Falklands Conservation

Criteria For Prioritising Rat Eradications From Islands in The Falklands

Discussion Draft, November 2001
Prepared by Derek Brown

Summary

Potential criteria for prioritising a rat eradication programme on the many islands in the Falklands are discussed.  Restoration of tussac communities and recovery of the Cobb’s wren population are key criteria, but many other criteria will also have a contributing influence in the selection process. The weighting of relative criteria and the whole process is somewhat dependent on yet to be formally established management objectives and goals. The relative achievability of any eradication is also an important consideration in the short-term future.

The prioritisation process is severely hampered by lack of information on many islands, and further survey work is clearly warranted. A short-term priority list is nonetheless established, not only for eradication projects but also for survey work to enhance the selection process.

Islands selected for possible eradications include North East Island, Rookery (Beaver), Green (Queen Charlotte Bay) and possibly the North West Islands and the Arch Island group. Many other islands are available, and the prioritisation process may need to remain flexible to accommodate new information as it arises and also the resources available at any given time.

The whole process of prioritisation of eradications and of funding and co-ordinating the eradication projects should occur from a central controlling body in order to streamline the process and maximise the conservation gains.

 

Contents

 

Background

As a result of recent trips to the Falkland Islands by New Zealand island management and pest eradication specialists, several islands from within the Falklands have most probably had rats successfully eradicated from them. The technique developed in New Zealand for this purpose is seemingly applicable to virtually all of the rat-inhabited islands in the Falklands. An island database has also been generated, and despite a relative lack of information it is considered that the majority of the Falkland’s 778 islands have rats present. Consequently there appears to be far more suitable island candidates for rat eradication than could practically be achieved in the foreseeable future, within realistic funding expectations. Even at the present moment, the respective owners of approximately 30 privately owned islands have indicated a desire to rid their islands of rats. A number of government-owned islands are also suitable candidates for rat eradication.

Protection of nature conservation values, education and tourism are all potential reasons for rat eradication. Nature conservation is the prime reason for rat eradication considered here. Clearly some form of selection criteria is necessary to prioritise the list of islands, in order to make the greatest conservation gains in the shortest practical timeframe.

The New Zealand rat eradication team was asked to develop criteria for selection of islands, and some recommendations about relative priorities for potential eradication projects, the results of which are presented in this discussion document.

 

Part I - Criteria

At a meeting with Falkland Conservation and Falkland Island Government officials in October 2001 it became clear that ownership per se was not a key issue for selecting which islands received priority for rat eradication. Nature conservation, education and tourism were all considered as valid reasons for rat eradication. However, general favour was found for the guiding objective to be that of maximising the overall conservation benefit.

Obviously before an island can even be considered it should be known to have rats present, and the owners should be willing to see an eradication occur. Thus, many islands cannot currently be considered until further survey work or information gathering occurs, and/or the owners indicate a desire to see the rats eradicated. The ‘pool’ of islands currently available for prioritisation is therefore relatively small in relation to the total number of islands potentially available.

NB. Only islands 250m or more from the mainland or other rat-inhabited islands have been considered. Islands closer than this are considered too high a risk due to the potential for natural re-invasion and should not initially be considered until further information on minimum re-colonisation distances becomes available.

The following is a list of possible criteria on which to base a priority assessment and listing:

1.      Suitability for re-establishment of threatened endemic species

2.      Suitability for re-establishment or population enhancement of other endemic taxa

3.      Suitability for other native wildlife, including those of restricted distribution or for which the Falklands population is of considerable importance.

4.      Restoration of natural tussac ecosystems, or other rare community types

5.      Risk to neighbouring rat-free islands of facilitating or increasing risk of invasion.

6.      Native species composition and diversity,  and presence of particularly rat-vulnerable taxa

7.       Proximity to currently rat-free islands, for ease of natural re-colonisation of endemic or native wildlife

8.      Linking sub-populations of endemic or native species

9.      Restoring local biodiversity to geographic areas within the Falklands

10.  Presence of unusual species or communities - representativeness versus uniqueness

11.  Size of the island – ability to hold sustainable populations of species.

12.  Quarantine security– if rats are taken off, what are the relative risks of  re-invasion?

13.  Security of ownership and/or future management intentions

14.  Ease of operation – cost per hectare, ease of access, terrain, size, vegetation, suitability for staff training, logistics, owner participation

15.  Non-target risks

16.  Educational potential and opportunities

17.  Tourism potential

18.  Potential to remove other predator species in a one-off operation

Discussion of the Individual Criteria

1.      Suitability for re-establishment of threatened endemic species

The only IUCN officially threatened taxa in the Falklands are:

-         Cobb’s wren, listed as an IUCN ‘vulnerable’ species, endemic to the Falklands at species level.

