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Keywords:

  • cat;
  • endemic species;
  • introduced species;
  • pig;
  • rat;
  • seabird

Abstract

  1. Top of page
  2. Abstract
  3. Arrival and spread of invasive alien predatory mammals
  4. Historical impacts of invasive mammalian predators
  5. Current impacts
  6. Control and eradication of introduced mammalian predators in the UKOTs
  7. Priorities for mammalian predator control and eradication
  8. Acknowledgments
  9. References

The UK has sovereignty over 16 Overseas Territories, which hold some of the world’s great seabird colonies and collectively support more endemic and globally threatened bird species than the whole of mainland Europe. Invasive alien mammalian predators have spread throughout most of the Territories, primarily since European expansion in the 16th century. Here we review and synthesize the scale of their impacts, historical and current, actions to reduce and reverse these impacts, and priorities for conservation. Mammalian predators have caused a catastrophic wave of extinctions and reductions in seabird colony size that mark the UKOTs as a major centre of global extinction. Mammal-induced declines of threatened endemics and seabird colonies continue, with four Critically Endangered endemics on Gough Island (Tristan da Cunha), St Helena and Montserrat directly threatened by invasive alien House Mice Mus musculus, Feral Cats Felis catus and rats Rattus spp. Action to reduce these threats and restore islands has been modest in comparison with other developed countries, although some notable successes have occurred and a large number of ambitious eradication and conservation plans are in preparation. Priority islands for conservation action against mammalian predators include Gough (which according to one published prioritization scheme is the highest-ranked island in the world for mammal eradication), St Helena and Montserrat, but also on Tristan da Cunha, Pitcairn and the Falkland Islands. Technical, financial and political will is required to push forward and fund the eradication of invasive mammalian predators on these islands, which would significantly reduce extinction risk for a number of globally threatened species.

Unbeknown to many of its citizens, the United Kingdom retains sovereignty over 16 Overseas Territories (hereafter UKOTs). With the exception of Gibraltar, Cyprus Sovereign Base Areas and British Antarctic Territory, the UKOTs comprise islands and archipelagos scattered across the Atlantic, Indian, Pacific and Southern Oceans and the Caribbean Sea (Fig. 1). As with other oceanic islands, they are of considerable importance to biodiversity (Procter & Fleming 1999), supporting 24 endemic breeding bird species, a similar number to the whole of Europe including its northern Atlantic islands (BirdLife International 2009). Six of the Territories lie within global biodiversity hotspots (Myers et al. 2000, Conservation International 2009), and 10 of them comprise or lie within Endemic Bird Areas or Secondary Areas (Stattersfield et al. 1998). They also include some of the world’s greatest seabird colonies, in particular on Gough Island (Tristan da Cunha) and South Georgia, where populations of tens of millions of albatrosses (Diomedeidae), burrowing petrels (Procellariiformes) and penguins (Spheniscidae) occur (Prince & Croxall 1996, Angel & Cooper 2006) (Fig. 2). The global Red List (BirdLife International 2009) indicates that this biodiversity is under substantial threat. The UKOTs currently support breeding populations of 34 globally threatened bird species, and 22 of the 24 endemic breeding bird species are threatened or near-threatened. Nevertheless, the UKOTs’ biodiversity remains surprisingly obscure in the scientific literature, and according to recent reports of the UK legislature (House of Commons Environmental Audit Committee 2008, House of Commons Foreign Affairs Committee 2008), receives rather little attention from the UK Government (which signs Multilateral Environment Agreements on the Territories’ behalf).

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Figure 1.  Location of UK Overseas Territories.

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Figure 2.  Summarized bird conservation value of the UK Overseas Territories. (a) Numbers of globally threatened and endemic bird species breeding on the Territory. Numbers above bars are number of bird species endemic to the Territory (BirdLife International 2009). (b) Estimated number of breeding seabird pairs. Data sources for seabird population estimates: All wider Caribbean Territories: Bradley and Norton (2009); Ascension: Rowlands (2006), Hughes et al. (2008); BIOT: Symens (1999); Falkland Islands: Woods and Woods (1997), Catry et al. (2003), Reid and Huin (2005); Pitcairn: de Brooke (1995a,b, 2006); South Georgia and South Sandwich Islands: Prince and Croxall (1996), Poncet (2006); St Helena: Ashmole and Ashmole (2000), McCulloch (2004); Tristan da Cunha: Angel and Cooper (2006), Ryan (2005).

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While habitat loss and climate change are undoubtedly important threats, the role of invasive alien species in the UKOTs requires particular attention, as the threat of invasive species is particularly severe for island endemic species and seabird colonies (Millennium Ecosystem Assessment 2005). Invasive predatory mammals are a particular threat to bird species (Moors & Atkinson 1984, Burger & Gochfeld 1994), and here we review their spread across the UKOTs, their impacts on birds, both historical and current, recent efforts to mitigate their impact and future conservation prospects. The Caribbean UKOTs of Turks and Caicos Islands (TCI), Cayman Islands, British Virgin Islands and Anguilla, for which data on distribution and impacts of mammalian predators are less clear (e.g. see Varnham 2006), and which have few extant endemic bird species, are considered in less detail. We omit Gibraltar, as it is part of the European mainland and inherently less likely both to be colonized by invasive mammalian predators and to be affected by them. We also exclude Crown Dependencies, the Cyprus Sovereign Base Areas, and British Antarctic Territory which are (debatably) not UKOTs in the strict sense.

