Eradicating Indian musk ( 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, 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 (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 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 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. Few vertebrate species were present on the island. The shrews were the only resident , while reptiles were represented by native Bou- ton’s skinks (Cryptoblepharus boutoni), and introduced night geckoes (probably Hemidactylus sp.). The resident Fig. 1 Map of Mauritius showing position of Ile bird fauna consisted of a single pair of introduced house aux Aigrettes and Ile de la Passe (adapted from sparrows (Passer domesticus), although passing seabirds Bullock 1986). occasionally landed there. Achatina snails appeared to be

343 Turning the tide: the eradication of invasive species absent, with no live animals encountered during three for the first three months (c. 50,000 trap nights). Trip traps weeks on the island, and only one empty shell. This con- alone were used for the remainder of the programme trasts with the situation on Ile aux Aigrettes where live (c.50,000 trap nights). Since trap numbers were never suf- snails are common and empty snail shells are abundant ficient to cover the whole surface of the island, traps were across the island. This part of the project had two main moved across the island as a rolling front. Starting at the aims. First, to learn more about shrew ecology, trapping, western end of the island, up to two thirds of the island and the acceptability of different baits before restarting was initially covered with traps. A maximum of 1100 traps the Ile aux Aigrettes project. The second, and more imme- were in use at any one time. For the first three sweeps the diate aim, was to protect a population of Bojer’s skinks traps at the furthest west part of the island were lifted and (Gongylomorphus bojeri) on a neighbouring island. This moved to the eastern side of the block of traps after 4-15 species is now restricted to six small Mauritian offshore days. Thereafter trapping periods were more variable de- islands, one of which, Ilot Vacoas, is only a few hundred pending on capture rate. This process was repeated for six metres from Ile de la Passe. MWF staff spent several days ‘sweeps’ of the island. Traps were baited with a variety of on this tiny island (less than 1 ha) in June 2000 and no substances to try to maximise attractiveness to the shrews. signs of shrews or other introduced mammals were found. Sweep one used rehydrated dried fish mixed with flour Shrews have been strongly implicated in the decline of and vegetable oil; sweep two rehydrated fish mixed with Bojer’s skinks on other islands (Jones 1993) and, since Ile vegetable oil; sweep three soaked sultanas; sweep four re- de la Passe and Ilot Vacoas are potentially joined by a hydrated dried fish mixed with cod liver oil; and sweeps land bridge at very low tides, it was imperative to remove five and six a mixture of peanut butter and oats. them from Ile de la Passe as soon as possible. Initially the programme appeared very successful and the METHODS AND RESULTS number of captures declined asymptotically in a classic extinction curve. For over six weeks no captures were Indian musk shrew eradication projects made, but at the end of November 1999 shrews began to be encountered once more. Captures continued at a low The Ile aux Aigrettes shrew trapping but steady rate for the next three months, and shrews were programme caught wherever traps were placed. Table 1 shows the cap- ture rate per 100 trap nights of each of the six trapping A large-scale trapping programme began in July 1999. A sweeps. The shrews caught in sweeps five and six included 12.5 x 12.5 m grid was marked out across the island using the first pregnant and lactating females caught during the blue polypropylene twine and traps were set at the inter- programme. Of the 54 shrews caught during the last three sections. This trap spacing was based on the findings of a months, 76% of the females were pregnant and/or lactat- mark-recapture study carried out on Ile aux Aigrettes by ing. The trapping programme was discontinued at the end Pilgrim (1996), which showed that shrews travelled up to of February 2000, when it became apparent that shrews 60m between captures. Her study found that 15 x 15 m were present wherever traps were placed and insufficient grids caught substantial numbers of shrews; we decided traps were available to cover the whole island. Weather on a 12.5 x 12.5 m grid to increase trapping intensity. This conditions were also affecting the efficiency of the traps, gave a total of 1651 trap points. Longworth traps and plastic with the percentage of traps found tripped but with no cap- tunnel traps of a similar design (‘Trip traps’ (Fig. 2), manu- ture increasing from around 4% in August to 23% in Janu- factured by Proctor Bros. Ltd., Pantglass Industrial Es- ary and February during the rainy season. Within months, tate, Bedwas, Caerphilly, Wales, CF83 8XD) were used shrews were as abundant as they had been before the trap- ping programme began, testament to the species’ phenom- enal powers of reproduction. Summary morphometric sta- tistics for this group of shrews, and those caught in subse- quent sections of the study, are shown in Appendix 1.

