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
2
Mauritian Wildlife Foundation, Black River, Mauritius, Indian Ocean 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 returned 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 Natural history of the Indian musk shrew The Indian musk shrew (Suncus murinus) is a highly adaptable insectivore, one of the largest members of the family Soricidae. Morphologically it is extremely variable, with body weights ranging from 33.2 g to 147.3 g in males and from 23.5 g to 80.0 g in females (Ruedi et al. 1996). Believed to have originated in the Indian subcontinent (Yosida 1982), its native range stretches across southern Asia from Afghanistan to the Malay archipelago and southern Japan. It has since been introduced into northern and eastern Africa, as well as much of the Middle East (Ruedi et al. 1996). Although this species can be found in forests and agricultural land, it is particularly common around areas of human activity. This association has contributed to its passive transportation to a number of oceanic islands, including Guam in the Pacific (Peterson 1956), and Madagascar, the Maldives, and Mauritius in the Indian Ocean (Wilson and Reeder 1993). The first reliable records of musk shrews in the western Indian Ocean date from the early 19th century (Hutterer and Trainer 1990), but the species is believed to have been on Mauritius since around 1760 (Cheke 1987).
The Indian musk shrew as an invasive species This species is now widespread and rapidly expanding its range, and represents a major ecological threat. Its commensal habit, combined with the prodigious capacity for reproduction common to many small mammal species, makes it a highly effective coloniser. Although nominally
an insectivore, the musk shrew is an opportunistic feeder and in some areas is known to feed predominantly on plant material (Advani and Rana 1981). On Mauritius, the 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 believed to have caused the extirpation of several species of endemic lizards from the mainland of Mauritius, Reunion, and many of their offshore islands (Jones 1988, 1993). Since their introduction to the nearby island of Rodrigues in 1997 they have colonised the whole island (approximately 109 km2) and have been strongly implicated in a sharp decline in the numbers of several invertebrate species, including two native centipedes and a field cricket. Beyond the Indian Ocean, the shrews are also causing widespread ecological damage. Since musk shrews were introduced to Guam in the early 1950s, the extirpation pattern of one lizard species, the pelagic gecko (Nactus pelagicus), has coincided almost exactly with the spread of the shrews. To a lesser extent, they are also believed to have affected two skink species Emoia cyanura and Emoia caeruleocauda (Rodda and Fritts 1992; Fritts and Rodda 1998). The musk shrew is fast becoming a pest species of global proportions, especially in disturbed, fragile, or small island ecosystems. The development of an effective method of eradicating or controlling this species is now a conservation priority. Unlike some commensal species introduced over a wide geographical range, such as rats (especially Rattus rattus and R. norvegicus) and house mice (Mus musculus), musk shrews are currently extremely difficult to control using poison. The second-generation anticoagulants such as brodifacoum, which are effective for controlling rodents, are relatively ineffective on shrews. Differences in feeding 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 quantities large enough to kill them (Morris and Morris 1991; Bell and Bell 1996). Toxicity studies suggest insectivores are generally less susceptible to commercially available poisons than rodents and herbivores. The few results available suggest musk shrews are killed by doses of around 47 mg/kg of brodifacoum, about 10 times the lethal dose for moles and 100 times the level needed to kill rats (Morris and Morris 1991). In addition, since shrews are not agricultural pests on the scale of rats and mice, there is far less commercial pressure on agrochemical companies to develop compounds specific to them. However, unlike rodent pests, notably R. norvegicus, shrews are not known to exhibit neophobia and indeed readily explore novel objects (Churchfield 1990; Gurnell and Flowerdew 1990). A previous study of the shrews on Ile aux Aigrettes, an offshore Mauritian island, showed them to enter live traps in large numbers (Pilgrim 1996). Therefore, in the absence of an effective chemical method, we decided that trapping was the most appropriate technique to eradicate musk shrews from the small offshore islands.
