Individual study: The implementation of a live-trapping programme in an attempted eradication of the Asian musk shrew Suncus murinus on Ile aux Aigrettes, Mauritius
Seymour A. & Varnham K. et al. (2005) Mechanisms underlying the failure of an attempt to eradicate the invasive Asian musk shrew Suncus murinus from an island nature reserve. Biological Conservation, 125, 23-35
The Asian musk shrew Suncus murinus native of southern Asia, is one of the largest members of the Soricidae (shrew) family. It has been introduced to many areas beyond its natural range, including Africa and the Middle East. A number of Oceanic islands important for their biodiversity have also been invaded, including Mauritius, Guam, Rodrigues and The Maldives.
The Asian musk shrew is highly adaptable and on some islands has been shown to have a severe impact on native plants and animals. The musk shrew is believed to be particularly detrimental to some reptiles, preying on smaller species directly, in addition to competing for food resources. In Guam it has been associated with the extinction of the gecko Nactus pelagicus, and declines of two skinks, Emoia cyanura and Emoia caeruleocauda. On 'mainland' Mauritius the extirpation of several endemic lizard species has been attributed to musk shrews, as well as feeding on both native and introduced invertebrates. On Mauritius it is also known to damage native plants through their digging activity which disturbs and exposes plant root systems.
On the small island nature reserve of Ile aux Aigrettes, off the south-east coast of Mauritius, it was decided to attempt a musk shrew eradication in preparation for the planned introduction of several endangered reptile species indigenous to Mauritius.
The island had already been successfully cleared of ship rats Rattus rattus and feral cats Felis catus by 1994 in preparation for the release of two endangered endemic bird species, Mauritius kestrel Falco punctatus and pink pigeon Columba mayeri populations.
Study site: The musk shrew Suncus murinus eradication attempt was undertaken on Ile aux Aigrettes, a 25 ha island nature reserve situated off the south east coast of Mauritius, between July 1999 and February 2000. Ile aux Aigrettes is a flat coralline island reaching 13 m at its highest point. Although soil depth is rarely more than 15 cm (mostly much less), it has the highest indigenous vegetation cover of all the inshore Mauritian islands, and also supports the largest area of native coastal lowland scrub forest remaining in Mauritius.
Eradication method: Anticoagulant poisons traditionally used to eradicate rats Rattus spp. and house mice Mus musculus were deemed unsuitable for musk shrew control. Insectivores such as shrews are less susceptible to these poisons and the high dose required to kill musk shrews would also be potentially fatal to pink pigeons Columba mayeri present on Ile aux Aigrettes, if bait was ingested. Eradication was therefore attempted using live traps.
Trapping programme: Initially, 912 Longworth traps and 200 trip traps were laid on the western side of the island, then gradually moved at 6-15 day intervals in a sweep encompassing the entire island. Four sweeps of the island were carried out using all 1,112 traps. The number of traps was decreased to 251-288 trip traps for sweep 5. Between 244-515 (variable number baited/set each day) trip traps were used for sweep 6 in addition to a reduced sweep area. The traps were baited with approximately 10 g of a range of baits:
Sweep 1 – re-hydrated dried fish, vegetable oil and flour
Sweep 2 – re-hydrated dried fish and vegetable oil
Sweep 3 – re-hydrated sultanas
Sweep 4 – re-hydrated dried fish and cod-liver oil
Sweeps 5 and 6 – peanut butter and oats
Traps were checked daily and any musk shrews found were humanly killed.
Bait trials: The data collected from each sweep were analysed to assess trapping success. Captive and field bait trials were then carried out in July 2000 in order to establish the most effective bait to use in future programmes. Baits tested during these trials were: egg, sardine, cheese, dog food, peanut butter, a non-native snail (African giant snail Achatina spp.) and foraged items.