-         The black-throated finch, a ‘near-threatened’ endemic race.

It is considered that this criterion should be a principal driving factor in island selection.

However, the black-throated finch is widespread over the entire Falklands group, and although rat eradications may generally benefit this species it does not appear to rely heavily on offshore islands or tussac communities for its survival.

Enhancement of the overall Cobb’s wren population is perhaps the key factor by which to drive island prioritisation, at least until its overall population has become secure enough to remove the species from IUCN listings. By focussing on the one key species, tussac island ecosystems will benefit in general. However, it is our impression that habitat restoration via rat eradication may have to be followed by active translocations of Cobb’s wren to create the new populations, as it is considered that the natural dispersal ability of this species is limited to a few kilometres. If the goal is species recovery, then active management is warranted and should be implemented as soon as practicable after success of a rat eradication has been confirmed (e.g. translocations of Cobb’s wren to Outer Island should be considered within the next few years).

Islands with apparently suitable Cobb’s wren habitat, i.e. boulder beaches and healthy tussac, should be of high priority.

2.      Suitability for re-establishment or recovery of other endemic taxa

Endemic but officially non-threatened taxa include:

-         short-eared owl, an endemic race

-         tussacbird, an endemic race

-         dark-faced ground tyrant, an endemic race

-         grass wren, an endemic race

-         Falkland thrush, a probable endemic race

-         Long-tailed meadowlark, an endemic race

-         Black-chinned siskin, most probably an endemic race

All of these species are relatively widespread around the Falklands, and most appear sufficiently mobile to reach most islands. However, for the most immediate population response, especially for tussacbird and short-eared owl, islands close to source populations would be of higher priority. The short-eared owl in particular would benefit most from rat eradications on islands that are likely to be quickly re-colonised by small seabird species such as storm petrels, diving petrels and prions.

The status and distribution of invertebrate species is not well known, but it is known that some larger invertebrate species such as camel crickets are rat-vulnerable. Further survey work is clearly warranted to identify ‘at-risk’ invertebrate taxa or communities, and their relative distribution within the Falkland Islands.

The effect of rats on endemic species of plant is unclear and may also warrant further examination. New Zealand examples suggest rats can have effects on the abundance of individual plant species and on community composition.

3.      Suitability for other native wildlife, including those of restricted distribution or for which the Falklands population is of considerable importance.

Native (rather than endemic) species of restricted distribution include:

There appears to be a negative correlation between rodent distribution and that of striated caracara. The majority of islands of importance as breeding sites for the species are rat-free. This is possibly related to the degree of modification and the relative isolation of such islands, but may also reflect the food resources available to the caracara from the populations of small seabirds on such rat-free islands.

There is stronger evidence that rat presence greatly affects the distribution, abundance and breeding success of the seabird species listed above.

The Falklands may collectively house the most important breeding sites in the world for thin-billed prions (Croxall 1984, cited in Stattersfield et al, Endemic Bird Areas of the World). Active protection or restoration of breeding habitat for this species would therefore have considerable international importance.

Distribution and status of the petrel, prion and shearwater species is unclear due to insufficient information from many islands and island groups. It is presumed that islands with remnant populations of seabirds, or those in close vicinity to rat-free islands with seabird populations will have the greatest prospect for re-colonisation by some of these species. Burrowing seabirds are probably an intrinsic component of most tussac island communities, but it is difficult in most cases to establish relative priorities on the basis of the scant existing information.

Other native species for which the Falklands are a stronghold include the ‘near-threatened’ ruddy-headed goose, as well as the black-browed albatross and various penguin species, but these are unlikely to be affected to any major extent by rats. Little information exists on the status of Falkland Island invertebrates.

4.      Restoration of natural tussac ecosystems, or other rare community types.

Tussac as a community type has been vastly reduced in area by grazing and burning over many years. It is a habitat of primary importance to many native and endemic species of wildlife, and one that in its natural (rat-free) state is rare. Those islands that still have both quality stands of tussac and a rat-free status are amongst the most important of the Falklands nature reserves (e.g. Beauchene, Kidney Island and the Jason Islands). Presence of extensive areas of healthy or actively regenerating tussac should be a highly valued aspect in determination of priorities.

