Falklands Conservation

Report on the Falklands Conservation Rat Eradication Project

August – October 2001

Derek Brown, 4 Main Road, Havelock, New Zealand
Lindsay Chadderton, c/o Department of Conservation, Hamilton, NewZealand
Kerry Brown, 31 Jollie Road, Twizel, New Zealand

Summary

The recent rat eradication trip to the Falklands by three New Zealand conservation staff appears to have been very successful. Key achievements included:

Awareness of pest eradication and general island management issues has been greatly enhanced, and the probable success on the islands treated during this project has highlighted the eradication potential within the Falklands. Efforts now need to be focussed on developing a strategic approach to future eradications and island restoration, and developing local expertise. A small number of local staff are sufficiently trained to be able to undertake eradications on smaller islands without further supervision, but caution is urged for larger, complex and or more ecologically important islands. We recommend that in the next few years the focus should be placed upon developing additional experience on small to medium-sized islands to build confidence and an better understanding of the logistics and issues associated with undertaking island eradications. New Zealand expertise should be used to mentor and provide an overview for these operations.  The highest priority island for eradication is North East Island due to the threat its rats pose to neighbouring Lively Island (the largest rat free island in the Falklands). At 305 ha, North East Island will represent a significant scaling up of the eradication programme and the on-site involvement of at least one experienced New Zealand operator is recommended. Local efforts will probably continue to be hampered by the shortage of suitable staff able to commit for extended periods of fieldwork required to complete this work.

Contents

Introduction

This visit by three conservation staff from New Zealand was the culmination of previous discussions and a previous visit to the Falklands in February/March 2001 where the feasibility of eradicating rats from islands in the Falklands was assessed (Brown and Chadderton 2001). This assessment concluded that many islands in the Falklands group appear suitable for rat eradication and that there was no obvious reason why eradication operations should not be successful provided that proven New Zealand methods were applied. It was recommended that eradication should be undertaken on four small islands (Top and Bottom Tussac Islands and Outer and Double Islands) using methods proven successful in New Zealand. Two techniques (bait station and hand broadcasting) were proposed, in order to test their relative local merits and to train local staff in both techniques, while also hopefully ensuring success.  Nevertheless, the principal aim of this project was to eradicate rats from the four selected islands.   Secondary objectives were eradication of rats from other small islands as resources allowed, and input wherever considered appropriate to assist in promoting the benefits of rat eradication and general conservation management of islands in the Falklands.

Rat Eradication From Top and Bottom Islands

Top and Bottom Tussac Islands are owned by the Falkland Island Government, and situated in Port William near Stanley. On these islands a systematic grid of bait stations were laid out and baited with and New Zealand-made ‘PestOff 20R’ rodent baits, that contain 20 parts per million of brodifacoum, an anticoagulant poison.

Bait stations were made from 600mm lengths of 100mm-diameter plastic piping, and these were fixed to the ground with 2-3 hoops made from fencing wire. Bait stations were installed on both islands on 28 August, and deliberately left unbaited for 2 weeks to allow rats to become accustomed to their presence to reduce rat neophobia towards the stations.

On Top Island (12 ha) bait stations were established on a 30m x 30m grid, along parallel compass lines running perpendicular to a central east-west dividing line (see Figure 1).  A total of 106 bait stations were set out. All bait stations were mapped and individually numbered. All station sites were marked using blue marker tape tied into an adjacent tussac to enable easy relocation. A 30m x 30m grid is a far smaller grid system than thought necessary just to achieve eradication. However, it was pursued in the belief that more detailed information on such issues as home range size and numbers of rats on the island could be obtained from analysis of rat activity around the higher density of stations.

On Bottom Island (8 ha) a total of 31 stations were installed (see Figure 2).  A total of 23 stations were placed at 50m intervals around the coast, with no station greater than 10m from the exposed rocky shore. A further 8 bait stations were placed on a line running along the central ridge of the island, again at 50m intervals. These stations were between 30 and 50m in a direct line from the coastal circuit of stations. The 50m spacing of bait stations is more typical of a Norway rat eradication campaign, and would prove a useful comparison against the higher density used on Top Island.


Figure 1: Top Tussac Island bait stations (30m x 30m spacing).
(NB. Station positions are indicative only, field calibration showed no point of the island was more than 30m from a bait station)

Baiting on both islands commenced on 11 September 2001. Bait take began on the first night on Top Island, with 9 stations active whereas the first take on Bottom Island occurred two nights after bait was put out. Subsequently both islands showed remarkably similar patterns of bait take (see Graph 1). Bait take peaked on day 3 on Top Island (with 56% of stations active) and day 6 on Bottom Island (65% active), and ceased entirely by day 10 and day 11 respectively.

