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Rat eradication in the Pitcairn Islands, South Pacific: a 25-year perspective

Island and Ocean Ecosystems, BRB

Available Online

Brooke, M.de L.

This essay offers a 25-year overview of eff orts to remove Pacific rats (Rattus exulans) from the four islands of the Pitcairn group. Following the 19911992 discovery that rats were severely reducing breeding success of gadfly petrels (Pterodroma spp.), Wildlife Management International proposed eradication. Eradication success was achieved using ground-based baiting on the small atolls of Ducie and Oeno in 1997, and there is now evidence of petrel recovery on Oeno, but two eradication attempts on inhabited Pitcairn (1997 and 1998) failed. By the early 2000s, the development of aerial baiting through the 1990s placed an eradication operation on the fourth island, Henderson, within reach. Preparatory fieldwork in 2009 allayed doubts in two key areas: the feasibility of maintaining a captive back-stop Henderson rail (Porzana atra) population, and bait uptake by crabs (Coenobita spp.). Royal Society for the Protection of Birds (RSPB) expertise secured the necessary funding of £1.5 million, and 75 tonnes of brodifacoum-containing bait were dropped in August 2011. Despite extensive mortality of free-living rails, the population, supplemented by released captive birds, returned to pre-operational levels in 23 years. Meanwhile those tending captive rails saw no rat sign before leaving Henderson in November 2011. Unfortunately, a rat was sighted in March 2012, and continuing rat presence confirmed in May 2012. Subsequently rat numbers have returned to pre-operational levels without any sign of population overshoot as observed on Pitcairn. Genetic analysis suggests around 80 rats, roughly 1 in 1,000, survived the bait drop. With no evidence of imperfect bait coverage or deficiencies in bait quality or brodifacoum resistance, it seems some animals chose not to eat bait. Choice tests on Henderson Island rats suggest some rats prefer natural foods over bait. This adverse situation may have been exacerbated because, in August 2011, natural fruits were more abundant than anticipated due to drought earlier in the year. To overcome rat preference for natural food, any second Henderson attempt might benefit from more attractive bait. Without such developments, a second attempt risks another failure. Hendersons biota will survive the delay.

Seasonal variation in movements and survival of invasive Pacific rats on sub-tropical Henderson Island: implications for eradication.

Island and Ocean Ecosystems, BRB

Available Online

Bond, A.L.

Churchyard, T.

Donaldson, A.

Duffield, N.

Havery, S.

Kelly, J.

Lavers, J.L.

McClelland, J.T.W.

Oppel, S.

Proud, T.

Russell, J.C.

Invasive rodents are successful colonists of many ecosystems around the world, and can have very flexible foraging behaviours that lead to differences in spatial ranges and seasonal demography among individuals and islands. Understanding such spatial and temporal information is critical to plan rodent eradication operations, and a detailed examination of an islands rat population can expand our knowledge about possible variation in behaviour and demography of invasive rats in general. Here we investigated the movements and survival of Pacific rats (Rattus exulans) over five months on sub-tropical Henderson Island in the South Pacific Ocean four years after a failed eradication operation. We estimated movement distances, home range sizes and monthly survival using a spatially-explicit Cormack-Jolly-Seber model and examined how movement and survival varied over time. We captured and marked 810 rats and found a median maximum distance between capture locations of 39 ± 25 m (0107 m) in a coastal coconut grove and 61 ± 127 m (01,023 m) on the inland coral plateau. Estimated home range radii of Pacific rats on the coral plateau varied between territorial (median: 134 m; 95% credible interval 106165 m) and roaming rats (median: 778 m; 2901,633 m). The proportion of rats belonging to the roaming movement type varied from 1% in early June to 23% in October. There was no evidence to suggest that rats on Henderson in 2015 had home ranges that would limit their ability to encounter bait, making it unlikely that limited movement contributed to the eradication failure if the pattern we found in 2015 is consistent across years. We found a temporal pattern in monthly survival probability, with monthly survival probabilities of 0.352 (0.0810.737) in late July and 0.950 (0.8460.987) in late August. If seasonal variation in survival probability is indicative of resource limitations and consistent across years, an eradication operation in late July would likely have the greatest probability of success.

Bait colour and moisture do not affect bait acceptance by introduced Pacific rats (Rattus exulans) at Henderson Island, Pitcairn Islands.

Island and Ocean Ecosystems, BRB

Available Online

Bond, A.L.

McClelland, G.T.W.

OKeefe, S.

Warren, P.

Rodent eradications are a useful tool for the restoration of native biodiversity on islands, but occasionally these operations incur non-target mortality. Changes in cereal bait colour could potentially mitigate these impacts but must not compromise the eradication operation. Changing bait colour may reduce mortality of Henderson crakes (Zapornia atra), an endemic globally threatened flightless bird on Henderson Island, Pitcairn Islands, South Pacific Ocean. Crakes had high non-target mortality in a failed 2011 rat eradication operation and consumed fewer blue than green cereal pellets. We examined which cereal bait properties influenced its acceptance by captive Pacific rats (Rattus exulans) on Henderson Island. We held 82 Pacific rats from Henderson Island in captivity and provided them with non-toxic cereal bait pellets of varying properties (blue or green, moist or dry). We estimated the proportion of rats consuming bait using logistic generalised linear mixed models. We found no effect of sex, females reproductive status, bait colour or bait moisture on rats willingness to consume baits. Rats bait consumption was unaffected by cereal bait properties (colour or moisture). The use of blue bait is unlikely to affect future eradication operational success but may reduce non-target mortality of Henderson crakes. Timing cereal bait distribution in relation to precipitation may also reduce crake mortality without compromising palatability to rats.