The current state of the vegetation has some bearing on suitability for a range of species – many rely on a healthy tussac community to survive. Preference should be given to islands with considerable proportions of healthy tussac that has been retired from grazing. (From a practical perspective however, eradication may be considerably easier and more cost-effective on an island without dense tussac)

5.      Risk to neighbouring rat-free islands of facilitating or increasing risk of invasion.

There are few examples of this, but the North-East / Lively Island situation is a classic illustration. North-East Island, with Norway rats, is barely 150m away from the closest point of Lively Island, the Falklands’ largest rat-free island. The inability of rats to have crossed the gap as yet may be simply attributable to chance events, and rat invasion of Lively could potentially happen at any time.

This criterion may also influence whether a whole cluster of islands is treated at once, rather than individually, to reduce any risks of natural re-invasion between close neighbouring islands (e.g. individual islands within the Arch Island group).

6.      Native species composition and diversity, and presence of particularly rat-vulnerable taxa.

Some species, such as sooty shearwater and tussacbird may survive in reduced numbers on islands with rats. If such species are currently present it increases the value of the island, as the response to rat removal will be almost immediate. There will be no time delay often inherent in the natural re-colonisation process, and there would be no need for assisted translocations for these species. This criterion is similar but distinct from point 7 (below).

7.      Proximity to currently rat-free islands, for ease of natural re-colonisation of endemic or native wildlife.

This may be particularly applicable in the case of Cobb’s wren, as it appears the species is a relatively poor flier. There is little evidence that it crosses large expanses of water, and its re-colonisation ability across water is unclear. However, single wrens have sometimes been observed at Beaver Island in winter (S.Poncet pers comm to R.Woods), originating possibly from islands near New Island some 2-3 kms away from Beaver. Similarly, wrens have been seen on West Point Island, probably originating from Gibraltar Rock, 2.4 km away or Carcass Island, 9 km away (R. Woods, pers comm), indicating some ability to travel a few kilometres over water. More distant islands may be out of reach for Cobb’s wren, but if active translocations are planned this criterion is not so relevant for Cobb’s wren at least.

However, close proximity to existing burrowing seabird colonies is likely to improve the prospect for rapid natural re-colonisation by such species, which are presumed to be considerably more difficult to translocate successfully.

Therefore, it would be beneficial if candidate islands are in relatively close proximity to rat-free islands, to act as an expansion of habitat for existing sub-populations of wrens or seabirds

It is considered most other passerines are sufficiently mobile to colonise all but the most isolated of islands. Nevertheless, whole community recovery will be much quicker if the island is near to currently rat-free islands.

8.      Linking sub-populations of endemic or native species.

Again, this may not be an issue apart from relatively poor fliers such as Cobb’s wren and tussacbird. For example, it is highly likely that the Kidney & Cochon Island populations of Cobb’s wren are physically and therefore genetically isolated from other sub-populations. As the natural re-establishment distance is small, perhaps only a few kilometres, and most populations of wren are in widely separated geographic groups, this criterion would be difficult to apply.

However, tussacbirds are thought to be able to traverse the gap between Kidney Island and Top Island (approximately 5 km), and are probably capable of travelling further than this, though some sub-populations may still be effectively isolated. Targetting a cluster of islands that collectively will support a large and viable population is therefore considered beneficial, as is any eradication that may create a link between currently isolated sub-populations.

9.      Restoring local biodiversity to geographic areas within the Falklands.

A counter-argument to expansion out from population centres (as per criteria 6-8) may be to focus on re-establishing species such as Cobb’s wren, tussacbirds or small seabirds to geographical groups of islands where they have previously been extirpated through the establishment of rats. Islands within the Falkland Sound or Choiseul Harbour may be examples of this. While possibly of lower short-term conservation gain, the educational and political gains to be made may be to the best long-term advantage. The re-introduction of such species to specific areas may provide landowners of the area with an appreciation of ‘what has been lost’ and may encourage further rat eradication efforts in that area to expand upon the newly re-established populations.  Such islands in sheltered waters near Stanley or larger settlements, with relatively easy access, may also provide the best opportunities for education.

10.  Presence of unusual species and/or communities or a wide diversity of habitats - representativeness versus uniqueness.

An island with special or unique features, e.g. rare vegetative communities, or a multitude of habitats such as freshwater ponds, feldmark, or Hebe shrubland may have priority over another island simply with ‘representative’ features common to many tussac islands within the Falklands. Currently information is generally lacking on such features, and their relative importance. Further information through field survey is required. Typically though, larger and taller islands tend to hold a wider variety of habitat, but will be technically more difficult to clear of rats, through complications in ensuring adequate ground coverage and the treatment of steep cliff areas.

11.  Size of the island’s habitats – the ability to hold sustainable populations of species.

The size of the potential habitat is an issue, particularly if the species is not highly mobile and the location is isolated. The island (or cluster of islands) should by preference have the ability to hold a self-sustaining population of the species particularly targetted for re-establishment, though this is generally relevant only to those species with poor dispersal abilities or very conservative breeding habits.