Arrival and spread of invasive alien predatory mammals

  1. Top of page
  2. Abstract
  3. Arrival and spread of invasive alien predatory mammals
  4. Historical impacts of invasive mammalian predators
  5. Current impacts
  6. Control and eradication of introduced mammalian predators in the UKOTs
  7. Priorities for mammalian predator control and eradication
  8. Acknowledgments
  9. References

The mammalian species of major concern in the UKOTs are the same species that have invaded islands worldwide (Moors & Atkinson 1984). These are three species of commensal rat (Ship Rat Rattus rattus, Norway Rat Rattus norvegicus and Kiore or Pacific Rat Rattus exulans), and the Feral Cat Felis catus. Feral Pigs Sus scrofa are also significant predators. The Small Indian Mongoose Herpestes auropunctatus is known from just one of the Caribbean UKOTs, British Virgin Islands, although it is widespread in the Caribbean region (Kairo et al. 2003), and the Patagonian Fox Dusicyon griseus has been introduced to the Falkland Islands (Hall et al. 2002). House Mice Mus musculus, although extremely widespread on islands, and found on almost all UKOTs, are not generally considered to be significant predators of birds and their nests (Burger & Gochfeld 1994, Chapuis et al. 1994). However, recent evidence from Gough Island has demonstrated that they can evolve devastating predatory behaviour (Cuthbert & Hilton 2004, Wanless et al. 2007). Feral Dogs Canis familiaris are present on all UKOTs except British Indian Ocean Territories (BIOT) and South Georgia and South Sandwich Islands (SGSSI). Domestic dogs undoubtedly predate birds on occasion, but it is not clearly established whether true feral populations exist anywhere in the Territories, although they are reported from Turks and Caicos, Cayman Islands and Montserrat (Sanders 2006, G. Hilton pers. obs.) along with historical occurrences on Tristan da Cunha (Cuthbert et al. 2009).

The vast majority of the land area of the Territories has been invaded by predatory mammals (Table 1). The few major islands free of mammalian predators include the South Sandwich Islands (310 km2) (which also lack terrestrial birds). Nightingale (400 ha) and Inaccessible Islands (1400 ha) (Tristan da Cunha) are mammal-free, although the latter did support a now-extirpated population of Feral Pigs. Among nearly 800 islands in the Falkland Islands, over 100 are believed to be free of mammalian predators, including several large ones such as Lively (5585 ha), Speedwell (5150 ha), George (2400 ha), and Carcass (1894 ha) (Falklands Conservation & RSPB unpubl. data). In the South Georgia territory, Annenkov (∼850 ha), Bird Island (∼400 ha), to Cooper Island (259 ha) and Willis Islands (∼250 ha) are mammal-free. In BIOT, Danger Island (66 ha) and Nelson Island (81 ha) are mammal-free (Symens 1999), as are Ducie (74 ha) and Oeno (62 ha) in the Pitcairn group. Apart from this, no islands larger than 50 ha in the UKOTs are known to be free of invasive mammalian predators (RSPB unpubl. data).

Table 1.   Arrival and current extent of invasive mammalian predators on UKOTs.
TerritoryTotal area (ha)Arrival of mammalian predatorsCurrent proportion of Territory occupied
SpeciesArrival date (AD)Rattus spp. (%)Feral Cat (%)Other species (%)Predator-free (%)
  1. aSubsequently extirpated.

  2. bGiven as this date in Varnham (2006) (with citation), but this seems unlikely given the generally much later spread of Rattus norvegicus worldwide.

  3. cDate of arrival of rodents in the Caribbean UKOTs is little known, but in general, Ship Rats spread through the Caribbean from the early 17th century (Varnham 2006) and Norway Rats from 1700 onwards (Atkinson 1985, Varnham 2006).

  4. dCalculation based on assuming that mice on Gough are predators of birds; for other Territories this assumption was not made and House Mouse presence is not reported.

  5. ePercentages are based on a database of Falkland Islands containing predator information for 92% of the total area of the archipelago (Falklands Conservation & RSPB unpubl. data). Predator distribution among remaining area is unknown. Data for cats in Falklands includes Patagonian Fox.

  6. fArea is the total area of South Georgia and surrounding islands. Percentage occupied by predators and predator-free is calculated based on the area free of permanent snow and ice only. Predator-free South Sandwich Islands (31 000 ha) are excluded from calculations.

  7. gComprehensive data for BVI were not found. Percentages occupied by cats and rats are minima based on sites of known presence. It seems probable that almost the entire land area is affected.

  8. hBased on the assumption that cats are present on Diego Garcia only; presence on outer islands is not known, whereas an eradication programme has been underway on Diego Garcia in recent years.

Ascension9700Feral Cat181599.95000.05
Ship Rat1654–1754
St Helena12 200Feral Cat16th century∼99.5∼99.500.5
Ship Rat1513–1665
Norway Rat1730–1800
Tristan da Cunha18 040Feral Cata (Tristan)Feral by 1824550House Mouse 9010d
Ship Rat (Tristan)1882
Feral Pig (Inaccessible)a1821
Feral Pig (Tristan)aBefore 1811
Falkland Islands1217 300Patagonian Fox1930s97.7e94.1e0e2.3e
Norway Rat1725–1833
Ship Rat (New Island)Late 18th century
Feral Cat1830s
South Georgia375 500fNorway Rat∼180096f0f0f4f
Bermuda5500Ship RatBy 161399.799.700.3
Norway RatMid-late 18th century
Feral CatUnknown
Montserrat10 900Ship RatUnknownc99.999.9Feral Pig ∼60; Feral Dog ∼99.90.1
Norway RatUnknownc
Feral CatUnknown
Feral Pig∼1998
Anguilla10 200Ship RatUnknownc98.9–99.6≥9400.4–1.1
Norway RatUnknownc
Feral CatUnknown
British Virgin Islands15 100Ship RatUnknownc≥83g≥77gSmall Indian Mongoose ≥42gNone knowng
Norway RatUunknownc
Feral CatUnknown
Feral Pig∼1998
Small IndianUnknown
Mongoose 
Cayman Islands26 300Ship RatSoon after 1503100100Feral Dog ∼1000
Norway RatDiscovery
Feral CatPresumably 1500sb 16th or 17th century
Turks & Caicos Islands50 000Ship RatUnknownc100?∼96.5Feral Dogs widespread∼1–2
Norway RatUnknownc
Feral CatUnknown
Pitcairn Islands4516Kiore∼80097003
Feral Cat (Pitcairn)Unknown
BIOT4360Ship RatBy 184095.362h04.7
Feral CatUnknown

