West, C.J. Consideration of rat impacts on weeds priorIsland to rat and invasives: cat eradication on eradication Raoul Island, Kermadecs and management Consideration of rat impacts on weeds prior to rat and cat eradication on Raoul Island, Kermadecs C. J. West Department of Conservation, PO Box 10-420, Wellington 6143, New Zealand. <[email protected]>. Abstract In anticipation of the planned eradication of rats (Rattus norvegicus and R. exulans) and cats (Felis catus) on Raoul Island in 2002, the exotic flora was evaluated to determine which species might become more invasive following the removal of rats. The interactions between the three mammal species targeted for eradication and the multiple weed species on the island were considered. A group of exotic species that had expanded their range vegetatively but not been observed fruiting in the presence of rats was identified. This group included grape (Vitis vinifera), shore hibiscus, fou (Hibiscus tiliaceus), rosy periwinkle (Catharanthus roseus) and airplant (Bryophyllum pinnatum). As a precaution, grape was targeted for eradication as this species would be dispersed effectively by tui (Prosthemadera novaeseelandiae), a native honeyeater, and blackbird (Turdus merula) if it began to fruit after rats were eradicated. Grape proved difficult to control but by mid-2002 all nine known grape infestations were reduced to zero density. In 2008/2009, no grape sprouts were found during searches of all known infestation sites. Since the eradication almost all species that did not fruit when rats were present are now fruiting. Keywords: Rattus exulans; Rattus norvegicus; Felis catus; invasive weeds; Vitis vinifera INTRODUCTION Plants and animals have been introduced to many islands comm.). Raoul Island has been intermittently occupied by around the world (Abbott et al. 2000; Sax and Gaines people since C. 960 A.D. and permanently occupied since 2008; Towns et al. 2006). Raoul Island is no exception, 1937 (Sykes et al. 2000). having been colonised through human agency since the th Raoul Island, a Nature Reserve managed by the first Polynesian voyagers arrived in the 10 century AD Department of Conservation, is the northernmost (29°15’ S, (Anderson 1980). Rattus exulans Peale (Pacific rat, kiore) 177° 55’ W) and largest (2934 ha) of the Kermadec Group, were well established in the mid 1800s and were assumed situated in the South Pacific Ocean north of New Zealand to be native (MacGillivray 1854). However, they are most (Fig. 1). It is an active volcano with rugged topography, likely to have been introduced by Polynesian voyagers subject to cyclones in the summer months and occasional many centuries before (Harper and Veitch 2006). Felis heavy rain that triggers landslips. The Raoul volcano catus L.(cat) accompanied the earliest human settlers and last erupted on 17 March 2006. Forest dominated by were definitely present in 1854 (MacGillivray 1854).Rattus pohutukawa (Metrosideros kermadecensis W.R.B.Oliv.) is norvegicus Berkenhout (Norway rat) colonised Raoul the main vegetation cover. Grasses, strand vines and ferns Island after the schooner “Columbia River” was wrecked dominate coastal cliffs and the beach ridges at Denham there in September 1921 (Harper and Veitch 2006). Bay and Low Flat. Exotic plants deliberately introduced were food and Approximately 20% of the vascular plant flora is utility species such as Cordyline fruticosa (L.) Goepp. (ti endemic (Sykes et al. 2000). This comparatively low pore, ki) and Aleurites moluccana (L.) Willd. (candlenut), level of endemism is principally due to the young age of introduced by Polynesians, and a range of fruiting trees, the Raoul volcano coupled with its remoteness. The high pasture grasses, vegetables and flowering garden plants degree of disturbance associated with active volcanism introduced by European settlers (Sykes et al. 2000). Many may also be a contributing factor. Many of the endemic plant species were accidentally introduced including plant species closely resemble species endemic to the New Bidens pilosa L. (beggar’s ticks) with Polynesians, Conyza Zealand mainland and offshore islands e.g., Kermadec bonariensis (L.) Cronquist (fleabane) with early European pohutukawa, Kermadec ngaio (Myoporum kermadecense settlers (Sykes et al. 2000) and more recently, Selaginella Sykes) and Kermadec fivefinger (Pseudopanax kraussiana (Kunze) A.Braun (selaginella), first recorded kermadecensis (W.R.B.Oliv.) Philipson). Little is known in 1999 (West 2002) and Soliva sessilis Ruiz and Pav. about elements of the flora that might be now extinct as (Onehunga weed) first recorded in 2008 (David Havell pers. a result of human occupation or eruption history: pollen Fig. 1 Location of Raoul Island and location of places mentioned in the text. Pages 244-247 In: Veitch, C. R.; Clout, M. N. and Towns, D. R. (eds.). 