The Mauritian offshore island shrew eradication projects The Ile aux Aigrettes habitat restoration project Ile aux Aigrettes (Fig. 1), a 25 ha designated nature reserve, is the site of a major habitat reconstruction programme under the control of the Mauritian Wildlife Foundation (MWF) and the Mauritian National Parks and Conservation Service (NPCS). The island is the subject of a management plan (Dulloo et al. 1997), describing the physical and biological character of the island, as well as a detailed description of past and future management objectives. Ile aux Aigrettes is a flat coralline island reach-
Fig. 1 Map of Mauritius showing position of Ile aux Aigrettes and Ile de la Passe (adapted from Bullock 1986).
ing a maximum height of 13 m. The surface of the island is covered with holes and pinnacles of jagged eroded coral, covered in places with shallow soil seldom more than 15cm deep. Despite this, it has the highest indigenous vegetation cover of all the inshore Mauritian islands. It contains the largest area of coastal lowland forest remaining in Mauritius, including species such as the critically endangered ebony (Diospyros egrettarum) as well as other endangered and vulnerable plant species. An intensive weeding programme is systematically removing invasive plant species, while a nursery situated on the island is producing native plants for replanting in the weeded areas. Following the eradication of feral cats (Felis catus) and rats (Rattus rattus), endemic pink pigeons (Columba mayeri) and Mauritian kestrels (Falco punctatus) have been reintroduced. It is also planned to introduce ecological analogues of extinct species. The first of these releases has just taken place, with an experimental introduction of Aldabran giant tortoises (Testudo gigantea) in place of the two extinct Mauritian species (Geochelone inepta and G. indicus). The next phase of the restoration plan is to establish several species of endangered endemic lizards, some of which are restricted to single island populations (all information in Dulloo et al. 1997). However, since shrews are known to both prey upon small lizards and compete with them for food (Jones 1988, 1993), it is necessary first to remove the musk shrews from the island. A second aim was to train staff from both MWF and NPCS in shrew trapping techniques. As a signatory to the Convention on Biological Diversity, Mauritius is pledged to the eradication or control of invasive species, but there is a severe shortage of people with expertise in invasive species control, especially in developing countries. Ile de la Passe 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 windswept 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 removed 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 island 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 mammals, while reptiles were represented by native Bouton’s skinks (Cryptoblepharus boutoni), and introduced night geckoes (probably Hemidactylus sp.). The resident bird fauna consisted of a single pair of introduced house sparrows (Passer domesticus), although passing seabirds 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 weeks on the island, and only one empty shell. This contrasts with the situation on Ile aux Aigrettes where live snails are common and empty snail shells are abundant across the island. This part of the project had two main aims. First, to learn more about shrew ecology, trapping, and the acceptability of different baits before restarting the Ile aux Aigrettes project. The second, and more immediate aim, was to protect a population of Bojer’s skinks (Gongylomorphus bojeri) on a neighbouring island. This species is now restricted to six small Mauritian offshore islands, one of which, Ilot Vacoas, is only a few hundred metres from Ile de la Passe. MWF staff spent several days on this tiny island (less than 1 ha) in June 2000 and no signs of shrews or other introduced mammals were found. Shrews have been strongly implicated in the decline of Bojer’s skinks on other islands (Jones 1993) and, since Ile de la Passe and Ilot Vacoas are potentially joined by a land bridge at very low tides, it was imperative to remove them from Ile de la Passe as soon as possible.
METHODS AND RESULTS Indian musk shrew eradication projects The Ile aux Aigrettes shrew trapping programme A large-scale trapping programme began in July 1999. A 12.5 x 12.5 m grid was marked out across the island using blue polypropylene twine and traps were set at the intersections. This trap spacing was based on the findings of a mark-recapture study carried out on Ile aux Aigrettes by Pilgrim (1996), which showed that shrews travelled up to 60m between captures. Her study found that 15 x 15 m grids caught substantial numbers of shrews; we decided on a 12.5 x 12.5 m grid to increase trapping intensity. This gave a total of 1651 trap points. Longworth traps and plastic tunnel traps of a similar design (‘Trip traps’ (Fig. 2), manufactured by Proctor Bros. Ltd., Pantglass Industrial Estate, Bedwas, Caerphilly, Wales, CF83 8XD) were used