With the exception of Pitcairn, to which Polynesian settlers apparently introduced Kiore over 1000 years ago (Weisler 1995), the invasion of the UKOTs by mammalian predators commenced with European exploration in the early 16th century (Table 1). New conservation threats from invasive mammals continue to the present: so far as is known, Feral Pigs began their spread on Montserrat following the abandonment of farmland in the wake of the volcanic eruptions of 1996–1998 (Montserrat Department of Environment pers. comm.).

Historical impacts of invasive mammalian predators

  1. Top of page
  2. Abstract
  3. Arrival and spread of invasive alien predatory mammals
  4. Historical impacts of invasive mammalian predators
  5. Current impacts
  6. Control and eradication of introduced mammalian predators in the UKOTs
  7. Priorities for mammalian predator control and eradication
  8. Acknowledgments
  9. References

Endemic species extinctions

BirdLife International (2009) reports that among 34 bird species believed to have been endemic to the UKOTs in 1500 AD, 10 are now likely to be extinct, which puts the UKOTs fifth globally among countries and associated territories with the highest number of avian extinctions, the highest ranking being USA and its Territories, France and its Territories (both with 29 species), New Zealand (18), Mauritius (16) and Australia and its Territories (13). The concentration of extinctions on islands is well known, but this analysis demonstrates the extent to which the Overseas Territories of developed nations are major extinction hotspots. Although direct evidence is largely lacking, eight of the 10 UKOT extinctions have been ascribed wholly or partly to invasive alien mammals (BirdLife International 2009).

Seven of the documented extinctions took place on St Helena, with one each on Tristan da Cunha, Grand Cayman and Ascension. In addition to those extinctions sufficiently well documented to have been recorded in the World Bird Database (BirdLife International 2009), there may also have been an endemic Night-heron Nycticorax olsoni lost from Ascension (Bourne et al. 2003) and several land-bird species lost from Bermuda since human colonization (in both cases in the early 16th century) (Wingate 1985). Furthermore, a number of earlier anthropogenic extinctions have also been suggested. Four species known from archaeological investigation on Henderson Island (Pitcairn) are now extinct – a Prosobonia sandpiper, a Ducula pigeon, a Ground-dove Gallicolumba leonpascoi and an archaic pigeon Bountyphaps obsoleta (Weisler 1995, Wragg 1995). These extinctions occurred following, and presumably as a result of, Polynesian occupation of the island and the arrival of commensal Kiore. Species extinctions may have occurred on Pitcairn Island, as on Henderson, during the period of Polynesian occupation (Benton & Spencer 1995).

Seabird colony depletion

It is difficult to estimate the size of seabird colonies that were lost in the early stages of human exploration of the UKOTs. Nevertheless, a number of lines of evidence point to widespread losses of vast numbers of seabirds (Table 2). On Henderson Island, inferences about historical seabird population sizes have been made based on backward extrapolation of current rates of decline and population sizes to the time at which rats colonized the island; they suggest that millions of pairs of gadfly petrels Pterodroma spp. may have been lost (de Brooke et al. 2010). On St Helena and Ascension, early historical accounts, sub-fossils and the evidence of large guano deposits provide evidence of the former presence of very large seabird colonies, again with inferred losses of hundreds of thousands or millions of pairs.

Table 2.   Inferred impacts of mammalian predators on breeding seabird community sizes in the UKOTs at selected islands where data exist to make comparisons.
Territory/IslandImpactsCurrent population
Henderson Island/Pitcairn‘Gadfly Petrels... probably much less numerous than formerly, by 1–2 orders of magnitude’ Back-calculating current Henderson Petrel population and decline rate suggests five million pairs at time of Kiore arrival (de Brooke et al. 2010)39 600 pairs of Pterodroma spp. (de Brooke 1995b)
St Helena‘combined numbers were probably in the millions’ (McCulloch 2004)1000–10 000 pairs of all species (Ashmole & Ashmole 2000, McCulloch 2004)
Ascension‘numbers are…probably only a tiny fraction of those originally present’ (Rowlands 2006)∼215 000 pairs of all species (Rowlands 2006, Hughes et al. 2008)
Tristan da Cunha‘before the introduction of rodents and cats…there were…probably millions of pairs’ (Angel & Cooper 2006)∼25 000 pairs of all species (Angel & Cooper 2006)
BIOTseabird density ∼20× greater on rat-free islands (= 8) than rat-infested (= 39) (Symens 1999)∼100 000–180 000 pairs of all species (Symens 1999, McGowan et al. 2007)
South Georgia‘in rat-infested coastal areas…smaller burrowing petrel species have been eliminated’ (Poncet 2006)∼35 million pairs of all species (Poncet 2006)

A further way of making inferences about lost populations is to compare community structure of ecologically similar predator-free and predator-infested islands, such as the islands of Tristan, Nightingale, Inaccessible and Gough in the Tristan da Cunha Group (Fig. 3). Based on this figure, it is reasonable to surmise that in the order of millions of smaller and medium-sized seabirds have been lost from Tristan, where introduced rats, Feral Cats and Feral Pigs are all likely to have had major impacts.