The precise level of such a ‘self-sustaining’ population is unknown in most cases, though existing local examples and theoretical modelling may assist in this analysis. Nevertheless, it is clear the greater the size or quality of the habitat the greater the overall benefit.

12.  Quarantine security – if rats are taken off, what are the relative risks of them re-establishing?

Islands that have regular human visitations, are still farmed or have resident people will have higher risk of accidental rodent introduction through the importation of necessary foods, equipment and supplies by boats. With appropriate quarantine measures this becomes a lesser issue, but consideration should be given to how easy it would be to implement effective quarantine procedures. With conscientious residents, inhabited islands may actually be at an advantage in that any potential rat invasion is likely to be discovered more quickly, and as a result is more likely to be acted upon swiftly before a population can establish. However, some quarantine measures may be beyond the control of the landowner, e.g. the extent of any rodent quarantine and control efforts on the supply and tourist boats visiting.

13.  Security of ownership and/or future management intentions.

The future management intention is a key criterion. As rat eradication is a conservation measure it should go hand in hand with general restoration and protection of habitats on those islands. For example, the Cobb’s wren is unlikely to benefit from rat eradication if the island is still being grazed and is largely devoid of tussac. Privately owned islands are also at a slight disadvantage as they are more likely to be sold on, perhaps to new owners with development intentions. Before any external support is provided for an island eradication, the current owner should confirm the future management intentions for the island in question, and preference should obviously given to the islands intended principally for conservation purposes.

14.  Ease of an eradication operation – cost per hectare, ease of access, terrain, size, vegetation, logistics, owner participation.

While not an ecologically based criterion, it nevertheless is an important consideration, especially in the short-term when the success and cost-effectiveness of rat eradication operations needs to be highly visible in order to win support and further finance. Small islands are likely to be more achievable and can be ‘picked off’ with a smaller pool of resources and personnel at any given time – e.g. four small islands may be easier to achieve than one large one of the same overall size. Operations on smaller islands tend to have less scope for errors and potential failure, and are more suitable for gaining local training opportunities and experience. In a similar vein, there may be significant differences in the ease of an eradication project on islands of the same size, due to logistical factors such as access, terrain, and ability for the owner to provide labour, financial support or to assist with access.  With a limited amount of external funding likely to be available, it may be pertinent to initially target those islands that can be done for the lowest cost per hectare. If there are no other important over-riding criteria, then this becomes a serious consideration.

Some islands or island groups appear to be more suited for aerial (helicopter) applications of bait. No appropriate helicopters or suitably experienced pilots are currently available, but could well become so in the foreseeable future. Exploration of this option is worthwhile, and could lead to re-prioritisation of islands and a far more rapid progress in terms of numbers of eradications achievable in any given time period.  Access to a helicopter and bait spreader would lend itself best to clearing larger islands and whole island groups (e.g. Beaver group) and would require a re-prioritisation process to access the most efficient and productive way to use this resource.

15.  Non-target risks

The presence or absence of vulnerable non-target species may influence priorities for selection of islands. Currently the indications are that there may not be any significant non-target issues to contend with, but further trials and monitoring is necessary to be confident of this. If during further eradication work a species (e.g. striated caracara or short-eared owl) shows itself to be vulnerable during rat eradications, its presence on a prospective island may be a negative influence on the islands relative priority, unless the effects can be securely mitigated.

16.  Educational considerations.

There may be distinct advantages to focus on areas where the greatest educational gains may be made, i.e. highly developed areas or near concentrations of people, to demonstrate the conservation gains that can be made in a highly visible location. For example, focussing on an island in an area where an eradication has not occurred before (e.g. Falkland Sound, Choiseul Sound, Port Stephens areas) may draw the interest of those in the vicinity, who may see that something positive for conservation is being carried out in their area. Islands accessible to local school children, and possibly also suitable for their participation, should be highly rated for the ‘hands-on’ educational opportunities they could provide. This criterion may be one that is satisfied by the selection of just one or two appropriate islands and its relative importance may be reduced after that.

17.  Tourism potential

An island that is rat-free clearly has advantages in terms of nature tourism, in that a greater range and abundance of species can normally be expected. Given that the rat-free tourism islands (e.g. Sealion and Carcass) are often fully booked and are amongst the most sought-after destinations in the Falklands, there is considerable scope to expand tourism opportunities by creating further rat-free sites. While creating economic and educational opportunities, conservation gains on ‘tourism’ islands may be more difficult to achieve than on purely ‘conservation’ islands through possible conflicts between the relative management needs. As examples, sensitive species may be deterred or disturbed by tourists, and there is probably a greater risk of accidental introduction of pests back onto the island through significantly higher amounts of traffic to the island.

