped between March 2009 and March 2010, following a mast beech seedfall. They consumed many lepidopteran larvae but fewer weta and more vegetative plant matter than in other habitats, as well as beech seed. Birds and mice made up only a relatively small proportion of the diet. A lizard was also confrmed as a prey item of R. rattus, for the frst time in New Zealand. The population included a high proportion of young rats and females that were breeding at an early age. A high percentage of females in breeding condition in each season, high uterine scar counts and consistent litter sizes throughout the year indicate high fecundity and year-round breeding. This breeding cycle is consistent with an ample food supply being available for rodents promoting a rat population irruption, which in turn may increase the predation pressure on native biodiversity. Keywords: age structure, black rat, mammalian pest, mast year, New Zealand, productivity, ship rat, stomach content analysis Introduction The ubiquitous rodent pest Rattus rattus L. (known variously as black rat, ship rat, house rat, or roof rat) is a predator of native biota in New Zealand and elsewhere (Innes 2005; Pender et al. 2013; Shiels et al. 2014). It can also have indirect effects on this biota through competition, spread of disease and other ecosystem disturbances (Shapiro 2005; Jones et al. 2008; Harris 2009; Banks & Hughes 2012; Shiels & Pitt 2014; Harper & Bunbury 2015). A better understanding of its diet and population dynamics in different habitats can inform management strategies. Rattus rattus eats a wide range of plant and animal foods (Shiels et al. 2013) but it is also described as a selective feeder (Clark 1981, 1982), with distinct individual diet preferences (Ruffno et al. 2011). It is considered more herbivorous than R. norvegicus, R. exulans or Mus musculus (Grant-Hoffman & Barboza 2010; Bridgman 2012; Shiels et al. 2013). As well as consuming vegetative parts of mature plants, its consumption of fruits, seeds and seedlings can reduce seedling establishment (Wilson et al. 2003; Grant-Hoffman & Barboza 2010). Predation by R. rattus also has the potential to reduce populations of large invertebrate species, reptiles, amphibians, forest birds and seabirds (Towns & Daugherty 1994; Innes et al. 1999, 2010; St Clair 2010; Ruscoe et al. 2013; Shiels et al. 2014). While lizards have been confrmed as prey items of R. rattus elsewhere (Clark 1981; Caut et al. 2008), they have not been reported in the stomach contents of this species in New Zealand. Although its diet has been described in various habitats and climatic regions of the world (Copson 1986; Tobin et al. 1994; Cole et al. 2000; Pisanu et al. 2011; Shiels & Pitt 2014; Shiels et al. 2014; Riofrío-Lazo & Páez-Rosas 2015) including New Zealand (studies summarised by Innes 2005 and Bridgman 2012), there have been few studies in Nothofagus beech forest. Massive seedfalls in some years (mast years) provide ample plant, invertebrate and vertebrate food sources for rats but there is little information on the dietary responses of rats to beech mast seeding. While R. rattus does not normally consume mammalian prey in New Zealand, McQueen and Lawrence (2008) found that mice (Mus musculus) contributed a large proportion of the diet of R. rattus in beech forest after a mast seeding. Rattus rattus usually breed in spring/summer in temperate forest habitats, producing on average 5 or 6 pups per litter (Innes 2005), and are able to produce four litters per breeding season (Best 1973). But with adequate food resources, they can breed year-round, leading to higher productivity (Daniel 1972; King & Moller 1997; Harper 2005). Thus they show a numerical response to a beech forest mast seeding (King & Moller 1997; Studholm 2000). These population irruptions are combined with changes in population structure, with an initial infux of young rats (King & Moller 1997). If these young rats eat more invertebrates relative to older rats, as shown for R. rattus in podocarp forests (Gales 1982; Sweetapple & Nugent 2007), they could further increase the threat to native fauna of the numerical response. The aim of this study was