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Figure 3.  Abundance of procellariiform seabirds in different size classes at different islands in the Territory of Tristan da Cunha. Small procellariiforms are typically acutely vulnerable to rats because they may be predated as adults as well as eggs/chicks, and may thus suffer reduced adult survival. Mid-sized procellariiforms tend to be chronically vulnerable because they mainly suffer lower productivity. Larger procellariiforms are, in general, invulnerable to rats (Burger & Gochfeld 1994). Note the very low numbers of birds at Tristan – the only island in the group with rats – in the small and medium body size classes. Gough Island, where procellariiforms suffer major mouse predation on chicks, shows no obvious size-class difference to predator-free Inaccessible and Nightingale Islands, possibly because Gough mice are, peculiarly, known to impact even the largest procellariiform species (Wanless et al. 2007). The island areas are in the range 4 km2 (Nightingale) to 99 km2 (Tristan). Data are from Richardson (1984), Ryan (2005), Angel and Cooper (2006) and references therein.

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In cases where an archipelago comprises a mixture of predator-free and predator-infested islands, it is difficult to determine whether some species of seabirds have been reduced in number, or have simply redistributed to predator-free islands, with archipelago-wide populations being limited by marine resources (Fig. 4). These questions are perhaps particularly pertinent for BIOT, South Georgia and Falkland Islands, which retain large populations of predator-sensitive seabirds on the remaining rat-free areas, and for which historical data on population sizes are lacking. Similarly, where mammalian predators have reached an island, it is conceivable that the total number or biomass of seabirds remains relatively constant, despite the loss of predator-sensitive species, through a shift towards predator-insensitive species. For example, the penguins and albatrosses of the South Atlantic Territories, and possibly the tropical booby Sula species appear to be relatively resistant to most mammalian predators (Burger & Gochfeld 1994, Jones et al. 2008) and could conceivably have benefited from reductions in populations of smaller, predator-sensitive species.

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Figure 4.  Density of commoner seabird species in BIOT in relation to the presence of rats. For most species, population densities are far higher among rat-free islands. This may imply that the presence of rats on ∼95% of the land area has caused a proportionate decrease in total seabird numbers. However, it is not easy to determine the extent to which redistribution of seabirds onto rat-free islands means that losses are smaller than this. Data from Symens (1999). Species with total BIOT populations >100 pairs are used. Population densities were calculated as total number in each island-class divided by the total area occupied by that island class. = eight rat-free and 39 rat-infested islands. Species: 1 = Wedge-tailed Shearwater Puffinus pacificus; 2 = Audubon’s Shearwater Puffinus lherminieri; 3 = Masked Booby Sula dactylatra; 4 = Red-footed Booby Sula sula; 5 = Brown Booby Sula leucogaster; 6 = Sooty Tern Sterna fuscata; 7 = Brown Noddy Anous stolidus; 8 = Lesser Noddy Anous tenuirostris; 9 = White Tern Gygis alba.

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Current impacts

  1. Top of page
  2. Abstract
  3. Arrival and spread of invasive alien predatory mammals
  4. Historical impacts of invasive mammalian predators
  5. Current impacts
  6. Control and eradication of introduced mammalian predators in the UKOTs
  7. Priorities for mammalian predator control and eradication
  8. Acknowledgments
  9. References

Endemic bird species

Of the 22 threatened or near-threatened endemic breeding birds of the UKOTs, the World Bird Database suggests that invasive mammalian predators are a threat to 21, and the most severe threat to 14 (BirdLife International 2009). For seven of the most highly threatened species, the entire population is in ongoing decline as a result of invasive predators. For the Critically Endangered Tristan Albatross Diomedea dabbenena, Gough Bunting Rowettia goughensis and Montserrat Oriole Icterus oberi, the impacts are acute, resulting in a high risk of imminent extinction (Hilton et al. 2003, 2005, Cuthbert et al. 2004, Ryan & Cuthbert 2008, Wanless et al. 2009). The situation is marginally less severe for the Critically Endangered St Helena Plover Charadrius sanctahelenae and Endangered Atlantic Petrel Pterodroma incerta, Henderson Petrel Pterodroma atrata, and Pitcairn Reed Warbler Acrocephalus vaughani only by virtue of slightly slower decline rates (de Brooke 1995b, Cuthbert 2004, BirdLife International 2009, de Brooke et al. 2010).

A second group of UKOT endemic species are not currently declining as a result of mammalian predators but are believed to be very sensitive to any future introductions. Some of these appear robust to the invasive predators with which they currently co-exist, such as the four Vulnerable endemic land-bird species on Henderson Island, which co-occur with Kiore (de Brooke 1995a), Gough Moorhen Gallinula comeri (also Vulnerable), which apparently tolerates House Mice on Gough Island (Cuthbert & Sommer 2004), Atlantic Yellow-nosed Albatross Thalassarche chlororhynchos (Endangered), which appears tolerant of rodents on Tristan da Cunha, and the non-threatened Falkland Steamerduck Tachyeres brachypterus, which is abundant throughout the islands (Woods & Woods 1997). Conversely, two of these species (both globally Vulnerable) have their natural range entirely on mammal-free islands – Inaccessible Rail Atlantisia rogersi (endemic to Inaccessible Island, Tristan da Cunha) and Grosbeak Bunting Nesospiza wilkinsi (endemic to Inaccessible and Nightingale Islands).