18.  Multiple pest species presence (the potential to remove other predator species in a one-off operation).

Rats are not the only pest species present in the Falklands, and often they occur in conjunction with other introduced pests such as rabbits or foxes. The presence of other such species limits the value of a ‘rats-only’ eradication, so on such islands there is a choice to either lower its priority as a result, or focus on a multi-species eradication. Rabbits are highly susceptible to the same bait used for rats, and foxes are almost certainly in the same category, though this has yet to be proven. However, it is known that a small percentage of rabbits survive such baiting operations, and intensive follow-up is required to ensure eradication. The same case probably applies to foxes until further information is available. While many multi-species eradication campaigns have been successful, the percentage of failures or part-failure (i.e. one of the two species is not eradicated) is higher than for single-species eradications. The rat-rabbit combination has rarely been attempted, and the rat-fox combination never, to our knowledge. Complications arise through having to re-calculate bait requirements, additional bait costs, and having to split the focus of the operation with possible compromises having to be made to accommodate the two different species. Additional to this is the knowledge that if the project fails to eradicate one of the species, few conservation gains will have been made. For this reason, islands with more than one pest species should be negatively weighted in terms of prioritisation, at least in the short term.

 

Part  II - Discussion

General Discussion

Unfortunately there is often a dire lack of information about many islands, and consequently about a range of important species, that hampers the ability to determine priorities for management action. For most islands in the Falkland Island database there is insufficient information currently available to make any assessment of their relative conservation value or of their priority for eradication of rats (in many cases even the presence of rats needs to be ascertained).

Some excellent survey work has been achieved in the past, e.g. Strange et al’s tussac survey and Woods’ striated caracara surveys, but clearly there is a pressing need to conduct general biological surveys of many islands for which information is minimal or entirely lacking. This should be a high priority for government and/or conservation agency funding. First priorities should be the islands currently in Government or conservation ownership, but should encompass as many islands and geographical areas as possible.

Key objectives for island surveys should include:

Some suggestions for islands requiring biological surveys include:

(NB. This is not a comprehensive list, but is indicative of some islands with possible restoration potential but for which there is very little ecological information, including whether rats are present or not. Islands known to hold rats but also requiring surveys or collection of further information are listed in the group of islands under the headings of “unknown priority – further information required” or “other possibilities”, in Section III of this document.)

Some species-focussed rather than island-focussed work is also desirable. For example, a better understanding of the dispersal ability and general ecology of key species such as Cobb’s wren would be beneficial, in order to manage the process logically and to maximise the conservation outcomes for the species. Issues such as the ability to successfully capture and translocate Cobb’s wren and tussacbird are also worthy of investigation, and it is recommended trials be carried out as soon as practical, perhaps using Outer Island as the trial, with possible source populations on the Passage Islands.

Nevertheless, there is sufficient information on which to set short-term priorities based on the islands known to be currently available for eradications. The islands known to be ‘available’ for eradication work are prioritised, on the basis of existing information, in Section III below.

The prioritisation of islands in terms of potential conservation gain is based on the assumption that there would be a single core funding or resourcing agency. This may not always be the case, as island-owners may in some circumstances circumvent the priority list by undertaking privately-sourced funding for eradication projects on their islands. While such efforts should be encouraged by Falkland Conservation and by the Falkland Government, they should not compromise the ability to carry out any scheduled priority programme. That is, skilled labour or resources should not be deferred to assist in non-priority islands unless there is some clear training or other value in doing so.

Over time, priorities may change (e.g. Cobb’s wren may recover enough to be removed from threatened species lists) and then priorities should be re-addressed. Information on other islands or further information on the current suite of islands will arise as further surveys are conducted. For example, rare invertebrates maybe found on some islands. This would necessitate their subsequent inclusion or reassessment in the prioritisation listing), Consequently priority lists will have to be regularly re-examined.

We have deferred from any attempt to establish any scoring system for the islands. Most islands have key information lacking, thus making any assignment of a score an incomplete and therefore inaccurate and potentially misleading process. The relative weighting of any given criteria is subjective as is the determination of ‘points’ within each criterion. Until considerably more information is available on most islands a scoring system is more likely to create debate about the scoring system rather than on the relative merits of the islands themselves.

 

Part III - The Priority List

Top Priority Islands

North-East Island (plus ‘Pete’s & ‘Hutchies’ islands) – a significant risk to Lively Island, and the only rat-inhabited island in the Lively Island cluster. Successful eradication would mean an entire geographical group would be rat-free, with significant benefits for maintaining viable breeding populations of many species. The island has a wide range of habitats (including recovering tussac) and species.