Effectiveness of the trapping programme: A total of 759 musk shrews were captured in the six trapping sweeps over Ile aux Aigrettes. The first four sweeps were successful in capturing 705 shrews and eradication appeared to have been more-or-less acheived with only four more caught in the following three months. However, sweeps 5 and 6 indicated a progressive increase in population size, with fairly large numbers of shrews again being caught (see Table 1, attached). Although simulation models predicted that sweeps 1 to 4 should have been sufficient to eradicate musk shrews from Ile aux Aigrettes, this was not the case.
Explanation for the failure of the trapping programme: Three possible explanations for the failure of the trapping programme were investigated: low efficiency of traps; presence of areas where trapping was less efficient (trapping refugia); and presence of trap-shy individuals.
Low efficiency of traps was dismissed as the high initial trapping success (around 90% of the estimated population being captured) did not fit this hypothesis. The trapping refugia hypothesis was also dismissed as the island was well-covered. The reason thought to be behind the failure was the presence of trap-shy individuals in the population. The increase in the number of shrews caught during sweeps 5 and 6 can be explained by either the trap-shy individuals becoming less trap-shy (for whatever reasons), or that trap-shy individuals have bred and produced trap-susceptible offspring. The presence of a trap-shy proportion of the musk shrew population was unexpected as most shrews are usually easily catchable (from past experience) in a range of live traps, including those used in this eradication attempt.
Success of bait type One key to success of future musk shrew eradication projects depends on finding an effective bait or attractant for the traps. This is important, as in bait trials it was found that the trip traps were only triggered 66% of the times a shrew entered the trap but that this rose to 100% if the shrew actually ate some of the bait (which triggered the trap).
Shrew stomach contents examined during the study were analysed and the vast majority were empty, confirming that bait placed in the traps was rarely eaten. Evidence, from bait trails and additional field observations, suggests shrews enter traps through curiosity rather than in response to the presence of bait. The meat of Achatina snails (these introduced snails are themselves scheduled for eradication from Ile aux Aigrettes) was eaten in 11 out of the 13 trials in which it was offered. This was by far the most effective bait and the only one with a higher relative success rate than that with no bait at all.
Trap efficiancy: The inefficiency of trip traps (only 66% triggered if bait not eaten) was highlighted during this study, however the cost of a trip trap is £1 (UK sterling), whereas the cost of the highly efficient Longworth trap is prohibitively expensive at around £35 (see Table 2, attached). Either an alternative trap needs to be found, or the efficiency of trip traps needs to be enhanced by the use of a more attractive bait, such as Achatina.
Trap placement: Trap placement was found to be more important than the presence of bait at the start of the trapping programme, therefore, traps need to be well placed and laid in areas where the shrews are most likely to be intercepted, such as alongside tree roots and the edges of buildings and paths, rather than following a rigerous grid pattern.
Explanation for successful eradication on Ile de la Passe: A live trapping eradication programme was subsequently carried out on the much smaller neighbouring islet of Ile de la Passe (2 ha) over 20 days in June 2000. This proved successful in the eradication of the entire musk shrews. There are several reasons attributed to the success of this trapping programme: the island is less than one tenth the area of Ile aux Aigrettes; it is less densely vegetated; and it is topographically simpler. The island was therfore much easier to trap and monitor. In addition trapping density was about twice that used on Ile aux Aigrettes, whilst the estimated musk shrew density at the commencement of trapping was about 50% lower. Traps were set simultaneously over the entire island resulting in all but one shrew caught within the first five days and a total of forty shrews were caught during the trapping programme.
Conclusions: Evidence from Ile aux Aigrettes indicates that live trapping was effective in substantially reducing the number of musk shrews at the start of an eradication programme. However, the presence of a few trap shy individuals, perhaps with a few refugia areas, shows that soley using trapping was ineffective in acheiving total eradication. Either trapping efficiency needs to be improved (through better trap design and use of more attractive baits), or a combination of methods may have to be used. Using trapping only, however, proved successful as an eradication method on Ile de la Passe, a small islet with a simple topography.
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