About 20 kg of bait was used on Top Island (1.6 kg/ha), compared with only 4.6 kg on Bottom Island (less than 0.6 kg/ha). The difference in quantity per hectare does not indicate higher rat densities or the greater number of rats on the larger Top Island. Rather, it probably reflects the difference in density of bait stations, meaning each rat was able to visit more bait stations on Top Island per night to take and cache bait.

 
Figure 2: Bottom Tussac Island bait stations (50m x 50m spacings)

The higher density of bait station on Top Island was expected to result in a quicker operation on this island, in that each individual rat would be able to access greater quantities of bait over a quicker timeframe.  However, despite having a station density at least twice that of Bottom Island the latter only finished one day later, and with appreciably less bait used per hectare.

 
Graph 1: Bait Take from Top and Bottom Tussac Islands

The shortness of both campaigns is consistent with the result reported for the eradication of Norway rats from Hawea Island in Fiordland of New Zealand where similar densities of stations (50m x 50m) were used (Taylor and Thomas 1989). The rapid result reflects the low densities of rats present on the islands at this time of year. Although impossible to precisely determine how many rats there were on the basis of bait take it is possible for estimations to be made. Based on New Zealand experience we can assume that it takes 3-5 days to kill the average rat.  It is also believed that dominant rats tend to exclude other rats from bait stations, and the continuous taking of baits from a station over 3-5 days indicates the activity of a single rat. Each rat, depending on home range size may dominate several bait stations. Based on the pattern of activity around the various bait stations it is considered that rat densities on Top and Bottom Islands were almost certainly no more than 2-2.5 rats per hectare, and perhaps considerably less than this. This figure is comparable with winter densities reported on Ulva Island in southern New Zealand (R. Taylor et al in prep), and with perceived winter densities on Campbell Island (D. Brown, pers obs). This equates to no more than 30 rats on Top Island and 20 on Bottom Island at the time of the operation. Density of rats is likely to increase dramatically over summer months, with the higher food resource generated from the abundance of penguin and seabird nesting activity present.

It is suspected that most tussac islands will have similar densities of rats during the winter periods, as food resources are likely to be similar from island to island, as vegetation types are often quite uniform. Slightly higher numbers of rats may occur where the coastal environment is particularly suitable for intertidal foraging. Both Top and Bottom Islands have tidal pools and a relatively accessible coastline, suggesting good foraging habitat and a higher than average rat population for a tussac island.

Poison stations were checked at intervals for 10 days after bait take had ceased. On Day 20 (9-10 days after cessation of take), ‘PestOff 20R’ was removed from all stations, and ‘Ratak’ waxy baits were placed in approximately every third station on Top Island and every second coastal station on Bottom Island. ‘Ratak’ baits were used as a longer-term, more weatherproof option, and as an added security measure if rats were for any reason avoiding the ‘Pestoff’ baits. Two baits were secured by string into each station, and a chew stick (15mm x 15mm untreated timber cut into x 160mm lengths) that had been presoaked for at least 5 days in peanut oil, was placed in the ground outside the station.

A further check of baits and chew sticks on 29 October (Day 48) confirmed no further bait take or evidence of surviving rats.

The absence of any bait take after day 11 on each island, and the similarity in take between the Falklands tussac islands and the pattern of take reported on Hawea Island where rats were successfully eradicated (Taylor and Thomas, 1986) suggests that both eradications attempts have been successful. Similar bait take records from many other successfully treated islands also back up this belief. Nevertheless, standard monitoring protocols (checks of bait stations and renewal and inspections of non toxic indicators i.e. chew sticks) should be adopted with checks continuing at least annually for the next 2 years to confirm this.

Rat Eradication, Outer & Double Islands

Outer and Double Islands are owned by Falkland Conservation and are situated within Queen Charlotte Bay, West Falkland. Outer & Double Islands were treated using the hand-broadcast method. This technique involved spreading the poison (again ‘PestOff 20R’) by hand in a systematic manner across the whole island. At set intervals a plastic scoop that held a known quantity of bait (approximately 200g of bait) was used to disperse the bait in an arc around the operator. The scoops ensured a set amount of bait could be spread at each stop (to enable a more even coverage), while also avoiding placement of human scent on the baits and in terms of operator safety negating any need for direct handling of the baits.