Green Island (Queen Charlotte Bay) - part of the Queen Charlotte Bay group of islands, including the recently treated Outer, Double and Harpoon Islands and the rat-free Shallow Harbour island. Green Island (17 ha) is the only rat-inhabited island remaining in the group (assuming recent eradications are successful) and collectively the islands would create a significant habitat for Cobb’s wren, tussacbird, short-eared owl, and seabirds such as sooty shearwater, in a fairly isolated geographical cluster of islands. Availability is dependent on the prior removal of cattle, which is intended but as yet not achieved.

Rookery Island (plus Cucumber and Rat Islands, Beaver group) – a low-lying island of 26 ha with medium quality tussac and extensive boulder beaches suitable for Cobb’s wren along its eastern coast. It is an island of good size, accessibility, terrain and vegetation to target as an achievable and valuable training exercise while also providing significant conservation benefit. Rat (1 ha) and Cucumber (3 ha) are in close proximity and should be done at the same time.

Second Priority Islands

First Passage - a large island (750 ha), with some limited tussac at its northern end, recovering since sheep were removed in 2001. In close proximity to the other Passage islands which are rat-free. Removal of rats from this island will create another complete geographical island ‘cluster’ free of rats. Re-colonisation of native species may be rapid due to the proximity of strong populations on the rat-free Passage islands. The Passage Islands could well have significant petrel, prion or shearwater breeding colonies.

New Island - this 2363 ha island is a very high priority in terms of conservation values, but the terrain of this island makes an eradication project considerably more difficult, and consequently may not be the best to take on without first developing appreciable experience on ‘easier’ islands. Use of a helicopter for bait spreading would be advantageous to ensure effective coverage of the many cliff areas, so the island may not be an option for ground-based operations at this stage. Thin-billed prions remain in good numbers on New Island, and it is perhaps the largest of only a few significant colonies for this species in the Falklands. This population may be under significant strain due to rat predation of chicks. Globally this population may be the most important single breeding site for the species. Relatively rapid re-colonisation of tussacbird and Cobb’s wren may be expected from the rat-free islands offshore. A very wide range of wildlife species is still present, including sooty shearwater, diving petrel and white-chinned petrels. New Island still has outstanding wildlife values, probably far higher than any other rat-inhabited island. This is in part due to the rat species present (ship rat rather than Norway rat), and part to the island’s inherent and traditional value as a seabird breeding ground. The presence of cats and rabbits is a further complication, and if rats are targetted these species (particularly cats) need to be incorporated into eradication plans.

Little Coffin Island, Skull Bay Island, Channel Island south and Channel Island north (in the Beaver Is group) – relatively small tussac-dominated islands (24, 5, 28 and 26 ha respectively) in one geographical area. Rat eradication would be relatively easily achievable on any or all of these islands, although the steep terrain may add some complications. The relatively healthy tussac communities mean significant conservation gains would result from successful eradications. However, most appear to have limited potential habitat for Cobb’s wren, and may be quite isolated from burrowing seabird populations for potential re-colonisation purposes. The Channel Islands are reported to have impressive flower meadow communities (S. Poncet, pers comm), and may be of considerable importance in this regard, but this requires confirmation.

Barton Island - an island of 7 ha near Keppel Island with excellent tussac, reputedly never grazed. However, Cobb’s wren habitat is limited. The island is in relatively close proximity to Big Channel Island, also considered for eradication, so that the two may collectively be considered as one habitat. Owned by D. Pole-Evans, who may undertake the eradication in a private capacity.

Lower Priority

Burnt Island and islet (Saunders Island) – Burnt Island (115 ha) is grazed and would have little value for tussac-inhabiting species, but the adjacent islet has 3-4 ha of reasonably healthy tussac. These islands may however be an ‘easy’ training exercise (in practical and logistical terms) that could be used to hone local eradication skills using the bait station method.  Such an operation may also be informative with regards to how rats are using open grassland area, which may have implication for other larger eradication operations with similar habitat.

Governor Island – a moderate sized island (250 ha) but with limited tussac, though this will be improving over time due to absence of recent grazing. Several beaches suitable as good Cobb’s wren habitat are present.