The Ile de la Passe shrew trapping programme

This trapping programme ran over 20 days in June 2000. We divided the island into four approximately equal sec- tions and used traps baited with a different substance in each section. These baits (cheese, dog food, sardines and mayonnaise) were moved every five days, so after 20 days Fig. 2 A ‘set’ Trip trap. When set, the door rests on the treadle in a horizontal position. The bait is placed in the end section of the trap, and the two sections clip together. Animals entering Table 1 Capture rate (shrew captures per 100 the trap press down on the treadle, releasing trap nights) for sweeps one to six. the door, which falls down into a vertical posi- tion behind them. When assembled the trap Sweep 1 2 3456 measures 40 x 50 x 165 mm. Capture rate 5.89 0.15 0.02 0.02 0.11 0.15

344 Varnham et al.: Eradicating Indian musk shrews each bait had been offered in each section for five days. This strategy was designed to pick up any stragglers who might have shown aversion to one bait type. In the event, all but one of the shrews were caught in the first five days, so the bait preference component was never tested. Forty shrews were caught in total. During a return visit made to the island in October 2000, 600 further trap nights revealed no sign of shrews. Night walks and careful searches for droppings, focusing on the buildings in which shrews had initially been concentrated, also proved negative. Small- scale ecological changes were apparent on this second trip, with one species of large cockroach being particularly abundant. It is likely that these invertebrates formed a sub- stantial part of the shrews’ diet and that their subsequent increase is due to this reduction in predation pressure. Fur- ther checks are planned after another four months and then at yearly intervals. However, so far we are cautiously op- timistic that we have carried out the world’s first success- ful eradication of musk shrews from an island. Fig. 3 Relative successes of different bait types, showing contribution made by males Comparison of the Ile aux Aigrettes and Ile de la (cross-hatched), females (white), and animals Passe projects of unknown sex (black).

The two projects differed in a number of respects, sum- marised in Table 2 below. The most notable differences fore we set up a series of captive bait trials in July 2000, are the length of the project and the size of the islands, aimed at testing the attractive qualities of the baits we had both about an order of magnitude greater on Ile aux Ai- used so far as well as series of novel substances. Bait trials grettes. Other differences with the Ile de la Passe project were carried out in a 2 m diameter enclosure, consisting were that the trapping density was about twice as high and of aluminium sheeting 0.6 m high, situated in a patch of that the traps were set simultaneously over the entire is- open ground. Experimental baits were placed in shallow land for the duration of the trapping programme. The shrew containers at regular intervals around the inside perimeter density was about 50% higher for Ile aux Aigrettes, even of the enclosure. The 82 shrews used in this part of the allowing for some of the Ile aux Aigrettes shrews to have project were all individually housed overnight without been born during the course of the six-month trapping pro- food, but provided with sufficient water and bedding ma- gramme. terial. Shrews were introduced into the centre of the en- closure, next to a dish of water and watched for 10 min- Experiments to improve trapping utes. Anything they ate during the course of the trial was efficiency recorded. Eight experimental baits were used in each trial; six baits were used in all 82 trials while the remaining two were ‘wild card’ baits, changed approximately every 10 Captive bait trials trials. This was done in order to test as many substances as possible. The stomachs of all the shrews caught on Ile aux Aigrettes and Ile de la Passe were removed at post-mortem; the vast Figure 3 shows the results of the bait trial. Forty-two shrews majority were empty. In addition, bait placed in the traps (52.1%) ate one or more of the baits, 23 (28.0%) ate items was rarely touched. Throughout the trapping programmes found on the floor of the enclosure (some individuals did on both islands we had not found an effective bait or at- both), while 28 animals (34.1%) ate nothing. The sex ra- tractant that unequivocally improved trap success. There- tio of shrews in this part of the study was strongly skewed, over 2:1 males to females (52 males, 25 females, five un- knowns). Figure 3 also shows the contribution males and Table 2 Summary of differences between the females made to each result. χ2 tests showed none of these Ile aux Aigrettes and Ile de la Passe projects. results to be different from those expected by chance with respect to gender. The results suggest the shrews show a Ile aux Aigrettes Ile de la Passe preference for familiar foods (e.g. snail, egg, and foraged items), which could be a profitable line of future bait re- Duration (days) 204 20 search. On trial 70, minced snail (Achatina spp.) was in- No. of trap nights 97822 4800 troduced as one of the wild card baits and proved very Area (hectares) 25 2 successful, being eaten in 11 of the 13 trials in which it No. of shrews 759 40 was offered. This result was surprising, since captive musk Shrews per hectare 29.2 20 shrews in Guam starved to death rather than eat Achatina, Traps per hectare 64 116 when housed with live specimens (Peterson 1956). How-