Double Island (9 hectares) was treated on the 1st of September 2001. Fifty kilograms of bait, or 5.55 kg/ha, was distributed in measured quantities at paced 25m intervals around the entire coast and then on several transects across the central portions of the island. Transects through the central portions of the island were judged by eye, and ran from one coast to the other. Transects were intended to be no more than 50m from the coast or from any other transect, and in reality were generally much closer than this, with four ‘inland’ transects completed on each half of the island, estimated to be no more than 200m at its widest point. Because the island is small and is basically split into two evenly-sized portions by a sandy isthmus, it was considered that eye judgement was sufficient to ensure total coverage, rather than use of grid markers or flags. Repeat transects were made over virtually all the island until the bait supply was completely used. It took approximately 1.5 hours for 2 people to spread the poison.

Outer Island was treated on 5-6 September 2001. The bait spreading was achieved using five people operating in two teams, and the process took an estimated total of 40 person-hours, with very dense tussac hampering rapid movement across the island. A total of 100 kg of bait was used on this 20-hectare island, or 5 kg/ha. (Note however, that the official area of these islands appears to be the total area of the island including bare rock around the coast. The actual vegetated areas appear to be appreciably less than this.)

Bait was cast inland at measured 25m intervals around the entire coastline, and at 25m intervals along a number of parallel lines 50m apart across the entire island. Start points 50m apart along the eastern coast for the lines were marked out previously. The lines were walked using compass bearings and a 25m measuring rope to ensure accuracy of coverage. On each line, one person would take an end of the rope, and walk on a compass line for 25 metres, as directed by the other person, holding the other end of the rope and compass at the start point. Once at the desired distance and bearing the lead person would spread the measured amount of bait, while the second person made their way to the spot for the process to be repeated. A prior attempt to set out a grid pattern of marker flags was abandoned due to interference and removal of markers by striated caracara. The two teams worked in view of each other on two adjacent lines, which provided a second accuracy check of direction of travel and therefore of comprehensive bait coverage.

A small additional amount of bait was hand-laid along the eastern coastline where tussac cover has been reduced due to historic fires. Bait was placed under remaining tussac stools or rock overhangs to minimise visibility to bird species.

Both islands were checked on 1 October 2001. There was virtually no bait remaining above ground, with the only bait located in several hours of searching being a small amount of bait cached under a tussac on Outer Island.  No evidence of rats was detected.

Gnaw-sticks (again presoaked in peanut oil) were placed out to detect any remaining rodent presence.  These should be checked and replaced on the next visit to the island, with at least annual checks for any sign of remaining rats occurring for the next 2 years.

Experimental ‘Eradications’ - Harpoon Island, Calf Islands, Horse Island and Rat Island

The opportunity to conduct ‘experimental’ eradications (with the idea of testing rat re-invasion ability) was taken on several islands theoretically within the natural swimming distance of Norway rats. Removal of rats from these islands would provide at least short-term conservation benefit, and as they were done while in the relative areas were achieved at negligible additional cost. Information gained on any subsequent re-invasion by rats will be beneficial in understanding natural rodent dispersal abilities in the Falklands and the relative vulnerability of any particular island.

Harpoon Island was treated by hand-broadcast on 1 October 2001, using 12.5 kg of bait, or 4.2 kg/ha. Approximately 400g was spread every 25m around the entire coast and on a single transect through the middle of this very narrow, long island. Distances were paced out and bait dispersed as for Double Island. Harpoon Island is more than 1 km from the nearest vegetated land (Fox Island), but there is a long rocky reef, much of which is only exposed at low tide. A gap of water between the reef and Harpoon is no more than 150m, and is filled with a very dense bed of kelp (Macrocystis pyrifera) throughout. The island had no terrestrial songbirds present, but had several juvenile striated caracara, nesting Magellanic penguins and a variety of common coastal bird species.

Calf Island (5 ha) and Calf Islet (<1 ha) were treated by hand-broadcast on 26 September 2001 using the same system as above. Approximately 3 kg of bait was used on the islet, and 20 kg on Calf Island. The tussac-covered Calf islet is approximately 200m from the mainland, and from the islet there are two similar-sized gaps of between 50-100m to a small rock reef and to Calf Island itself. Calf Island has been previously grazed and has sparse tussac around the fringes. Magellanic penguins, turkey vultures and upland geese were breeding. Only one pair of ground tyrants and a single long-tailed meadowlark were seen.