for the first three months (c. 50,000 trap nights). Trip traps alone were used for the remainder of the programme (c.50,000 trap nights). Since trap numbers were never sufficient to cover the whole surface of the island, traps were moved across the island as a rolling front. Starting at the western end of the island, up to two thirds of the island was initially covered with traps. A maximum of 1100 traps were in use at any one time. For the first three sweeps the traps at the furthest west part of the island were lifted and moved to the eastern side of the block of traps after 4-15 days. Thereafter trapping periods were more variable depending on capture rate. This process was repeated for six ‘sweeps’ of the island. Traps were baited with a variety of substances to try to maximise attractiveness to the shrews. Sweep one used rehydrated dried fish mixed with flour and vegetable oil; sweep two rehydrated fish mixed with vegetable oil; sweep three soaked sultanas; sweep four rehydrated dried fish mixed with cod liver oil; and sweeps five and six a mixture of peanut butter and oats. Initially the programme appeared very successful and the number of captures declined asymptotically in a classic extinction curve. For over six weeks no captures were made, but at the end of November 1999 shrews began to be encountered once more. Captures continued at a low but steady rate for the next three months, and shrews were caught wherever traps were placed. Table 1 shows the capture rate per 100 trap nights of each of the six trapping sweeps. The shrews caught in sweeps five and six included the first pregnant and lactating females caught during the programme. Of the 54 shrews caught during the last three months, 76% of the females were pregnant and/or lactating. The trapping programme was discontinued at the end of February 2000, when it became apparent that shrews were present wherever traps were placed and insufficient traps were available to cover the whole island. Weather conditions were also affecting the efficiency of the traps, with the percentage of traps found tripped but with no capture increasing from around 4% in August to 23% in January and February during the rainy season. Within months, shrews were as abundant as they had been before the trapping programme began, testament to the species’ phenomenal powers of reproduction. Summary morphometric statistics for this group of shrews, and those caught in subsequent sections of the study, are shown in Appendix 1. The Ile de la Passe shrew trapping programme
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 the trap press down on the treadle, releasing the door, which falls down into a vertical position behind them. When assembled the trap measures 40 x 50 x 165 mm. 344
This trapping programme ran over 20 days in June 2000. We divided the island into four approximately equal sections 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
Table 1 Capture rate (shrew captures per 100 trap nights) for sweeps one to six. Sweep 1 Capture rate 5.89
2 0.15
3 0.02
4 0.02
5 0.11
6 0.15
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. Smallscale 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 substantial part of the shrews’ diet and that their subsequent increase is due to this reduction in predation pressure. Further checks are planned after another four months and then at yearly intervals. However, so far we are cautiously optimistic that we have carried out the world’s first successful eradication of musk shrews from an island. Comparison of the Ile aux Aigrettes and Ile de la Passe projects The two projects differed in a number of respects, summarised in Table 2 below. The most notable differences are the length of the project and the size of the islands, both about an order of magnitude greater on Ile aux Aigrettes. Other differences with the Ile de la Passe project were that the trapping density was about twice as high and that the traps were set simultaneously over the entire island for the duration of the trapping programme. The shrew density was about 50% higher for Ile aux Aigrettes, even allowing for some of the Ile aux Aigrettes shrews to have been born during the course of the six-month trapping programme.
Experiments to improve trapping efficiency Captive bait trials The stomachs of all the shrews caught on Ile aux Aigrettes and Ile de la Passe were removed at post-mortem; the vast majority were empty. In addition, bait placed in the traps was rarely touched. Throughout the trapping programmes on both islands we had not found an effective bait or attractant that unequivocally improved trap success. There-
Table 2 Summary of differences between the Ile aux Aigrettes and Ile de la Passe projects. Ile aux Aigrettes Duration (days) No. of trap nights Area (hectares) No. of shrews Shrews per hectare Traps per hectare
204 97822 25 759 29.2 64
Ile de la Passe 20 4800 2 40 20 116
Fig. 3 Relative successes of different bait types, showing contribution made by males (cross-hatched), females (white), and animals of unknown sex (black).