A third group of six species have suffered major historical range reductions, but have persisted on mammal-free areas within their range. Tristan Bunting Nesospiza acunhae (Vulnerable) has been extirpated from Tristan da Cunha and Tristan Thrush Nesocichla eremita (Near Threatened) nearly so, but both persist on the much smaller mammal-free Nightingale and Inaccessible Islands (Angel & Cooper 2006). Cobb’s Wren Troglodytes cobbi (Falkland Islands) and South Georgia Pipit Anthus antarcticus appear to be completely intolerant of rats (Prince & Croxall 1996, Hall et al. 2002). Their extinction has been averted by the continued presence of rat-free remnants on their respective Territories. However, the spread of rats through their ranges is ongoing. For example, Bleaker Island (2070 ha), a major predator-free area in the Falklands, was invaded by Norway Rats as recently as the mid 1980s, with the subsequent loss of Cobb’s Wren (Falklands Conservation & Falkland Islands Government 2008). In South Georgia, Saddle Island (∼100 ha) was invaded by Norway Rats, perhaps across an ice bridge, during the last decade (S. Poncet pers. comm.), and in places where glaciers formerly came down to sea level, creating physical barriers, glacial retreat is likely to expose new areas of the main island to rat colonization (Poncet 2006, BirdLife International 2009). By 1951, Bermuda Petrel Pterodroma cahow, formerly widespread and abundant on Bermuda, was reduced to 18 pairs on <1 ha of offshore islets by mammalian predators; extinction was averted because these low-quality nesting areas were free of all but occasional rat incursions, and the population is now recovering due to conservation action (see below) (Wingate 1985, BirdLife International 2009). Similarly, Ascension Frigatebird Fregata aquila was extirpated from the main island of Ascension by cat predation and now breeds only on a 3-ha offshore stack (Ashmole & Ashmole 2000, Ratcliffe et al. 2008), although cat eradication from the main island appears to have created the conditions for recovery (see below). Finally, Spectacled Petrel Procellaria conspicillata, endemic to Inaccessible Island, was driven to near extinction by Feral Pigs by the early 20th century; its survival and subsequent recovery is apparently due to the serendipitous eradication of the Pigs for non-conservation reasons before extinction occurred (Ryan et al. 2006).

Seabird communities

It might seem reasonable to assume that, except where invasive species continue to spread to new areas (see above), seabird populations in the UKOTs have now reached stable equilibria with respect to the impacts of mammalian predators. In most cases, data are inadequate to determine whether this is the case, but several lines of evidence suggest that some populations and communities may still be in decline. de Brooke et al. (2010) have shown that where breeding success of long-lived petrel species is chronically reduced by invasive predators, population declines can continue over many centuries before extirpation finally occurs. This is particularly likely to occur where predation is largely on eggs/chicks rather than on adults – as is typically the case where rodents are the only predators, and/or the seabirds are not very small (Jones et al. 2008). The implications of this for seabirds on the UKOTs’ two World Heritage Sites – Gough and Henderson Islands – are potentially severe. Thus it is thought that populations of Pterodroma petrels on Henderson are continuing their downward trajectory following the Polynesian introduction of Kiore a millennium ago (de Brooke 1995b). For Henderson Petrel, which is not known to have source populations on other islands, extinction will eventually occur, and Phoenix Petrel P. alba may already have been extirpated from the island (Murphy & Pennoyer 1952). Similarly, heavy mouse predation on chicks of Tristan Albatross and Atlantic Petrel is sufficient to drive population declines even though the mice have been present for up to 200 years (Cuthbert 2004, Wanless et al. 2009). It is not clear what proportion of Gough’s wider seabird community of 20 extant species is also in long-term decline. Angel and Cooper (2006) speculate that smaller burrow-nesting species, and in particular those that have chicks in the nest during winter, are likely to be particularly susceptible – which might implicate winter-breeding Great-winged Petrel Pterodroma macroptera and Grey Petrel Procellaria cinerea as well as the small storm-petrels (Hydrobatidae) and diving-petrels (Pelecanoididae). Angel and Cooper (2006) also suggested that the remnant petrel populations reported as having survived long-term rat and cat predation on Tristan da Cunha in the early 1970s (Richardson 1984) were still likely to be declining, and some may already have been lost, except where immigration from nearby predator-free sources maintains small sink populations.

Ratcliffe et al. (2008) showed that where seabirds are confined to nesting on small predator-free islands near to (often much larger) islands that have predators, predation of adult seabirds that attempt to settle on the predator-inhabited islands can limit the size of the population as a whole. This probably applied to the endemic Ascension Frigatebird prior to cat eradication on Ascension’s main island (Ratcliffe et al. 2008), and might plausibly apply even for large seabirds on St Helena, parts of the Falkland Islands and several of the Caribbean Territories, where cats, Patagonian Foxes and mongooses are variously present on large islands close to remaining seabird colonies. It might also affect small seabirds that are vulnerable to predation of adults by rats at sites such as Ascension, Tristan da Cunha, South Georgia and BIOT.

There are certainly anomalous situations in the UKOTs (as elsewhere) where seabirds unexpectedly tolerate mammal presence, the most striking of which is at New Island (2363 ha, Falkland Islands), where two million pairs of Thin-billed Prion Pachyptila belcheri persist, and appear to have high breeding success, in the presence of Ship Rats and Feral Cats (Catry et al. 2007). Similarly, the impacts of rodents on a number of tropical seabird species seems to be rather variable and unpredictable (Jones et al. 2008). For sites where tropical seabird populations persist in the presence of rats, such as BIOT (Symens 1999, McGowan et al. 2007), Dog Island (Anguilla) (Hodge & Holliday 2006) and Henderson Island (de Brooke 1995b), data are simply not adequate to determine whether their continued presence indicates population stability, or whether in fact the populations are declining and/or sustained by immigration (and therefore acting as a sink to the wider population).