Rat Islet (shown as an un-named island of c.0.5 ha on 1:50,000 maps, but adjoining a peninsula of Saunders Island and near Half Tide Island in Burnt Harbour) was treated on the same day and by the same team and process as on Calf Island. A total of 1.5 kg of bait was hand-broadcast on this island. Rat Island is virtually connected at low tide to Saunders Island, via an extensive rocky reef. This non-vegetated and partially tidal reef extends for a distance of more than 500m between Rat Island and Saunders. Rat Island has a population of the rare ‘sea blite’ shrub.

Horse Island (5 ha) was treated using 8 bait stations set out in a circuit just inland of the coastline, at approximately 80 m intervals (100 paces). They were installed complete with bait, on 28 September 2001, and checked thereafter by David Pole-Evans. The stations were first checked two days later and all bait was taken. Poison was consistently taken for a further five days, until 5 October, when activity ceased. Approximately 2.5 kg of bait (0.5 kg/ha) was used in this operation. Bait stations and poison baits have been maintained since, with no further take noted. Horse Island is 300m from Saunders Island, but much of this gap contains intertidal rock reef, that is exposed at low spring tides. Magellanic penguins and upland geese were breeding. King cormorants roost on Horse Island, and juvenile striated caracara were present in low numbers. A pair of grass wren appeared, symbolically, the day after rat bait take ceased (D. Pole-Evans, pers comm).

Horse and Rat Islands appear to be highly vulnerable to re-invasion. However, how long it takes rats to re-invade will be an interesting and informative piece of knowledge for future rat control in the Falklands. Calf Island, its islet and Harpoon Island are less likely to be re-invaded, but on the basis of New Zealand data they are theoretically within invasion distances. Whether they are eventually re-invaded will help establish the maximum distances rats are capable of covering to re-invade islands within the Falklands. This will assist in the setting of eradication criteria in relation to how far offshore an island needs to be to be relatively secure from potential natural re-invasion. For instance Harpoon Island will provide clues as to whether rats will travel along extensive open rock reefs, and with Calf Islet and Island whether they can in fact swim 200m or so in the cold waters of the Falklands. Circumstantial evidence from the North-East / Lively Island situation suggests this distance may be sufficient to prevent most invasion possibilities.

With appropriate management (eg. permanent traps or bait stations) small ‘near-shore’ islands such as these may be able to be maintained in a predominantly rat-free state. While they are obviously not as securely ‘rat-free’ as an island further offshore would be, the on-going rodent control on these islands should create a conservation benefit that can continue in perpetuity if control efforts are maintained.

Shallow Harbour Island and Skip Rock

The un-named island (‘Shallow Harbour island’) owned by Ali and Marlane Marsh in Shallow Harbour of Queen Charlotte Bay was confirmed as rat free. There had been reports that rats were present but no sign of rodents was found. Poison bait stations left in situ for three weeks were untouched. There may have been previous rat presence, but the island is small enough (at <2ha) that random chance events may have led to a natural extinction of the population (i.e. development of a single sex population leading to recruitment failure).

Skip Rock, in Byron Sound, south of Saunders Island was visited courtesy of David Pole-Evans, and this small (<1ha) island was also confirmed as rat-free. It had extensive coverage of healthy tussac and good numbers of coastal birds, but apart from a very few Magellanic penguin burrows, no evidence of burrowing seabirds.

Effects of the Operations on Non-Target Species

Bird counts were made prior to, during and after the eradication campaigns on Outer Island and on Top and Bottom Islands. These were conducted not only to act as a bench-mark to measure the response to rat eradication, but also to detect if any species of birds were affected by consumption of the bait either directly (primary poisoning) or through ingestion of dead or dying rats (secondary poisoning).

Bird counts on Outer Island three weeks after the broadcast of baits (all effects should have manifested themselves by this time) showed that the same number of pairs of dark-faced ground tyrants, long-tailed meadowlarks and Falkland thrushes remained on the island. (One pair of tyrants seen on Double Island on 1/10/01 but not prior to this was presumed to have flown across from Outer Island). Nesting activity was evident amongst these species. Grass wrens also survived the operation, but no accurate count could be obtained, as they were not singing strongly on the day of the later count. No negative changes were noted in any population of ducks, geese, gulls or sheathbills. Numbers of turkey vultures and striated caracara were higher than initial counts but lower than the maximum number seen in anecdotal observations in the days preceding the bait spread. However, there was little indication that effect of baiting was responsible for any change in numbers (ie. no dead birds were observed). Turkey vultures were most probably less conspicuous due to the onset of nesting, and many of the striated caracara had probably dispersed back to breeding grounds in the period between counts. Striated caracara previously aggregating at Shallow Harbour farm had also ‘moved on’ during this time (A. Marsh, pers comm). However, evidence of one scavenged rat was found, with a quantity of plucked rat fur being found amongst the tussac. The feeding sign could not be attributed to any particular species, but indicated that at least one rat was probably taken by a predatory or carrion-feeding bird.