Bleaker Island (plus First & Second islands) – a very large (2070 ha) but relatively easily treated island, with topography, vegetation and vehicle access all lending themselves to facilitate a seemingly straightforward ground-based operation. There is a wide range of habitats, including wetlands and a small area of tussac on the islets and the main island, including some active tussac restoration. Large areas will continue to be grazed, reducing the value for tussac restoration and some associated species. Tourism may also be a focus for the owners, with possible conflicts between tourism and conservation needs. Relatively rapid re-colonisation of seabirds may be expected from the presumably rat-free islands situated offshore. However, there may be limited Cobb’s wren habitat until the tussac recovers and expands. Cobb’s wren may be present on the offshore islands but this needs to be determined through surveys.

Staats Island – grazed by guanaco and foxes are present, making recovery of vegetation or native wildlife unlikely unless both these species were also removed. The rugged terrain and 400-hectare size of this island is more suited to helicopter application of bait when this becomes possible. Foxes would most probably be killed through secondary poisoning, and it would be advantageous to target both rats and foxes in the one operation.

Tea Island – Foxes are present, and the rugged terrain of this 300 ha island is more suited to helicopter application of bait when this becomes possible. Excellent potential Cobb’s wren habitat and good tussac on the eastern portion of the island, but eradication of foxes would also be necessary to permit conservation gains to be made.

Stick-in-the-Mud Island – a small (4 ha) tussac-covered island where eradication would be easily achievable, but the distance from Weddell is marginal (only 150m) and rat re-invasion could occur. However, it would be sensible given its small size and the relatively small degree of effort required to treat the island if in the Beaver Island vicinity for other eradications, but only as a test of re-invasion distances and rates.

Beaver Island, plus close outlying islands (Chain, Stinker, & Split) – this is a very large island (c.4,000 ha), logistically difficult or impractical to treat without use of helicopters to spread the bait. The smaller islands must be treated at the same time as they are almost certainly within rat re-invasion distance.

Unknown Priority - Further information required

North-West islands – Likely to be of high priority, being of a moderate size with generally good tussac, but whether these islands have suitable cobble beaches or other habitat for a viable Cobb’s wren population needs to be confirmed. The two islands may need to be treated as one sizeable (>100ha) unit as they could be within rat re-invasion distance of each other. However, the islands could be of significant geographical importance, to re-establish a natural and rat-free tussac island community in the Falkland Sound area, where currently no confirmed ‘rat-free’ islands are known, apart from George and Barren Islands, to the extreme south of the Sound. The North-West islands are large enough to hold good populations of many species. These are the only Falkland Conservation-managed islands of any size that are known to be inhabited by rats. These islands should be re-prioritised following the planned surveys in the 2001/02 summer.

Arch Island group (Big Arch, Natural Arch, Pyramid Rock, Clump, Tussac and Sand Bay Islands, Albemarle Rocks and Peat Island) – rats are known present on the larger islands and are probably present on all, though this needs confirmation. The extent and quality of tussac and wren habitat needs investigation, though it is suspected that collectively this island group has high conservation value and even greater potential. Peat Island, Tussac Island and the Albemarle Rocks group may be treated as separate operations, but it is logistically sensible and recommended that all in the Arch group proper are treated all at the same time. Likely to be of high priority, but surveys are required to confirm this.

Tussac Island (Arch Is group) – an 18 ha island with excellent tussac. Can be done separately to the rest of the Arch Island group due to sufficient isolation from them. Extent of suitable Cobb’s wren habitat unknown, and should be confirmed. Likely to be a high or second priority island, and this is dependent on extent of potential wren habitat. Logistically it would be sensible to treat in conjunction with Sand Bay Island (see below), or alternatively as an adjunct to any operations in the Beaver Island area.

Sand Bay Island (un-named island on 1:50,000 maps, in Arch Island group) – a 19 ha government-owned island with excellent tussac and extensive habitat suitable for Cobb’s wren. It appears to be approximately 300m from Big Arch Island (this needs confirmation), which may be only marginally safe to prevent natural rodent re-invasion. Apart from this factor, it probably deserves a high priority on its individual merits, but it may be logistically easier to treat the entire Arch Island group at the one time if a helicopter becomes available.

Bense and Little Bense – quality of tussac needs checking due to presence of rabbits, which may also complicate eradication efforts. Maximum benefit would accrue only if rabbits were also targetted for eradication, and they may prove more difficult than rats, based on New Zealand experience. Suitable cobble beaches or other habitat for Cobb’s wren needs to be ascertained. Both islands would need to be done simultaneously as they are tidally linked, making it a large operation (150+ ha). Probably of lower priority because of the presence of rabbits, but if surveys show good wren habitat and tussac quality this rating could improve.

East Island – an island of appreciable size (140 ha), offering opportunity to re-establish wildlife populations in an isolated geographical area. The quality and extent of tussac needs investigation, as the island is still being grazed by horses, and may still be so in future. Some question remains as to rodent status on the island and this needs to be verified. The reported presence of tussacbirds suggests a rat-free island is nearby, and re-colonisation by some rat-vulnerable species such as this may be rapid if eradication occurs.