345 Turning the tide: the eradication of invasive species ever, it was an excellent result from the point of view of of the traps were left unbaited. Traps of each type were Mauritian conservation, since Achatina are also introduced. alternated throughout the grid. The traps were checked They are highly abundant across both mainland Mauritius every day for seven days (1008 trap nights), and 200 shrews and Ile aux Aigrettes, and could provide a valuable source were caught. of bait, either in a live trapping programme, or as a carrier for a suitable chemical agent. Musk shrews on Ile aux Ai- The traps baited with egg actually caught fewest shrews; grettes were observed attacking and eating live whole 58 individuals, compared with 78 in sultana-baited traps snails, as well as readily taking minced dead snails. and 70 in unbaited traps. However, these differences were not significant (χ2 = 5.32, df = 2, p>0.05). Shrews caught Captive trap trials in the early stages of trapping programmes are likely to be the most inquisitive individuals, who enter traps out of Captive trap trials were carried out in July 2000 on 52 curiosity rather than in response to the presence of bait. shrews to see how they responded to baited and unbaited Had we continued this experimental trial for longer, we traps, and to see if foods that showed promise in the bait would have started to catch shrews moving in from trial were effective at attracting animals into traps. Eight untrapped areas (Pilgrim 1996). However, with a larger Trip traps were set around the perimeter of the enclosure, trapping grid it is possible that the different bait substances between the dishes of food. Four were baited (with egg, may have had an effect on the long-term trapping rate. cheese, dog food, or sardines), and four left unbaited. The This study underlines the fact that a bait which proves suc- number of baited and unbaited traps entered by each shrew cessful in a captive trial will not necessarily be successful in a 10 minute period was recorded, along with whether under field conditions. the shrew tripped the trap mechanism or not. DISCUSSION The results are shown in Table 3. Shrews actually entered unbaited traps more frequently than those containing bait, Recommendations although the difference was not significant (χ2 = 2.73, df = 1, p>0.05). More disturbing was the fact that on one Trap placement third of occasions, shrews entered and left a Trip trap with- out causing the mechanism to fire. The most important The results of the captive trap trials, which showed no sig- finding was that if a shrew actually ate the bait present nificant difference in capture rate between baited and inside the trap (n = 17, 21.8% of trapping events involv- unbaited traps, suggest that shrews enter traps out of curi- ing baited traps), the trap tripped in every case. This ap- osity rather than in response to the presence of bait. This peared to be due to the shrews moving further into the trap idea is also supported by the results of the field trial. So, at if they ate the bait, which was placed at the back of the least at the beginning of trapping programmes, trap place- trap. So, at least in the case of Trip traps, their perform- ment appears to be more important than the presence or ance can be enhanced if they contain palatable bait. type of bait. Traps must be placed where the animals are most likely to go – along the edges of buildings, rocks, Field bait trials tree roots and paths.

The captive bait trials had identified some potential baits, Duration of trapping but the trap trials suggested this might not be enough to improve trap success. Consequently, in July 2000 we set Traps need to be left down for long periods, and ideally up a field trial on Ile aux Aigrettes of some of these baits the whole area should be trapped simultaneously. Some to see if they made any difference to capture rates. A grid animals do not go into traps for months, and these are the of 144 Trip traps was set out using the original 12.5 x animals we need to target. The failure of the Ile aux Ai- 12.5 m grid system in an area of relatively mature ebony grettes removal programme was almost certainly due to forest, the largest section of continuous habitat available. missing a very small number of shrews. Setting traps across One-third of the traps were baited with boiled egg, the the whole of the island to be cleared should also increase most successful bait known at the time (this trial was set the chance of trapping stragglers. Because of the extended up before the attractive properties of minced snail were time period needed, it is also important to trap when the discovered), one third with sultanas, which had a zero suc- shrews are not breeding. On Ile aux Aigrettes pregnant cess rate in the captive bait trials, and the remaining third and/or lactating females were only found between Novem- ber and April.