fore we set up a series of captive bait trials in July 2000, aimed at testing the attractive qualities of the baits we had used so far as well as series of novel substances. Bait trials were carried out in a 2 m diameter enclosure, consisting of aluminium sheeting 0.6 m high, situated in a patch of open ground. Experimental baits were placed in shallow containers at regular intervals around the inside perimeter of the enclosure. The 82 shrews used in this part of the project were all individually housed overnight without food, but provided with sufficient water and bedding material. Shrews were introduced into the centre of the enclosure, next to a dish of water and watched for 10 minutes. Anything they ate during the course of the trial was 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 trials. This was done in order to test as many substances as possible. Figure 3 shows the results of the bait trial. Forty-two shrews (52.1%) ate one or more of the baits, 23 (28.0%) ate items found on the floor of the enclosure (some individuals did both), while 28 animals (34.1%) ate nothing. The sex ratio of shrews in this part of the study was strongly skewed, over 2:1 males to females (52 males, 25 females, five unknowns). Figure 3 also shows the contribution males and females made to each result. χ2 tests showed none of these results to be different from those expected by chance with respect to gender. The results suggest the shrews show a preference for familiar foods (e.g. snail, egg, and foraged items), which could be a profitable line of future bait research. On trial 70, minced snail (Achatina spp.) was introduced as one of the wild card baits and proved very successful, being eaten in 11 of the 13 trials in which it was offered. This result was surprising, since captive musk shrews in Guam starved to death rather than eat Achatina, when housed with live specimens (Peterson 1956). How345
Turning the tide: the eradication of invasive species ever, it was an excellent result from the point of view of Mauritian conservation, since Achatina are also introduced. They are highly abundant across both mainland Mauritius and Ile aux Aigrettes, and could provide a valuable source of bait, either in a live trapping programme, or as a carrier for a suitable chemical agent. Musk shrews on Ile aux Aigrettes were observed attacking and eating live whole snails, as well as readily taking minced dead snails. Captive trap trials Captive trap trials were carried out in July 2000 on 52 shrews to see how they responded to baited and unbaited traps, and to see if foods that showed promise in the bait trial were effective at attracting animals into traps. Eight Trip traps were set around the perimeter of the enclosure, between the dishes of food. Four were baited (with egg, cheese, dog food, or sardines), and four left unbaited. The number of baited and unbaited traps entered by each shrew in a 10 minute period was recorded, along with whether the shrew tripped the trap mechanism or not. The results are shown in Table 3. Shrews actually entered unbaited traps more frequently than those containing bait, although the difference was not significant (χ2 = 2.73, df = 1, p>0.05). More disturbing was the fact that on one third of occasions, shrews entered and left a Trip trap without causing the mechanism to fire. The most important finding was that if a shrew actually ate the bait present inside the trap (n = 17, 21.8% of trapping events involving baited traps), the trap tripped in every case. This appeared to be due to the shrews moving further into the trap if they ate the bait, which was placed at the back of the trap. So, at least in the case of Trip traps, their performance can be enhanced if they contain palatable bait. Field bait trials
of the traps were left unbaited. Traps of each type were alternated throughout the grid. The traps were checked every day for seven days (1008 trap nights), and 200 shrews were caught. The traps baited with egg actually caught fewest shrews; 58 individuals, compared with 78 in sultana-baited traps and 70 in unbaited traps. However, these differences were not significant (χ2 = 5.32, df = 2, p>0.05). Shrews caught in the early stages of trapping programmes are likely to be the most inquisitive individuals, who enter traps out of curiosity rather than in response to the presence of bait. Had we continued this experimental trial for longer, we would have started to catch shrews moving in from untrapped areas (Pilgrim 1996). However, with a larger trapping grid it is possible that the different bait substances may have had an effect on the long-term trapping rate. This study underlines the fact that a bait which proves successful in a captive trial will not necessarily be successful under field conditions.
DISCUSSION Recommendations Trap placement The results of the captive trap trials, which showed no significant difference in capture rate between baited and unbaited traps, suggest that shrews enter traps out of curiosity rather than in response to the presence of bait. This idea is also supported by the results of the field trial. So, at least at the beginning of trapping programmes, trap placement appears to be more important than the presence or type of bait. Traps must be placed where the animals are most likely to go – along the edges of buildings, rocks, tree roots and paths.
The captive bait trials had identified some potential baits, but the trap trials suggested this might not be enough to improve trap success. Consequently, in July 2000 we set up a field trial on Ile aux Aigrettes of some of these baits to see if they made any difference to capture rates. A grid of 144 Trip traps was set out using the original 12.5 x 12.5 m grid system in an area of relatively mature ebony forest, the largest section of continuous habitat available. One-third of the traps were baited with boiled egg, the most successful bait known at the time (this trial was set up before the attractive properties of minced snail were discovered), one third with sultanas, which had a zero success rate in the captive bait trials, and the remaining third
Duration of trapping
Table 3 Summary of trapping events (captive trap trials).
