Suncus Murinus, Soricidae) from Mauritian Offshore Islands
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Eradicating Indian musk shrews (Suncus murinus, Soricidae) from Mauritian offshore islands K. J. Varnham1*, S. S. Roy2, A. Seymour2, J. Mauremootoo1, C. G. Jones1, and S. Harris2 1 Mauritian Wildlife Foundation, Black River, Mauritius, Indian Ocean 2 School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, U.K. *Corresponding author. E-mail: [email protected] Abstract The Indian musk shrew (Suncus murinus), an efficient and rapid coloniser, has spread from its original home in India to become an ecological threat of global importance. A project to eradicate musk shrews from a 25 ha Mauritian offshore island began in July 1999. Due to the shrew’s low susceptibility to anticoagulant poisons, we relied on live trapping. Seven months of continual trapping initially appeared to have been successful but the population soon re- turned to its original level. A second experimental eradication on a smaller island (2 ha), carried out over three weeks in June 2000, allowed us to monitor the eradication process more closely, and a return visit has revealed no further signs of shrews. Studies of bait preference and trap use, in field and captive situations, gave further insights into how to attract shrews into traps. The invasive land snail Achatina fulica proved by far the most successful bait. Captive trap trials revealed design problems in the type of traps used in both eradication attempts, which resulted in one third of animals escaping capture. Keywords Invasive species; live trapping; poisoning; bait selection; island eradications; impact on native species. INTRODUCTION an insectivore, the musk shrew is an opportunistic feeder and in some areas is known to feed predominantly on plant Natural history of the Indian musk material (Advani and Rana 1981). On Mauritius, the shrew shrew is known to prey upon native and introduced invertebrates, as well as damaging seeds and young plants by digging. Through predation or competition, musk shrews are be- The Indian musk shrew (Suncus murinus) is a highly adapt- lieved to have caused the extirpation of several species of able insectivore, one of the largest members of the family endemic lizards from the mainland of Mauritius, Reunion, Soricidae. Morphologically it is extremely variable, with and many of their offshore islands (Jones 1988, 1993). body weights ranging from 33.2 g to 147.3 g in males and Since their introduction to the nearby island of Rodrigues from 23.5 g to 80.0 g in females (Ruedi et al. 1996). Be- in 1997 they have colonised the whole island (approxi- lieved to have originated in the Indian subcontinent (Yosida mately 109 km2) and have been strongly implicated in a 1982), its native range stretches across southern Asia from sharp decline in the numbers of several invertebrate spe- Afghanistan to the Malay archipelago and southern Japan. cies, including two native centipedes and a field cricket. It has since been introduced into northern and eastern Af- Beyond the Indian Ocean, the shrews are also causing wide- rica, as well as much of the Middle East (Ruedi et al. 1996). spread ecological damage. Since musk shrews were intro- Although this species can be found in forests and agricul- duced to Guam in the early 1950s, the extirpation pattern tural land, it is particularly common around areas of hu- of one lizard species, the pelagic gecko (Nactus pelagicus), man activity. This association has contributed to its pas- has coincided almost exactly with the spread of the shrews. sive transportation to a number of oceanic islands, includ- To a lesser extent, they are also believed to have affected ing Guam in the Pacific (Peterson 1956), and Madagas- two skink species Emoia cyanura and Emoia car, the Maldives, and Mauritius in the Indian Ocean caeruleocauda (Rodda and Fritts 1992; Fritts and Rodda (Wilson and Reeder 1993). The first reliable records of 1998). The musk shrew is fast becoming a pest species of musk shrews in the western Indian Ocean date from the global proportions, especially in disturbed, fragile, or small early 19th century (Hutterer and Trainer 1990), but the island ecosystems. The development of an effective method species is believed to have been on Mauritius since around of eradicating or controlling this species is now a conser- 1760 (Cheke 1987). vation priority. The Indian musk shrew as an invasive Unlike some commensal species introduced over a wide species geographical range, such as rats (especially Rattus rattus and R. norvegicus) and house mice (Mus musculus), musk This species is now widespread and rapidly expanding its shrews are currently extremely difficult to control using range, and represents a major ecological threat. Its poison. The second-generation anticoagulants such as commensal habit, combined with the prodigious capacity brodifacoum, which are effective for controlling rodents, for reproduction common to many small mammal species, are relatively ineffective on shrews. Differences in feed- makes