Table 3 Summary of trapping events (captive Trap type trap trials). The results of the captive trap trials show that in one third Baited Unbaited Total of cases shrews entered and left Trip traps without being captured. Longworth small mammal traps, used in the first Tripped 47 63 110 four trapping sweeps of the island, were far less likely than Untripped 31 24 55 Trip traps to be found tripped without captures - an aver- Total 78 87 165 age of 1.1% of traps per 100 trap nights for Longworths,

346 Varnham et al.: Eradicating Indian musk shrews as opposed to 20.6% for Trip traps. However, Longworth supplies, especially camping equipment, building supplies, traps are also far more expensive (about £35 each, as op- and large amounts of food. Ile de la Passe is a popular posed to £1 each for Trip traps), and were only on loan to spot for picnicking Mauritians, especially at weekends and the project. Further research is needed into alternative trap public holidays in summer. During the three weeks of the types to see if a cheap, effective alternative can be found. Ile de la Passe eradication project approximately 50 peo- In the meantime, however, Trip traps may have a useful ple visited the island. Some people bring bags of firewood, role to play in future eradication attempts in combination as well as large quantities of food and camping equipment, with other more effective ‘mopping-up’ techniques. These all possible hiding places for stowaway shrews. The ab- may include poisoning (in the event of a suitable agent sence of shrews on nearby Ilot Vacoas is probably due in being found) and possibly the use of specially trained dogs. part to the fact that it makes a less attractive picnic site, being small, flat, and difficult to land on. Continued moni- Bait type toring of Ile de la Passe is needed to ensure the island remains clear of shrews. If they are found to have re-in- It remains to be seen if all shrews enter traps solely out of vaded, they should be removed again as soon as possible. curiosity, or if good bait can eventually encourage the more MWF have the equipment and staff to do this at short no- cautious shrews to enter traps. At the moment, Achatina tice and the island could probably be cleared again by two snail seems the most promising candidate. As mentioned people in two weeks. above, Achatina snails are also introduced to Mauritius, and are also scheduled for eradication from Ile aux Ai- Ile aux Aigrettes is a different situation. If shrews were grettes. If further bait trials confirm their effectiveness as eradicated in the future it will represent a huge investment bait for trapping shrews, it may be wise to combine the of time and resources. Equipment, food supplies, and eradication plans for these two species. One possibility building materials must be thoroughly checked for the pres- may be to reduce the population of Achatina through hand- ence of shrews before being brought to the island. It would picking and/or poisoning and then to start trapping shrews also be sensible to have a cordon sanitaire of traps and/or using untainted Achatina bait when snail numbers are low. suitable poison around the jetty and nearby Warden’s However, it is probably sensible to use a variety of differ- house. The permanent presence of MWF staff on the is- ent baits during any future trapping programme in order to land means that re-invaders would hopefully be found appeal to as many shrews as possible. Another possibility quickly before they had time to breed out of control. is using the shrews’ own natural scents as attractants, as this species is highly dependent on its sense of smell. Ani- Coordinating multi-species eradications mals in breeding condition have conspicuous scent glands on their flanks, responsible for their characteristic musky The timing of eradication projects on islands with multi- odour. Both sexes possess these glands (Dryden and ple invasive species needs careful consideration. The Conaway 1967) and use them for scent marking and com- shrews on Ile aux Aigrettes were seldom seen prior to the munication (Balakrishnan and Alexander 1980). Traps con- eradication of black rats (R. rattus), and it was only after taining adult male shrews could often be detected while the removal of this competitor that they multiplied to pest still several metres away due to the pungent odour they proportions. Similar findings have been reported from produced. It may be possible to use the shrews’ flank glands areas of mainland Mauritius subject to rat control, where as a chemical attractant in traps. shrew numbers increased in inverse proportion to rat num- bers (D. Hall pers. comm.). We therefore recommend that One finding from the trapping on Ile aux Aigrettes was every effort is made to ascertain the presence of shrews on that some shrews will enter traps on the first night, while islands where rat eradications are planned. This would others will avoid them for months on end despite the high allow the shrews to be specifically targeted in either a prior density of traps. It is equally likely that not all shrews re- or parallel eradication attempt. spond in the same way to all bait types. In eradication ef- forts we must strive to target every individual shrew. No The future of trapping for island shrew single bait type or trap type is likely to appeal to all shrews eradications and future eradication attempts must bear this in mind.