2011. Island invasives: eradication and management. IUCN,244 Gland, Switzerland. West: Rat impacts on weeds analyses may shed some light on species turnover in this highly disturbed island ecosystem. The 2006 eruption has caused the extinction of at least one native species – Ophioglossum petiolatum Hook. (stalked adder’s tongue) – that was known from a small area in Green Lake crater that is now both under water and buried in a thick mud deposit. More than half the vascular plant flora of Raoul Island is introduced and a number of vines, trees and shrubs are transformer weeds (Pyšek et al. 2004). The majority of the introduced species are herbaceous and associated with disturbed ground along roads and open tracks, the accommodation for staff, meteorological station and abandoned rough pasture along the northern terraces of the island. There is an active surveillance programme, and this is how the selaginella and Onehunga weed were detected. Selaginella, in particular, could become a problem; however, prompt action has prevented this. Weed eradication has been a focus of management Fig. 2 Raoul Island food web from 1984–2002. Relationships on Raoul Island since 1972, with 29 species targeted for derived from published sources (Fitzgerald et al. 1991; Harper and Veitch 2006) and from observations (C.R. eradication in 1996 (West 2002). Goats were eradicated Veitch pers. comm., and pers. obs.). from Raoul Island in 1984 (Sykes and West 1996). The impact of goats on the native and exotic flora was evident both before and after they were eradicated: no exotic plant Island was constructed, based on published information species are known to have increased significantly after and observations (Fig. 2). Particular attention was paid to goats were eradicated (Parkes 1984; Sykes and West 1996). known prey and competitors of rodents and cats and those Rats and cats were eradicated from Raoul Island in 2002 organisms known to eat or disperse seeds. It was considered and 2004 respectively (Broome 2009), thus removing all that the plant eradication regime on Raoul was resourced introduced mammals. The need to identify the effect of rat sufficiently to be able to cope with any increase in the eradication on the exotic flora was considered well before abundance or range of the target species if they responded the rat eradication operation (West 2002; Sykes and West positively to rat eradication, since a key action in achieving 1996) in order to avoid unintended outcomes (Zavaleta et eradication is to detect all individuals before they produce al. 2001; Caut et al. 2009). ripe fruit/seeds (West 2002). The effect of the Pacific rat on seeds and seedlings of The list of exotic vascular plant species present on Raoul native tree species of northern New Zealand islands has Island, excluding those currently targeted for eradication been documented (Campbell and Atkinson 1999; Campbell or control of seedlings (species of historic value), was and Atkinson 2002; Towns et al. 2006). Rats were shown to reviewed to identify those that might become invasive after eat seeds and seedlings, thus depressing the populations of rats were eradicated. The eradication of goats showed that at least 11 tree species. Norway rats also have been shown the primary vegetation response was a significant increase to suppress regeneration of native tree species by eating in density and abundance of native plant species, indicating seeds and seedlings (Allen et al. 1994; Towns et al. 2006). that forest regeneration was not impaired by the majority of Rats eat many plant parts including flowers, seeds, fruits the exotic species which are herbaceous (a full list of exotic and seedlings (Atkinson and Towns 2005; Innes 2005). species can be seen in Sykes et al. 2000). Woody trees and There is no published information on impacts of rats on shrubs, vines and clonal semi-woody perennials pose the exotic plant species in New Zealand. However, the factors greatest threat to the forest communities on Raoul Island. that predispose native species to predation by rats were The primary question, therefore, was would a species, if assumed to apply to exotic species as well: seedlings of all not checked through seed or seedling predation, expand its species, and plants with fleshy fruit and/or fruit with large population to the detriment of native plant communities edible seeds were considered to be vulnerable (Towns et on the island? Many species were eliminated at this point, al. 2006). particularly light-demanding, herbaceous species. Non- Cats on Raoul Island primarily preyed upon rats, forest communities, where light-demanding weeds might particularly the Pacific rat, and secondarily upon birds, thrive, are in comparatively harsh environments (e.g., some of which are effective seed dispersers (Fitzgerald coastal cliffs, rocky ridges, heated ground in the crater, et al. 1991). Rats also prey upon some of the same seed back dunes) and have not indicated susceptibility to weed dispersing bird species, mainly introduced passerines invasion to date, with the exception of the dune slack at (Towns et al. 2006). On Raoul, the principal fruit-eating Denham Bay where airplant has spread vegetatively. and seed-dispersing bird species are tui (Prosthemadera The reproductive status and dispersal potential of the novaeseelandiae Gmelin), blackbird (Turdus merula remaining species were then considered.