Control and eradication of introduced mammalian predators in the UKOTs

  1. Top of page
  2. Abstract
  3. Arrival and spread of invasive alien predatory mammals
  4. Historical impacts of invasive mammalian predators
  5. Current impacts
  6. Control and eradication of introduced mammalian predators in the UKOTs
  7. Priorities for mammalian predator control and eradication
  8. Acknowledgments
  9. References

Rat and cat eradications from islands have become a major conservation industry in recent years, with recent reviews showing that around 300 and 50 islands have been cleared of the two taxa, respectively (Nogales et al. 2004, Howald et al. 2007). A number of control and eradication actions have taken place in the UKOTs (Table 3). One of the earliest and most important rat eradications took place in the UKOTs: the pioneering removal of rats from Nonsuch Island in Bermuda, which played an important role (with other interventions) in allowing the recovery of the then Critically Endangered Bermuda Petrel (Wingate 1985, BirdLife International 2009). Similarly, one of the world’s largest cat eradication programmes took place on Ascension Island during 2001–2006. From data in Nogales et al. (2004) and Howald et al. (2007), it is evident that among developed nations with substantial island possessions, New Zealand and Australia have made far more progress in removing mammalian predators from islands than have the UK, the USA or France (Table 4).

Table 3.   Control and eradication attempts for invasive mammalian predators in UKOTs.
TerritorySpeciesActivityDateBird population response
Ascension IslandFelix catusEradication from main island (9700 ha)2001–2004Recolonization by at least five seabird species; Sooty Tern population increasing
Tristan da CunhaF. catusEradication from Tristan da Cunha (9837 ha)1970sNo published data
Tristan da CunhaSus scrofaEradication from Tristan da Cunha and Inaccessible Islands (1400 ha)Early 20th century (for hunting)Probable recovery of Spectacled Petrel on Inaccessible Island
Falkland IslandsRattus norvegicusEradication from ∼25 islands (∼465 ha)2001–2009No published information. Evidence of increasing land-bird populations; no Cobb’s Wren colonization (R. Woods unpubl. data)
Falkland IslandsDusicyon griseusEradication on Tea Island (310 ha)2008Too early for bird population response
Bermuda Eradicated from Nonsuch Island (5.8 ha)Before 1985Increased productivity and population recovery of Bermuda Petrel
Turks & CaicosRattus rattusEradication from four small cays (5.7 ha)2002No published information
Turks & CaicosF. catusEradication from Long Cay (111 ha)1999No published information
Cayman IslandsF. catus & Canis familiarisControl in two Grand Cayman Blue Iguana Cyclura lewisi sitesFrom 2001No published information for birds
MontserratR. rattus & R. norvegicusControl in 13 ha of Centre Hills forest2006–2008No published information
MontserratS. scrofaControl in Centre Hills forest (∼1200 ha)2002–2004No data published
British Virgin IslandsR. rattusEradicated from Sandy Cay (5.7 ha)2002No data published
BIOTR. rattusFailed eradication on Eagle Is (244 ha)2006No impact expected
BIOTF. catusControl towards eradication, Diego Garcia (2720 ha)∼2001 onwardsApparent increase in Red-footed Booby colony
PitcairnF. catusEradication from Pitcairn (660 ha); subsequently reintroduced1997No published information
PitcairnRattus exulansEradication from Oeno (62 ha) and Ducie (74 ha); failed on Pitcairn1997No published information
Table 4.   Eradications of invasive mammalian predators among islands of developed nations and their overseas territories.
 Island eradication coverage
RodentsCats
No. of islandsTotal area (ha)No. of islandsTotal area (ha)
  1. Data are from Nogales et al. (2004) and Howald et al. (2007). There have been many operations subsequent to these publications, but no comprehensive database has been published.

  2. aFigures in parentheses are for the Overseas Territories of the country. For USA, Hawaii is included as part of the ‘metropolitan’ USA.

Australia343328513 884
Francea34 (20)1191 (1097)1 (1)145 (145)
NZ8927 012106937
UKa22 (18)1418 (236)2 (2)771 (771)
USAa12 (3)1114 (296)3 (3)721 (721)

A major scaling-up of rodent eradication efforts is being planned for the UKOTs, with feasibility studies and operational plans completed or in development for major sites such as Gough Island (House Mouse, 6400 ha (Parkes 2008)), Tristan da Cunha (Ship Rat and House Mouse, 9837 ha (Brown 2007a,b)), Henderson Island (Kiore, 3700 ha (de Brooke & Towns 2008)), and even South Georgia (D. Christie, Government of South Georgia pers. comm.). However, the funding required for eradications on this scale (>£1 million, see Martins et al. (2006)) has not yet been obtained for any of these potential operations.

Priorities for mammalian predator control and eradication

  1. Top of page
  2. Abstract
  3. Arrival and spread of invasive alien predatory mammals
  4. Historical impacts of invasive mammalian predators
  5. Current impacts
  6. Control and eradication of introduced mammalian predators in the UKOTs
  7. Priorities for mammalian predator control and eradication
  8. Acknowledgments
  9. References

The largest island hitherto cleared of rats is 11 300-ha Campbell Island (New Zealand), of cats is 29 000-ha Marion Island (South Africa), of mice is 710-ha Enderby Island (New Zealand) and of pigs is 58 465-ha Santiago (Galapagos Island, Ecuador, Cruz et al. (2005)), with maximum sizes increasing rather rapidly over time (Towns et al. 2006). The eradication of House Mice from Gough Island is clearly a high priority (see above; Cuthbert & Hilton 2004, Angel & Cooper 2006, Wanless et al. 2007), but the largest island cleared of mice is an order of magnitude smaller than Gough. However, an eradication of mice was conducted on 1200-ha Coal Island (New Zealand) in 2008, with the outcome currently unconfirmed (N. Torr pers. comm.), and in 2010 there will be an attempt to remove House Mice, Ship Rats and Rabbits Oryctolagus cuniculus from 12 200-ha Macquarie Island (Australia) in a single operation (Anon 2007).