Pleasant Island – The distance of this island offshore may be sufficient but could be marginal to prevent re-invasion of rats by swimming. Investigation of the precise margin between the island and the mainland is needed. Further experience may help define what is a ‘safe’ distance in the Falklands. Geographically it may be in an ideal position for development of a public/educational role – it is a sizeable island (150ha) in a relatively sheltered harbour for easy access, and only a short distance offshore. Quality and abundance of tussac needs to be assessed, as does potential habitat for Cobb’s wren.

Big Channel Island – a 35 ha island near Keppel Island with some tussac around the cliff edges. Limited suitable habitat for Cobb’s wren. The owner (D. Pole-Evans) is possibly keen for eradication to occur, but has some concerns over effects of removal of rats (as a prey item) on the short-eared owl population.

Other Possibilities

Ten Shilling Bay Islands, East and West – considered to be of ‘high conservation value’ but owners (Robertson’s of Port Stephens) have not been approached regarding their support for eradication or their future management intentions. Both islands would have to be done simultaneously as they are linked by reefs. They are sizeable islands (42 & 30 ha), with good tussac cover and suitable Cobb’s wren habitat (R. Woods, pers comm). If the owners are agreeable to rat eradications and intend managing the islands for conservation purposes, they can be re-assigned as a high priority.

Outer Triste Islands – 3 islands of 18 ha, 9 ha and 1 ha, with some tussac and good beaches for wren habitat. Tussacbirds present in low numbers. Check ownership (FIC or Falkland Landholdings?) and establish if they are willing for eradication to occur. If so, likely to be of high priority.

Pitt Island (Weddell) – a 45 ha island known to have rats. Some tussac but quality of this, and extent of the potential habitat for wrens need investigation. Owner (Annie Gisby) probably supportive of rat eradication but this should be reconfirmed. Priority to be confirmed after survey.

Possible Timetable

There are so many variables to take into account that it is extremely difficult at this stage to set a long-term work programme. Funding levels, staff availability and experience, the potential availability of helicopters, logistical opportunities and other factors all contribute to how much may be able to be achieved in any given year.

It is also important not to forget commitments or potential additional work resulting from prior eradications. Checks for rodent presence should be made once a year for two years following an eradication project. These should take priority over further eradications, to ensure methods being implemented are indeed successful and do not have any negative implications on other wildlife.

Biological surveys of many islands are also critical to answer important questions relating to eradication planning and prioritisation.

Nevertheless, given the relative importance of the rat eradication work, it is essential that progress continues, and momentum continued. A realistic and achievable schedule for the next two to three years is suggested below.

Year 1:

Eradications:  Rookery Island (plus Rat and Cucumber), then North East Island using hand-broadcast techniques. Burnt Island and islet using bait stations. After successful completion of these local staff should be sufficiently trained and experienced to undertake most eradications on islands up to several hundred hectares in size. Green Island could also be done if cattle have been removed. The owner of Barton Island may undertake an eradication attempt there without any external support.

Checks:           Outer, Double, Harpoon, Top, Bottom, and Calf Island.

Surveys:          North West Islands, South West Horse islets, Cat, Rookery (all Falkland Conservation islands in northern Falkland Sound). Islands in the Beaver group (e.g. Little Coffin, Skull Bay and Channel Islands)

Year 2:

Eradications:  Little Coffin, Skull Bay and Channel Islands as one group or North West Islands and any others in northern Falkland Sound, or Ten Shilling Bay Islands.

Checks:           Outer, Double, Harpoon, Top, Bottom, and Calf Island (final confirmations), Rookery, North East and Burnt (first year checks).

Translocations:            Cobb’s wren and tussacbird to Outer Island

Year Three and beyond:

Eradications:  Arch Island group as first priority, dependent on results of surveys. Other options include any listed as options in Year 2 but not yet exercised.

Other work:     Completion of any monitoring projects from Years 1 and 2.

Major fund-raising for an eradication attempt on a larger island e.g. New or Bleaker Island can occur at any time, but should be stand-alone projects, i.e. not compromising the ability to fund the sequence of eradications on smaller islands, at least in the short-term future.

Efforts should be made to continue to work through the eradication priority list as funding and staff resources allow.

Continue surveys of islands as possible, with an emphasis on comprehensive coverage of geographic areas, and a standardised method of survey and recording.

Continue translocations of Cobb’s wren as deemed necessary and as habitat becomes available (i.e. is confirmed rat-free).

Recommendations

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