it a highly effective coloniser. Although nominally ing habits and susceptibility make it difficult to get shrews Pages 342-349 In Veitch, C. R. and Clout, M. N. (eds.). Turning the tide: the eradication of invasive species. IUCN SSC Invasive Species Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK. Varnham et al.: Eradicating Indian musk shrews to consume either acute-acting or anticoagulant poison in ing a maximum height of 13 m. The surface of the island quantities large enough to kill them (Morris and Morris is covered with holes and pinnacles of jagged eroded coral, 1991; Bell and Bell 1996). Toxicity studies suggest insec- covered in places with shallow soil seldom more than 15cm tivores are generally less susceptible to commercially avail- deep. Despite this, it has the highest indigenous vegeta- able poisons than rodents and herbivores. The few results tion cover of all the inshore Mauritian islands. It contains available suggest musk shrews are killed by doses of around the largest area of coastal lowland forest remaining in 47 mg/kg of brodifacoum, about 10 times the lethal dose Mauritius, including species such as the critically endan- for moles and 100 times the level needed to kill rats (Mor- gered ebony (Diospyros egrettarum) as well as other en- ris and Morris 1991). In addition, since shrews are not dangered and vulnerable plant species. An intensive weed- agricultural pests on the scale of rats and mice, there is far ing programme is systematically removing invasive plant less commercial pressure on agrochemical companies to species, while a nursery situated on the island is produc- develop compounds specific to them. However, unlike ro- ing native plants for replanting in the weeded areas. Fol- dent pests, notably R. norvegicus, shrews are not known lowing the eradication of feral cats (Felis catus) and rats to exhibit neophobia and indeed readily explore novel (Rattus rattus), endemic pink pigeons (Columba mayeri) objects (Churchfield 1990; Gurnell and Flowerdew 1990). and Mauritian kestrels (Falco punctatus) have been re- A previous study of the shrews on Ile aux Aigrettes, an introduced. It is also planned to introduce ecological ana- offshore Mauritian island, showed them to enter live traps logues of extinct species. The first of these releases has in large numbers (Pilgrim 1996). Therefore, in the absence just taken place, with an experimental introduction of of an effective chemical method, we decided that trapping Aldabran giant tortoises (Testudo gigantea) in place of the was the most appropriate technique to eradicate musk two extinct Mauritian species (Geochelone inepta and G. shrews from the small offshore islands. indicus). The next phase of the restoration plan is to es- tablish several species of endangered endemic lizards, The Mauritian offshore island shrew some of which are restricted to single island populations eradication projects (all information in Dulloo et al. 1997). However, since shrews are known to both prey upon small lizards and com- The Ile aux Aigrettes habitat restoration project pete with them for food (Jones 1988, 1993), it is neces- sary first to remove the musk shrews from the island. A Ile aux Aigrettes (Fig. 1), a 25 ha designated nature re- second aim was to train staff from both MWF and NPCS serve, is the site of a major habitat reconstruction pro- in shrew trapping techniques. As a signatory to the Con- gramme under the control of the Mauritian Wildlife Foun- vention on Biological Diversity, Mauritius is pledged to dation (MWF) and the Mauritian National Parks and Con- the eradication or control of invasive species, but there is servation Service (NPCS). The island is the subject of a a severe shortage of people with expertise in invasive spe- management plan (Dulloo et al. 1997), describing the cies control, especially in developing countries. physical and biological character of the island, as well as Ile de la Passe a detailed description of past and future management ob- jectives. Ile aux Aigrettes is a flat coralline island reach- Following the Ile aux Aigrettes project we were keen to try our methods on a smaller and more manageable island, where the whole process could be monitored more closely and where we could manipulate trapping densities and trap coverage. An opportunity was presented when shrews were discovered on the nearby islet of Ile de la Passe (Fig. 1), some 4km north-east of Ile aux Aigrettes. This tiny wind- swept coralline island has an area of about 2 ha, less than one tenth the size of Ile aux Aigrettes, and is much less densely vegetated and topographically simpler. This re- moved the problems of size and accessibility we faced on Ile aux Aigrettes. In fact, the vegetation of Ile de la Passe was simple in the extreme, consisting almost entirely of short grass (Stenotaphrum sp.) and the occasional small bush of Tournefortia argentea. The outer edge of the is- land consists of exposed highly eroded jagged coral, but most of the island is smooth grassland, a habitat type not found on Ile aux Aigrettes.