Bird counts on Top and Bottom Island also showed no detectable effect on passerines. Grass wren numbers appeared constant throughout and after the baiting, with an estimated four pair on Bottom Island and two pairs on Top Island. There was evidence of breeding on Top Island on 1/10/01 with the discovery of a neatly-opened half egg-shell in one wren territory, indicating a successful hatching. The small number of tyrants and thrushes present on both islands appeared to remain static. A small number of black-chinned siskins were also present throughout, and although highly mobile and difficult to count there was no detectable change in numbers.

Two short-eared owls were seen on Top Island, and at least one was regularly observed throughout the operation, and was still present at the last check on 29 October. The second owl was observed 9 days into the operation, but its subsequent fate is unknown. This does not necessarily infer it has succumbed to poison, as it could easily have been missed during subsequent surveys (the owls were not seen on a daily basis), or alternatively could have been inconspicuous due to nesting or could have left the island. Nevertheless, individual owls in New Zealand are known to suffer from secondary poisoning, though populations have not been significantly affected. Both short-eared and barn owls in the Falklands are likely to prey upon rats, and are vulnerable regardless of what technique is used. It will be important to record any sightings of short-eared owls on Top Island or any of the other treated islands in the next few months.

The turkey vulture population appeared unchanged, as did populations of gulls, sheathbills, ducks and geese.

Overall, it appears that the effects on non-target species were very low, and it was very encouraging to see no obvious effects on any passerine species. However, it remains a point of concern that at least one rat was known to die above ground, and another was either preyed upon or scavenged after probably having fed upon the poison. Rats may therefore be available to some predatory or scavenging birds, and two species of importance in this regard are the short-eared owl and the striated caracara. Until further evidence is available, diligence and careful observation is warranted for any eradication project where either of these species is present.

It will be important to monitor the bird populations of the islands where eradication of rats has occurred, in order to measure the level and time-span of any response to removal of the rats. Such information is vital to demonstrate the effects rats are having on the wildlife, and adds weight to justification for any future eradication proposals.

In order to gather some baseline information on use of Bottom Island by burrowing seabirds, a transect was conducted around the entire island. All the burrows in a one-metre wide transect were recorded by one person, while a second person counted a slightly more random search pattern, estimated to be two metres wide on average. The latter was, where possible, on the extreme edge of the tussac vegetation, while the 1 metre wide transect was always slightly inland of this. The transect was conducted in 50m ‘runs’, using the coastal rat bait stations as start and stop points. The total length of the transect was therefore a reasonably accurate 1150 metres. Burrows were divided into ‘penguin’ and ‘shearwater’ by subjective assessment of the size of the burrow entrance, with ‘penguin’ burrows noticeably larger than the ‘shearwater’ burrows (Table 1).

 

Magellanic penguin

Sooty shearwater

1 metre transect
(1m x 1150m)

25 burrows
(a density of 0.02 / m2 )

91 burrows
(a density of 0.08 /m2 )

c.2 metre transect
(est. average of 2m x 1150m)

21 burrows
(a density of 0.009 / m2)

198 burrows
(a density of 0.08 / m2 )

Table 1: Burrow Counts, Bottom Tussac Island, 17/9/01.

We assumed all ‘shearwater’ sized burrows were in fact those of sooty shearwaters, but it is possible white-chinned petrels or other species may also be present.

Media and Public Relations, and Open Days on Top and Kidney Islands

The trip generated considerable local interest, with numerous articles and letters in the ‘Penguin News’ in relation to the rat eradication work and the public open days. The local radio station also devoted several half-hour sections to the project, and to the involvement and reaction of a group of Stanley students who visited the island during the eradication and assisted with bait station checks.

Two public ‘open days’ were held on Saturday 22nd and Sunday 23rd September. These involved 5 launch trips to visit both Kidney Island (as an example of a rat-free island) and Top Island, to view the differences on a rat-inhabited island and to see how the eradication programme worked. These trips were fully booked with additional people unable to obtain a booking due to the limitations on space in the launches. Nevertheless, approximately 75 members of the public availed themselves of the opportunity to visit and view the eradication process first hand. The feedback was generally extremely positive.