Trap type
Tripped Untripped Total 346
Baited
Unbaited
47 31 78
63 24 87
Total 110 55 165
Traps need to be left down for long periods, and ideally the whole area should be trapped simultaneously. Some animals do not go into traps for months, and these are the animals we need to target. The failure of the Ile aux Aigrettes removal programme was almost certainly due to missing a very small number of shrews. Setting traps across the whole of the island to be cleared should also increase the chance of trapping stragglers. Because of the extended time period needed, it is also important to trap when the shrews are not breeding. On Ile aux Aigrettes pregnant and/or lactating females were only found between November and April.
The results of the captive trap trials show that in one third of cases shrews entered and left Trip traps without being captured. Longworth small mammal traps, used in the first four trapping sweeps of the island, were far less likely than Trip traps to be found tripped without captures - an average of 1.1% of traps per 100 trap nights for Longworths,
Varnham et al.: Eradicating Indian musk shrews as opposed to 20.6% for Trip traps. However, Longworth traps are also far more expensive (about £35 each, as opposed to £1 each for Trip traps), and were only on loan to the project. Further research is needed into alternative trap types to see if a cheap, effective alternative can be found. In the meantime, however, Trip traps may have a useful role to play in future eradication attempts in combination with other more effective ‘mopping-up’ techniques. These may include poisoning (in the event of a suitable agent being found) and possibly the use of specially trained dogs. Bait type It remains to be seen if all shrews enter traps solely out of curiosity, or if good bait can eventually encourage the more cautious shrews to enter traps. At the moment, Achatina snail seems the most promising candidate. As mentioned above, Achatina snails are also introduced to Mauritius, and are also scheduled for eradication from Ile aux Aigrettes. If further bait trials confirm their effectiveness as bait for trapping shrews, it may be wise to combine the eradication plans for these two species. One possibility may be to reduce the population of Achatina through handpicking and/or poisoning and then to start trapping shrews using untainted Achatina bait when snail numbers are low. However, it is probably sensible to use a variety of different baits during any future trapping programme in order to appeal to as many shrews as possible. Another possibility is using the shrews’ own natural scents as attractants, as this species is highly dependent on its sense of smell. Animals in breeding condition have conspicuous scent glands on their flanks, responsible for their characteristic musky odour. Both sexes possess these glands (Dryden and Conaway 1967) and use them for scent marking and communication (Balakrishnan and Alexander 1980). Traps containing adult male shrews could often be detected while still several metres away due to the pungent odour they produced. It may be possible to use the shrews’ flank glands as a chemical attractant in traps. One finding from the trapping on Ile aux Aigrettes was that some shrews will enter traps on the first night, while others will avoid them for months on end despite the high density of traps. It is equally likely that not all shrews respond in the same way to all bait types. In eradication efforts we must strive to target every individual shrew. No single bait type or trap type is likely to appeal to all shrews and future eradication attempts must bear this in mind.
supplies, especially camping equipment, building supplies, and large amounts of food. Ile de la Passe is a popular spot for picnicking Mauritians, especially at weekends and public holidays in summer. During the three weeks of the Ile de la Passe eradication project approximately 50 people visited the island. Some people bring bags of firewood, as well as large quantities of food and camping equipment, all possible hiding places for stowaway shrews. The absence of shrews on nearby Ilot Vacoas is probably due in part to the fact that it makes a less attractive picnic site, being small, flat, and difficult to land on. Continued monitoring of Ile de la Passe is needed to ensure the island remains clear of shrews. If they are found to have re-invaded, they should be removed again as soon as possible. MWF have the equipment and staff to do this at short notice and the island could probably be cleared again by two people in two weeks. Ile aux Aigrettes is a different situation. If shrews were eradicated in the future it will represent a huge investment of time and resources. Equipment, food supplies, and building materials must be thoroughly checked for the presence of shrews before being brought to the island. It would also be sensible to have a cordon sanitaire of traps and/or suitable poison around the jetty and nearby Warden’s house. The permanent presence of MWF staff on the island means that re-invaders would hopefully be found quickly before they had time to breed out of control. Coordinating multi-species eradications The timing of eradication projects on islands with multiple invasive species needs careful consideration. The shrews on Ile aux Aigrettes were seldom seen prior to the eradication of black rats (R. rattus), and it was only after the removal of this competitor that they multiplied to pest proportions. Similar findings have been reported from areas of mainland Mauritius subject to rat control, where shrew numbers increased in inverse proportion to rat numbers (D. Hall pers. comm.). We therefore recommend that every effort is made to ascertain the presence of shrews on islands where rat eradications are planned. This would allow the shrews to be specifically targeted in either a prior or parallel eradication attempt.