An obvious obstacle to island restoration in the UKOTs is that eradication methods for islands inhabited by humans are in their infancy (Merton et al. 2000, Brown 2007a), meaning that eradications (especially of rodents) – and subsequent quarantine – from the ‘main islands’ such as Ascension, St Helena, Tristan, most of the Caribbean Territories and Pitcairn – would be breaking new ground. However, alternative options do exist for reducing mammal impacts at such sites. Large-scale mammalian predator control is now successfully deployed across large areas of New Zealand and Australia (Saunders 2000, Gillies 2002); for example, the Te Urewera Mainland Island in Northern New Zealand covering several thousand hectares. Mammal exclusion through fencing has also achieved some notable successes, such as the 3400-ha Maungatatauri Sanctuary (New Zealand) (http://www.maungatrust.org) and even the 225-ha Karori Sanctuary, which has excluded mammals, including House Mice, from a natural area in the capital city, Wellington (Lukis & Bell 2007). This indicates that almost all of the uninhabited islands (or those with human populations substantially <1000 such as Pitcairn, many of the Falkland Islands, Tristan da Cunha, and some in the Caribbean) in the UKOTs are broadly within the compass of what has already been achieved for eradication of mammalian predators – the exceptions being the very large main islands of the Falklands and South Georgia. Meanwhile, there is potential for predator control and exclusion at selected locations within other Territories, such as Ascension, St Helena and Montserrat.

Given this broad range of possibilities, some thought should clearly be given to prioritization. Apart from birds, there are many other native and endemic populations, communities and habitats for conservationists to consider in a prioritization exercise – perhaps especially the considerable numbers of endemic reptiles and plants in the UKOTs (Procter & Fleming 1999). Other taxon specialists are required to undertake similar analyses in the UKOTs, allowing a more comprehensive assessment. However, purely in the context of control and eradication of mammalian predators of bird species, a number of priority conservation actions suggest themselves (Table 5).

Table 5.   Island restoration priorities for bird conservation in the UK Overseas Territories.
Island/TerritoryActionThreatened species to benefitOther potential bird benefitsReferences
  1. Islands are presented in descending order of priority, based on the number of threatened bird species to benefit, and their level of threat. Threatened species listed are only those for which invasive species are a threat, and hence which are expected to benefit. Species in bold are endemic to the island in question. Cr = critically endangered, En = endangered, Vu = vulnerable, NT = near threatened (BirdLife International 2009).

  2. aMouse impacts on Grey Petrel unconfirmed, but suspected as it is a winter-breeder (Angel & Wanless 2006).

  3. bThought to have been extirpated, but potential re-colonist.

  4. cControl/exclusion would probably benefit Atlantic Petrel and other colonial seabirds, but impacts on the more dispersed and wide-ranging land-birds would probably be minimal.

  5. dExtirpated; could potentially be reintroduced, though would probably be a different subspecies.

Gough (Tristan da Cunha)House Mouse eradicationTristan albatross (Cr) Grey Petrel (NT)a Atlantic Petrel (En) Gough Bunting (Cr)Unknown part of one of the world’s greatest seabird colonies would recoverAngel and Cooper (2006), Parkes (2008)
MontserratPig, cat and rodent control/exclusionMontserrat Oriole (Cr) Forest Thrush Cichlerminia lherminieri (Vu) Hilton et al. (2003, 2005)
St HelenaCat and rodent control/exclusionSt Helena Plover (Cr)Depending on location of site, potential recovery of seabird populationsBirdLife International (2009)
Henderson Island (Pitcairn)Kiore eradicationHenderson Petrel (En) Phoenix Petrel (En)b Murphy’s Petrel Pterodroma ultima (NT)Recovery/recolonization of other seabirds Potential recovery from population limitation by four endemic landbirdsde Brooke and Towns (2008), de Brooke et al. (2010)
PitcairnCat and Kiore eradicationPhoenix Petrel (En)b Pitcairn Reed-warbler (En)Recovery/recolonization of other seabird populations.de Brooke and Towns (2008)
Tristan da CunhaShip Rat and House Mouse eradication or localized control/exclusioncAtlantic Petrel (En) Tristan Bunting (Vu)d Tristan Thrush (NT)Recovery of other vulnerable seabird populationsAngel and Cooper (2006), Brown (2007a)
Falkland IslandsPatagonian Fox, cat and rodent eradication from feasible islandsCobb’s Wren (Vu)Recovery of vulnerable seabird populationsFalklands Conservation & Falkland Islands Government (2008)
South GeorgiaNorway Rat eradication from offshore islands, parts or all of main islandSouth Georgia Pipit (NT)Recovery of vulnerable seabird populationsBirdLife International (2009)
AscensionRat control/exclusion/eradication on main island (Increased rate of) recovery of vulnerable seabird populations 
BIOTShip Rat eradication from some or all infested islands Recovery of vulnerable seabird populationsSymens (1999), Carr (2006)
TCI, BVI, Anguilla, CaymanCat, mongoose and rodent eradication from offshore islands Recovery of vulnerable seabird populationsVarnham (2006)