Two school groups also visited, with a group from the MPA school visiting Kidney Island on 19 September and a group from the Stanley Community School who over-nighted on Top Island on 21-22 September and assisted in checks of bait stations.

The media publicity had the added bonus of drawing attention to the possibilities of rat eradication and the owners of several privately owned islands contacted the eradication team to discuss the potential for their islands. This resulted in the opportunity to undertake some trial eradications on small islands near Saunders Island (see earlier section). Time limitations and availability of boats prevented inspections of two other larger islands, (East and Pleasant), whose owners expressed strong interest at the prospect of rat eradication on these islands.

Island Databases

The island database was updated. It now contains a total of 599 islands including all of the named islands in the Falkland group, plus most of the vegetated but un-named islands. This database lists the name, location and size of the island, the owner, the pest species present, area of tussac, and the key wildlife species present, particularly any colonial nesting seabirds, rare species and rodent-vulnerable species.

Analysis of the database has revealed that 78 islands in the Falklands are thought to contain rats, while 96 are rat-free or most probably rat-free. Unfortunately for most islands (425 in total) information is currently not available. It is suspected however that the vast majority of this latter category will have rats present, due to their location and past history (most islands close inshore to the main islands with a history of farming, appear to have rat populations). While some islands will be too close to shore to consider rat eradication as a feasible option, it appears that in all probability there are at least two hundred islands for which rat eradication could be contemplated.

This database has been well received as it brings together a summary of existing information and identifies knowledge gaps. It should be used to highlight survey needs, and should be updated as any further information becomes available. It can be modified to suit specific needs.  It will be a useful management tool and its regular maintenance (updating fields, etc) should be a priority.

A more detailed ‘second-tier’ data field was completed for the islands owned or managed by Falklands Conservation. This summarises the existing knowledge of each island, and includes details such as flora and fauna lists, access and camping details, dates of last visits, lists any reports relating to the island, and identifies management issues. Again, this highlights the relative lack of information available on many conservation islands, but the database should provide a good basis on which to build up knowledge on each of the islands, following any future visitations.

Generic Eradication Guidelines

As part of the aim to enable local staff to undertake eradications without overseas assistance, a generic eradication guideline document has been produced. This is intended to lead people through the stages of planning and conducting a rat eradication. It describes the methods used to achieve eradications, and the various ‘do’s and don’ts’ to maximise the chance of a successful result. It should also be of value for private land-owners contemplating eradication of rats from their own islands.

Further information is presented in the draft document “Guidelines for eradication of rats from islands in the Falklands group”.

Rodent Identification and Monitoring Kit

Few individuals appeared to be familiar with how to detect rodent sign, or how to distinguish the different rat species. The former means that rodent presence or absence on some islands visited may not be able to be confirmed due to lack of expertise in the visiting party in terms of identifying definite rodent sign. The latter has created some confusion in reports (i.e. which species are present on several islands), which has implications for eradication planning and prioritisation.

The rodent identification and monitoring kit provides a basic information package, that should enable local people to use it to greatly improve skills in identifying rats and their sign.

Monitoring for rat presence is a key component of eradication operations. Monitoring over a period of up to two years following an eradication operation is a necessary step in confirming whether an eradication attempt has been successful. This kit can be used for determining whether any rats remain after an attempted eradication, or for establishing whether rats are present on any particular island where the current rodent status is unknown. 

Prioritising Islands For Eradication

A draft document is being prepared outlining the possible criteria for prioritising the islands identified as available for eradication. A meeting with conservation and government staff identified that a variety of criteria can be applied, but that protection and restoration of endemic species and the natural tussac communities should be primary factors.

However, upon analysis it is clear there remain a number of questions that need to be answered before a logical prioritisation process can occur. Any analysis or priority assignment is currently limited to those islands where sufficient biological information is known. Most islands cannot readily be included in assessments due to lack of even basic information on habitats, native species and presence/absence of rodents. In addition the lack of information on both the distribution of important native species, and their colonisation ability further hampers any analysis.

There are many possible criteria, many unknown factors, and some subjective assessments, so consequentially we may see the priorities arranged quite differently by various individuals or agencies. It is recommended a short-term strategy is prepared, based on existing information and islands currently available for eradication.