The future of trapping for island shrew eradications
Potential for re-invasion
On small or topographically simple islands
At present there are no measures in place on either island to prevent new musk shrews becoming established. Limited experiments by Morris and Morris (1991) suggest that shrews are unlikely to reach Ile aux Aigrettes by natural means, due to the strength and direction of the current between the island and mainland. The likelihood of their reaching Ile de la Passe is even more remote given its considerable distance from the mainland. The most likely route for shrews re-invading either island is via shipment of bulky
We appear to have successfully cleared shrews from one small island, showing that the technique can work. However, the chance of failure increases dramatically as the number of shrews increases; obviously, the more shrews there are, the greater the chance of missing one or more animals. On very small or topographically simple islands, trapping may be sufficient in itself to eradicate a population of musk shrews, as appears to have been the case on Ile de la Passe.
347
Turning the tide: the eradication of invasive species Where cheap, dedicated manpower is readily available The labour requirements and associated costs will also increase as the work area increases, and in many situations will be prohibitive. The failure of the Ile aux Aigrettes project was due in part to not having sufficient traps and human resources in place on the island to react quickly enough to the recurrence of shrews. Trapping programmes of this scale need large numbers of staff who are available for the duration of the programme. Where the use of poison is constrained Poison bait trials are required as a matter of urgency to see if an appropriate chemical control method can be identified. Finding an effective poison could revolutionise musk shrew control. However, trapping could still play an important role on islands like Ile aux Aigrettes, where the presence of endangered species constrains the use of large quantities of poison, or where the risk of secondary poisoning through scavenging of carrion is unacceptably high. In combination with other methods The future of shrew eradication is likely to rely on a combination of methods, perhaps incorporating trapping and poisoning. Trapping has been shown to be a highly-effective way of dramatically reducing populations of shrews relatively quickly – 75% of shrews caught on Ile aux Aigrettes were caught within eight days, and over 90% within 30 days. On larger islands, trapping may have a role to play as an efficient way of quickly reducing musk shrew numbers locally, but other methods may prove more effective at catching remnant individuals. One possibility may be tracking with dogs specifically trained on the scent of the target species. This method has been used extensively in New Zealand to target the last individuals in eradications of possums (Brown and Sherley 2002) and wallabies (Mowbray 2002), where the populations had already been knocked down through the use of poison. It may be possible to adapt the method to track shrews, using dogs to locate individuals surviving any future large scale trapping or poisoning programmes. However, the use of dogs with this species, which is not naturally preyed upon by canids, is as yet untested and may prove impractical.
ACKNOWLEDGMENTS We would like to thank the many people who helped with the project, especially the many MWF, NPCS, and University of Bristol workers who helped with the colossal amount of fieldwork undertaken. Thanks also to Drs P. 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
REFERENCES Advani, R. and Rana, B. D. 1981. Food of the house shrew, Suncus murinus sindensis in the Indian Desert. Acta Theriologica 26: 133-134. Balakrishnan, M. and Alexander, K. M. 1980. A study of scent marking and its olfactory inhibition in the Indian musk shrew, Suncus murinus viridescens. Bonner Zoologische Beiträge 31: 2-13. Bell, B. D. and Bell, E. 1996. Mauritius offshore islands project phase II. Implementation of management recommendations. Unpublished report from Wildlife Management International Ltd., New Zealand. Brown, K. P. and Sherley, G. H. 2002. The eradication of possums from Kapiti Island, New Zealand. In Veitch, C. R. and Clout, M. N. (eds.). Turning the tide: the eradication of invasive species, pp. 46-52. IUCN SSC Invasive Species Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK. Bullock, D. J. 1986. The ecology and conservation of reptiles on Round Island and Gunner’s Quoin, Mauritius. Biological Conservation 37: 135-156. Cheke, A. S. 1987. An Ecological history of the Mascarene Islands, with particular reference to extinctions and introductions of land vertebrates. In Diamond, A. S. (ed.). Studies of Mascarene Island Birds, pp. 5-89. Cambridge University Press. Churchfield, S. 1990. The natural history of shrews. London, Christopher Helm. Dryden, G. L. and Conaway, C. H. 1967. The origin and hormonal control of scent production in Suncus murinus. Journal of Mammalogy 48: 420-428. Dulloo, M. E.; Verburg, J.; Paul, S. S.; Green, S. E.; de Boucherville Baissac, P. and Jones, C. G. 1997. Ile aux Aigrettes Management Plan, 1997-2000. Mauritian Wildlife Foundation, Technical Series No. 1/97. Fritts, T. H. and Rodda, G. H. 1998. The role of introduced species in the degradation of island ecosystems. Annual Review of Ecology and Systematics 29: 113-140. Gurnell, J. and Flowerdew, J. R. 1990. Live trapping small mammals: a practical guide. Reading, The Mammal Society.