Potential for re-invasion On small or topographically simple islands

At present there are no measures in place on either island We appear to have successfully cleared shrews from one to prevent new musk shrews becoming established. Lim- small island, showing that the technique can work. How- ited experiments by Morris and Morris (1991) suggest that ever, the chance of failure increases dramatically as the shrews are unlikely to reach Ile aux Aigrettes by natural number of shrews increases; obviously, the more shrews means, due to the strength and direction of the current be- there are, the greater the chance of missing one or more tween the island and mainland. The likelihood of their animals. On very small or topographically simple islands, reaching Ile de la Passe is even more remote given its con- trapping may be sufficient in itself to eradicate a popula- siderable distance from the mainland. The most likely route tion of musk shrews, as appears to have been the case on for shrews re-invading either island is via shipment of bulky Ile de la Passe.

347 Turning the tide: the eradication of invasive species Where cheap, dedicated manpower is readily REFERENCES available Advani, R. and Rana, B. D. 1981. Food of the house shrew, The labour requirements and associated costs will also Suncus murinus sindensis in the Indian Desert. Acta increase as the work area increases, and in many situa- Theriologica 26: 133-134. tions will be prohibitive. The failure of the Ile aux Ai- grettes project was due in part to not having sufficient traps Balakrishnan, M. and Alexander, K. M. 1980. A study of and human resources in place on the island to react quickly scent marking and its olfactory inhibition in the Indian enough to the recurrence of shrews. Trapping programmes musk shrew, Suncus murinus viridescens. Bonner of this scale need large numbers of staff who are available Zoologische Beiträge 31: 2-13. for the duration of the programme.

Where the use of poison is constrained Bell, B. D. and Bell, E. 1996. Mauritius offshore islands project phase II. Implementation of management rec- Poison bait trials are required as a matter of urgency to see ommendations. Unpublished report from Wildlife Man- if an appropriate chemical control method can be identi- agement International Ltd., New Zealand. fied. Finding an effective poison could revolutionise musk shrew control. However, trapping could still play an im- Brown, K. P. and Sherley, G. H. 2002. The eradication of portant role on islands like Ile aux Aigrettes, where the possums from Kapiti Island, New Zealand. In Veitch, presence of endangered species constrains the use of large C. R. and Clout, M. N. (eds.). Turning the tide: the quantities of poison, or where the risk of secondary poi- eradication of invasive species, pp. 46-52. IUCN SSC soning through scavenging of carrion is unacceptably high. Invasive Species Specialist Group. IUCN, Gland, Swit- zerland and Cambridge, UK. In combination with other methods Bullock, D. J. 1986. The ecology and conservation of rep- The future of shrew eradication is likely to rely on a com- tiles on Round Island and Gunner’s Quoin, Mauritius. bination of methods, perhaps incorporating trapping and Biological Conservation 37: 135-156. poisoning. Trapping has been shown to be a highly-effec- tive way of dramatically reducing populations of shrews Cheke, A. S. 1987. An Ecological history of the Mascarene relatively quickly – 75% of shrews caught on Ile aux Ai- Islands, with particular reference to extinctions and in- grettes were caught within eight days, and over 90% within troductions of land vertebrates. In Diamond, A. S. (ed.). 30 days. On larger islands, trapping may have a role to Studies of Mascarene Island Birds, pp. 5-89. Cam- play as an efficient way of quickly reducing musk shrew bridge University Press. numbers locally, but other methods may prove more ef- fective at catching remnant individuals. One possibility Churchfield, S. 1990. The natural history of shrews. Lon- may be tracking with dogs specifically trained on the scent don, Christopher Helm. of the target species. This method has been used exten- sively in New Zealand to target the last individuals in Dryden, G. L. and Conaway, C. H. 1967. The origin and eradications of possums (Brown and Sherley 2002) and hormonal control of scent production in Suncus wallabies (Mowbray 2002), where the populations had al- murinus. Journal of Mammalogy 48: 420-428. ready been knocked down through the use of poison. It may be possible to adapt the method to track shrews, us- Dulloo, M. E.; Verburg, J.; Paul, S. S.; Green, S. E.; de ing dogs to locate individuals surviving any future large Boucherville Baissac, P. and Jones, C. G. 1997. Ile aux scale trapping or poisoning programmes. However, the Aigrettes Management Plan, 1997-2000. Mauritian use of dogs with this species, which is not naturally preyed Wildlife Foundation, Technical Series No. 1/97. upon by canids, is as yet untested and may prove imprac- tical. Fritts, T. H. and Rodda, G. H. 1998. The role of intro- duced species in the degradation of island ecosystems. ACKNOWLEDGMENTS Annual Review of Ecology and Systematics 29: 113-140. We would like to thank the many people who helped with the project, especially the many MWF, NPCS, and Uni- Gurnell, J. and Flowerdew, J. R. 1990. Live trapping small versity of Bristol workers who helped with the colossal mammals: a practical guide. Reading, The Mammal amount of fieldwork undertaken. Thanks also to Drs P. Society. Craze and P. Baker and to referees Drs J. Daltry and G. Rodda for their useful comments on various drafts of this paper. Finally, thanks to Mr. R. Varnham for drawing Fig. 2.