It is suggested that the driving forces behind the short-term priorities be firstly suitable training opportunities for local staff, while also focusing on most cost- and time-effective enhancement of the overall population of Cobb’s wren. As Cobb’s wren favour healthy tussac communities such a factor also ‘captures’ by default the restoration of tussac communities as a whole, which was also seen as a very important criterion.

A longer-term plan can be developed in future as better information is gathering for other islands with the Falklands.

A full discussion and a preliminary priority list for island eradications is presented in the draft document “Criteria for prioritising rat eradications from islands in the Falklands”.

There was a consensus amongst Falklands conservation staff and the New Zealand team that North East Island stands out as the highest priority for rat eradication based on existing information because it is only 150m from the highly significant Lively Island, the largest rat-free island remaining in the Falklands.  However, it is strongly recommended at least one smaller island is undertaken as a further trial and training exercise before attempting North East which at 305 ha is of considerable size for a ground operation.

Inspections Of Potential Islands For Future Eradication Projects

Several islands were visited to assess their future eradication potential:

North-East Island (305 ha)– This island is generally flat and open country, with only a small amount of taller tussac at the eastern end of the southern portion. The island has a good range of habitat types, including tall tussac, short turf, sand dunes, rocky beaches and ephemeral wetlands. Cassin’s falcon, snipe, sanderling, two tussacbirds and a short-eared owl were observed, along with breeding pied oystercatcher, steamer duck, crested duck, and meadowlark. Large numbers of Magellanic penguins and upland geese were observed. The island could be cleared by either method, but the hand-broadcast technique would be most cost-effective. This would require a minimum team of four people, and at least 1800 kg of bait (approx. £3750 worth). Such an operation would probably have to occur over several consecutive days due to the scale of the operation.  It represents a significant step up in terms of the size of islands for which eradications in the Falklands that have been attempted to date.  If it is to be attempted in the near future we would recommend that strong consideration is given to having at least one experienced New Zealand operator on site for guidance and assistance in the eradication attempt.

Burnt Island and islet (115 ha) – The island is presently stocked with a small group of cattle, and the island has virtually no tussac. It would suit a bait station operation. The lack of tussac means grid spacing could be extended to a 100m x 100m grid, in inland areas, with a 50m spacings around the coastline. The tussac islet should also have the smaller 50 x 50m spacings.  A bait station operation would provide a good indication of rat densities in non-tussac grasslands, and the results will be useful in developing techniques on larger islands (e.g. Bleaker). Total cost of bait for such an operation would be approximately £400, for 200kg of bait. A few extra bait stations may need to be purchased but Falkland Conservation has approximately 170 on hand, which may be sufficient for the task if they are available. Most are currently in use on Top and Bottom Islands but should be available from early 2002. On Burnt Island there were significant colonies of Magellanic penguins present, plus dotterels and a range of passerines including pipits. Gulls are reported to breed on the eastern spit. The tussac-covered islet has Magellanic penguins, grass wrens, and reported nesting colonies of South American terns and possibly of giant petrels.

Green Island (17 ha) – A small number of cattle are still present, but there are plans for their removal as soon as practical. Ideally rat eradication should occur as soon as possible after removal of the cattle, while access through the tall tussac is still relatively easy. The island is almost entirely covered with impressively tall tussac stools, which are still in reasonable condition despite the presence of the cattle. A hand-broadcast technique would be the best way to approach this island, and there is enough bait currently stored at Shallow Harbour farm to achieve this task.

Future Eradication Work

As a result of the two visits made by the New Zealand rat eradication team, many islands have been identified as being available or potentially available for rat eradications.

North-East Island (305 ha) is a clear priority, being perilously close to Lively Island, the largest rat-free island remaining in the Falklands. The adjacent small islands of Pete's and Hutchy's islands would also need to be treated at the same time.

As a result of the apparent success of the operations on Calf and Horse Islands, David Pole-Evans has expressed a keenness to undertake rat eradication on two more of his tussac islands, Barton (7 ha) and Big Channel (35 ha), near Keppel Island.

Burnt Island and its adjacent islet are also owned by David Pole-Evans, and are another eradication option, but this would probably require some external labour assistance to complete this 115 ha site.  It offers excellent learning opportunity with regards to rat densities in open grassland areas.

Rookery Island (25 ha), Rat Island (1 ha), Cucumber (3 ha) and possibly others such as the Channel Islands in the Beaver Island locality (all owned by Sally and Jerome Poncet) may all be highly suitable as ‘training grounds’ for development of further local experience in rat eradications. It is highly recommended that at least one smaller island as Rookery has an eradication attempt, to develop further confidence and experience before undertaking North-East Island.