de Brooke et al. (2007) and Ratcliffe et al. (2009) described systems for prioritizing island eradications based on the magnitude of benefits to threatened bird species and the estimated financial cost. A re-analysis of de Brooke et al.’s (2007) global analysis incorporating changes in species’ Red List status since the paper was published results in Gough being the highest priority island in the world for mammal eradication. As the authors pointed out, this analysis offered only a partial prioritization, because it ignored effects on non-threatened bird species, perhaps especially on seabird communities (as opposed to the threatened species therein, which were included), which are particularly important on UKOTs (and other islands). However, as Gough is also ‘arguably the world’s greatest seabird island’ (Collar & Stuart 1985), and it is suspected that several of the non-threatened seabird species are also being impacted by mice (Angel & Cooper 2006), the conclusion that Gough should be top of the world’s island restoration priorities is likely to hold. Mouse eradication on Gough would greatly reduce the probability of extinction for three UKOT endemic bird species, of which two are Critically Endangered. Activities on St Helena, Montserrat, Henderson, Pitcairn, Tristan da Cunha and the Falkland Islands would also significantly benefit globally threatened endemics (Table 5). The Centre Hills of Montserrat also support the world’s only population of the Critically Endangered Montserrat Galliwasp Diploglossus montiserrati (Reptilia: Diploglossidae) and one of the last remaining populations of Mountain Chicken Leptodactylus fallax (Amphibia: Ranidae), also Critically Endangered. Both are believed to be highly sensitive to mammalian predators and action against such predators is strongly recommended in recent action plans (Martin et al. 2007, L. Martin, M.N. Morton, G.M. Hilton, R.P. Young, G. Garcia, G. Gray & S. Mendes in prep., Young 2008). Major conservation benefits would probably also accrue on many islands that lack sensitive threatened bird species if mammalian predators were eradicated, controlled or excluded, as there is potential for at least partial recovery of the massive lost seabird colonies of Ascension, St Helena, South Georgia, BIOT and possibly also the Caribbean Territories.

Clearly, other actions against invasive species, such as preventing further incursions and rapidly responding to any that do occur, are of great conservation value, in addition to the control and eradication priorities discussed here. Predator-free islands that support endemic land-birds – Inaccessible and Nightingale Islands (Tristan da Cunha) – are major priorities for quarantine and rapid-response, and action has recently been taken to reduce the possibility of rodent incursion onto Nightingale (C. Stringer pers. comm.). Of comparable importance would be the prevention of more aggressive rodent species (Ship Rats or Norway Rats), or even cats, reaching Henderson and Gough Islands.

We have not attempted a detailed costing of the activities listed in Table 5 here. However, published assessments indicate one-off costs of rodent eradication from Tristan da Cunha at 2.5 million GBP (Brown 2007b), Gough at 1.5 million GBP (Parkes 2008), and Henderson at 1.33 million GBP (de Brooke & Towns 2008), while Martins et al. (2006) provide models that broadly predict costs of mammal eradication as a function primarily of island size. Costings for mammal control and exclusion are less well developed. Overall, RSPB (2007) estimated that £3.92 million per year was needed between 2007 and 2011 to control introduced mammals for biodiversity conservation. Existing levels of expenditure have not been collated across the UKOTs as a whole, although they are well below the levels estimated to be required to achieve biodiversity conservation goals (RSPB 2007).

In conclusion, this synthesis highlights the enormous loss of avian biodiversity that has occurred in the UKOTs over several centuries of occupation by mammalian predators, and the magnitude of the ongoing threat to what remains. However, reviewing what has been achieved in terms of eradication and control in recent years gives a strong indication that restoration activities could realistically deliver major long-term gains for UKOT birds.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Arrival and spread of invasive alien predatory mammals
  4. Historical impacts of invasive mammalian predators
  5. Current impacts
  6. Control and eradication of introduced mammalian predators in the UKOTs
  7. Priorities for mammalian predator control and eradication
  8. Acknowledgments
  9. References

This review was made possible through long-term collaboration with a large number of colleagues at RSPB, especially Sarah Sanders, Clare Stringer, Mark Bolton, Erica Sommer, Paul Walton, Brian Summers and Andrew Darlow. Also in the UK, we have benefited greatly over several years from collaboration with Mike Brooke (Cambridge University), Tamas Szekely, James StClair and Fiona Burns (Bath University), Andy McGowan and Brendan Godley (Exeter University), Colin Clubbe and Martin Hamilton (Royal Botanic Gardens – Kew), Richard Young, Matthew Morton and Gerardo Garcia (Durrell Wildlife), Tara Pelembe, David Stroud and Vin Fleming (JNCC). Colleagues at University of Cape Town – Peter Ryan, Ross Wanless, John Cooper and Andrea Angel - have been particularly helpful. Above all we would like to thank our partners in the UKOTs – Pete Carr (BIOT), Gerard Gray, Calvin Fenton, Jervain Greenaway (Montserrat), Darren Christie (South Georgia), Helen Otley, Ann Brown, Robin Woods (Falklands), Rhon Connor (Anguilla), Trevor Glass, James Glass (Tristan), Eddie Duff, Emma Bennett, Vince Thompson (St Helena), Susanna Musick, Adrian Bowers, Raymond Benjamin (Ascension), Jay Warren and Simon Young (Pitcairn). We are grateful to the organizers of the BOU symposium on invasive species, especially Steve Dudley and Niall Burton.

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  4. Historical impacts of invasive mammalian predators
  5. Current impacts
  6. Control and eradication of introduced mammalian predators in the UKOTs
  7. Priorities for mammalian predator control and eradication
  8. Acknowledgments
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