Varnham et al.: Eradicating Indian musk shrews Hutterer, R. and Trainer, M. 1990. The immigration of the Asian house shrew Suncus murinus into Africa and Madagascar. In Peters, G. and Trainer, M. (eds.). Vertebrates in the tropics. Proceedings of the international symposium on vertebrate biogeography and systematics in the tropics, pp. 309-319. Alexander Koenig Zoological Research Institute and Zoological Museum, Bonn. Jones, C. G. 1988. A note on the Macchabée Skink with a record of predation by the Lesser Indian Mongoose. Proceedings of the Royal Society of Arts and Science of Mauritius 5 (1): 131-134.
Peterson, G. D. 1956. Suncus murinus, a recent introduction to Guam. Journal of Mammalogy 37: 278-279. Pilgrim, A. 1996. A study of the introduced shrew (Suncus murinus) on Ile aux Aigrettes, Mauritius. Unpublished M.Sc. thesis, Imperial College of Science, Technology and Medicine, University of London. Rodda, G. H. and Fritts, T. H. 1992. The impact of the introduction of the colubrid snake Boiga irregularis on Guam’s lizards. Journal of Herpetology 26: 166-174.
Jones, C. G. 1993. The ecology and conservation of Mauritian skinks. Proceedings of the Royal Society of Arts and Science of Mauritius 5 (3): 71-95.
Ruedi, M.; Courvoisier, C.; Vogel, P. and Catzeflis, F. M. 1996. Genetic differentiation and zoogeography of the Asian house shrew Suncus murinus (Mammalia: Soricidae). Biological Journal of the Linnean Society 57: 307-316.
Morris, P. A. and Morris, M. J. 1991. Removal of shrews from the Ile aux Aigrettes. Unpublished report to the Mauritian Wildlife Appeal Fund.
Wilson, D. E. and Reeder, D. M. 1993: Mammal species of the world: a taxonomic and geographic reference. Washington, Smithsonian Institute Press.
Mowbray, S. C. 2002. Eradication of introduced Australian marsupials (brushtail possum and brushtailed rock wallaby) from Rangitoto and Motutapu Islands, New Zealand. In Veitch, C. R. and Clout, M. N. (eds.). Turning the tide: the eradication of invasive species, pp. 226-232. IUCN SSC Invasive Species Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK.
Yosida, T. H. 1982. Cytogenetical studies on Insectivora, 2. Geographical variation of chromosomes in the house shrew, Suncus murinus (Soricidae), in east, southeast and southwest Asia, with a note on the karyotype, evolution and distribution. Japanese Journal of Genetics 57: 101-111.
Appendix 1 Summary statistics for shrews caught on Ile aux Aigrettes and Ile de la Passe. Sample
Ile aux Aigrettes (Removal) Males Females Ile de la Passe (Removal) Males Female Ile aux Aigrettes (Captive bait trial) Males Females Ile aux Aigrettes (Field bait trial) Males Females * ** #
Sex ratio (M : F)
No. of shrews
Weight (g)
Head + body length (mm)
Mean
Range
Mean
Range
0.72
759* 317 441
18.1 21.2 15.9
10-48 12-48 10-28
100.2 105.1 96.7
75-131 84-131 75-117
1.22
40 22 18
22.0 24.7 18.7
10-42 10-42 12-26
105.8 108.7 102.2
76-129 76-129 90-116
2.04
82** 51 25
23.7 25.9 19.6
15.46 15-46 15-27
105.3 108.7 98.8
85-139 89-139 85-115
1.4
200# 112 80
22.7 25.2 19.2
14-36 16-36 14-27
103.7 107.2 98.8
78-126 89-126 78-115
Includes one shrew of unknown sex Includes five shrews of unknown sex Includes eight shrews of unknown sex
349