Pleasant Island (150ha, in Port Pleasant, near Fitzroy settlement) and East Island (140ha, in Port Fitzroy) are also possibilities, as the owners have indicated a desire to see rats eradicated. Little is known about these islands in terms of vegetation, rat densities, and wildlife values, as field inspections were unable to be carried out in the time available.

A number of other islands are potentially available for eradication work, including:

The full list of islands potentially available for eradication work and their relative priorities are discussed in the document “Criteria for prioritising rat eradications from islands in the Falklands”.

Discussion

Rat eradication is clearly feasible in the Falkland Islands, and the level of support and enthusiasm indicated during this visit suggests that a systematic programme of rat eradications is desirable and achievable. Significant potential conservation gains can be assumed to derive from such operations if they occur. However, considerable further work is required to clearly demonstrate these benefits through monitoring, and to determine if there are any undesirable effects on other wildlife.

The ability to establish clear management directions and priorities for conservation islands in the Falklands is hampered by the dearth of information on the fauna and flora of many of the islands within the Falklands archipelago. Comprehensive biological surveys are a high priority to provide sufficient baseline information from which to make informed management decisions. As an example, the actual status and distribution of many species (particularly burrowing petrel species) is unclear, and this restricts ability to formulate clear island management strategies and priorities to best assist in recovery of populations of these species.

Rat eradication is only one small portion of a much wider conservation management strategy, and should not be seen as the goal itself, but rather as a means to achieve desired conservation outcomes. Preservation of endemic, threatened and native species, and the restoration of natural communities, particularly of the much-diminished tussac island ecosystem, are likely to be the key objectives resulting from rat eradication efforts.

Recommendations

Acknowledgements

Special thanks to Darren Christie and Nic Huin of Falkland Conservation for their invaluable assistance with the eradications and general fieldwork and for their company in the field and Stanley. We hope Darren’s aquatic heroics will receive a more appropriate reward! Many thanks are due to Becky Ingham and all the staff of Falklands Conservation for their good humour and excellent logistical support.  Ali Marsh was of enormous assistance in the trials of getting to and from the islands in Queen Charlotte Bay, and also in the actual eradications on those islands. Ali and Marlane’s warm hospitality made Shallow Harbour feel like a home away from home.

Thanks to David Pole-Evans for the transport, accommodation and generous hospitality provided whilst on Saunders Island and its offshore islets.

Tony Blake, Dave Clark, Zach Stevenson, and Mike and Scotty on the Speedwell FIC launch all provided transport at various times to and from Top and Bottom Islands. Scotty’s enthusiastic assistance with the many landings was above and beyond the call of duty (as well as his waders) and deserves special mention.

Shane Clark provided safe passage to and from North East Island, and Ian Bury once more provided tools and equipment as well as permission to visit North East Island.

Thanks also to Des Vanguard of the PWD who assisted for several days on Top Island, and to the students and teachers of the Community School who also helped out.  Finally thanks to Tom Eggeling, Gordon Liddle, Sally Poncet and Robin Woods, for sharing their collective wisdom and experience.

References:

Brown, D.A.; Chadderton, L.C.  2001. Investigation of the potential for rat eradication in the Falkland Islands. Report on preliminary visit, 27 Jan-3 March 2001. Unpublished report to Falklands Conservation.

Taylor, R. H.; Thomas, B.W. 1989. Eradication of Norway rats (Rattus norvegicus) from Hawea Island, Fiordland, using brodifacoum. New Zealand Journal of Ecology 12: 23-32.

Taylor, R.H.; Chadderton, L.; Thomas, B.W. in prep. An eradication campaign against Norway rats on Ulva Island; and a trial of the ‘rolling front’ technique.

APPENDIX 1. Bird Counts from the Eradication Islands

Top Island
(Rat eradication occurred between 12 and 21 Sept. 2001) 

Formal Bird Counts:

 

12/09/01

1/10/01

29/10/01

Rock shag

147

80

80

King cormorant

2

1

60

Black-crowned night heron

1

1

 

Upland goose

4

2

7 males ‘standing off’ (females nesting?)

Kelp goose

 

6

2 males

Steamer duck

5

4

0

Turkey vulture

11

2

15

Crested caracara

 

 

1

Pied oystercatcher

0

0

2

Black oystercatcher

2

0

0

Sheathbill

10

7

3

Dolphin gull

2

 

 

Kelp gull