348 Varnham et al.: Eradicating Indian musk shrews

Hutterer, R. and Trainer, M. 1990. The immigration of the Peterson, G. D. 1956. Suncus murinus, a recent introduc- Asian house shrew Suncus murinus into Africa and tion to Guam. Journal of Mammalogy 37: 278-279. Madagascar. In Peters, G. and Trainer, M. (eds.). Ver- tebrates in the tropics. Proceedings of the international Pilgrim, A. 1996. A study of the introduced shrew (Suncus symposium on vertebrate biogeography and systemat- murinus) on Ile aux Aigrettes, Mauritius. Unpublished ics in the tropics, pp. 309-319. Alexander Koenig Zoo- M.Sc. thesis, Imperial College of Science, Technology logical Research Institute and Zoological Museum, and Medicine, University of London. Bonn. Rodda, G. H. and Fritts, T. H. 1992. The impact of the Jones, C. G. 1988. A note on the Macchabée Skink with a introduction of the colubrid snake Boiga irregularis on record of predation by the Lesser Indian Mongoose. Guam’s lizards. Journal of Herpetology 26: 166-174. Proceedings of the Royal Society of Arts and Science of Mauritius 5 (1): 131-134. Ruedi, M.; Courvoisier, C.; Vogel, P. and Catzeflis, F. M. 1996. Genetic differentiation and zoogeography of the Jones, C. G. 1993. The ecology and conservation of Asian house shrew Suncus murinus (Mammalia: Mauritian skinks. Proceedings of the Royal Society of Soricidae). Biological Journal of the Linnean Society Arts and Science of Mauritius 5 (3): 71-95. 57: 307-316.

Morris, P. A. and Morris, M. J. 1991. Removal of shrews Wilson, D. E. and Reeder, D. M. 1993: Mammal species from the Ile aux Aigrettes. Unpublished report to the of the world: a taxonomic and geographic reference. Mauritian Wildlife Appeal Fund. Washington, Smithsonian Institute Press.

Mowbray, S. C. 2002. Eradication of introduced Austral- Yosida, T. H. 1982. Cytogenetical studies on Insectivora, ian marsupials (brushtail possum and brushtailed rock 2. Geographical variation of chromosomes in the house wallaby) from Rangitoto and Motutapu Islands, New shrew, Suncus murinus (Soricidae), in east, southeast Zealand. In Veitch, C. R. and Clout, M. N. (eds.). Turn- and southwest Asia, with a note on the karyotype, evo- ing the tide: the eradication of invasive species, pp. lution and distribution. Japanese Journal of Genetics 226-232. IUCN SSC Invasive Species Specialist Group. 57: 101-111. IUCN, Gland, Switzerland and Cambridge, UK.

Appendix 1 Summary statistics for shrews caught on Ile aux Aigrettes and Ile de la Passe.

Sample Sex ratio No. of Weight (g) Head + body length (mm) (M : F) shrews Mean Range Mean Range

Ile aux Aigrettes (Removal) 0.72 759* 18.1 10-48 100.2 75-131 Males 317 21.2 12-48 105.1 84-131 Females 441 15.9 10-28 96.7 75-117

Ile de la Passe (Removal) 1.22 40 22.0 10-42 105.8 76-129 Males 22 24.7 10-42 108.7 76-129 Female 18 18.7 12-26 102.2 90-116

Ile aux Aigrettes (Captive bait trial) 2.04 82** 23.7 15.46 105.3 85-139 Males 51 25.9 15-46 108.7 89-139 Females 25 19.6 15-27 98.8 85-115

Ile aux Aigrettes (Field bait trial) 1.4 200# 22.7 14-36 103.7 78-126 Males 112 25.2 16-36 107.2 89-126 Females 80 19.2 14-27 98.8 78-115

* Includes one shrew of unknown sex ** Includes five shrews of unknown sex # Includes eight shrews of unknown sex

349