Recovery of introduced Pacific rats following a failed eradication attempt on subtropical Henderson Island, South Pacific Ocean

Island and Ocean Ecosystems, BRB

Available Online

Bond, A.L.

Churchyard, T.

Cuthbert, R.J.

Duffi eld, N.

Havery, S.

Kelly, J.

Lavers, J.L.

McClelland, G.T.W.

Oppel S.

Proud, T.

Torr, N.

Vickery, J.A.

Rodent eradications in tropical environments are often more challenging and less successful than those in temperate environments. Reduced seasonality and the lack of a defined annual resource pulse influence rodent population dynamics differently than the well-defined annual cycles on temperate islands, so an understanding of rodent ecology and population dynamics is important to maximise the chances of eradication success in the tropics. Here, we report on the recovery of a Pacific rat (Rattus exulans) population on Henderson Island, South Pacific Ocean, following a failed eradication operation in 2011. We assessed changes in the rat population using capture rates from snap-trapping and investigated seasonality by using capture rates from live-trapping. Following the failed eradication operation in 2011, rat populations increased rapidly with annual per capita growth rates, r, of 0.485.95, increasing from 6080 individuals to two-thirds of the pre-eradication abundance within two years, before decreasing (r = -0.25 -0.20), presumably as the population fluctuated around its carrying capacity. The long-term changes in rat abundance may, however, be confounded by short-term fluctuations: four years after the eradication attempt we observed significant variation in rat trapping rates among months on the plateau, ranging from 36.6 rats per 100 corrected trap-nights in mid-June to 12.6 in late August. Based on mark-recapture, we also estimated rat density fluctuations in the embayment forest between 20.4 and 42.9 rats ha-1 within one month in 2015, and a much lower rat density on the coral plateau fluctuating between 0.76 and 6.08 rats ha-1 in the span of two months. The causes for the short-term density fluctuations are poorly understood, but as eradication operations on tropical and subtropical islands become more frequent, it will be increasingly important to understand the behaviour and ecology of the invasive species targeted to identify times that maximise eradication success.

Invasive rats on tropical islands: their population biology and impacts on native species

Available Online

Bunbury, Nancy

Harper, Grant. A,

The three most invasive rat species, black or ship rat Rattus rattus, brown or Norway rats, R. norvegicus and Pacific rat, R. exulans have been incrementally introduced to islands as humans have explored the worlds oceans. They have caused serious deleterious effects through predation and competition, and extinction of many species on tropical islands, many of which are biodiversity hotspots. All three rat species are found in virtually all habitat types, including mangrove and arid shrub land. Black rats tend to dominate the literature but despite this the population biology of invasive rats, particularly Norway rats, is poorly researched on tropical islands. Pacific rats can often exceed population densities of well over 100 rats ha?1 and black rats can attain densities of 119 rats ha?1, which is much higher than recorded on most temperate islands. High densities are possibly due to high recruitment of young although the data to support this are limited. The generally aseasonally warm climate can lead to year-round breeding but can be restricted by either density-dependent effects interacting with resource constraints often due to aridity. Apparent adverse impacts on birds have been well recorded and almost all tropical seabirds and land birds can be affected by rats. On the Pacific islands, black rats have added to declines and extinctions of land birds caused initially by Pacific rats. Rats have likely caused unrecorded extinctions of native species on tropical islands. Further research required on invasive rats on tropical islands includes the drivers of population growth and carrying capacities that result in high densities and how these differ to temperate islands, habitat use of rats in tropical vegetation types and interactions with other tropical species, particularly the reptiles and invertebrates, including crustaceans.

On the verge of a biological crisis: the state of invasive species in the Pacific

Island and Ocean Ecosystems, BRB

Available Online

Cas Vanderwoude

A report prepared by the Invasive Partnership for the Pacific Islands Forum Leaders Meeting, Palau July 2014 as requested by the Micronesian Chief Executives in Resolution 19-01

Feasibility Assessment for the Removal of Pacific Rats (Rattus exulans) from Late Island. Reports 1 & 2

Bonham, J.

Griffiths, R.

Island Conservation

Feasibility Assessment for the Removal of Pacific Rats (Rattus exulans) from Late Island

Available Online

Bonham. J

Griffiths. R.

Late is an isolated and uninhabited island located about 55 km WSW of the island of Vava'u, in the Kingdom of Tonga. Late supports a tropical broad-leaf forest ecosystem, one of the most threatened ecosystem types in the world and one of the best remaining tracts of diverse native forest in Tonga. Owing to its relatively unmodified forest communities, Late is also a global stronghold for two IUCN listed species of bird, one native mammal, and six species of reptile.

Comprehensive Desk-top Review of Biodiversity, Conservation and Invasive Species Information for the Kingdom of Tonga

Island and Ocean Ecosystems, BRB

Available Online

Pagad. Shyama

Situated between Fiji to the west and Samoa to the northeast, the Kingdom of Tonga (referred to as Tonga) is comprised of 171 scattered islands of which less than 50 are inhabited. The islands are mainly composed of limestone formed from uplifted coral. Current critical environmental concerns have arisen due to deforestation; damage to coral reefs and the introduction and spread of invasive alien species. Anthropogenic pressure has resulted in extensive modification of all ecosystems on the limestone islands of this group. Only uninhabited and steep volcanic islands still support large tracts of forest.

Status of birds, peka (flying foxes) and reptiles on Niue island

Available Online

Butler, D.J.

Powlesland, R.G.

Westbrooke, I.M.

During 1021 September, quantitative surveys were carried out of birds and flying foxes using the same techniques as applied in earlier surveys, and searches carried out for a rare parrot and lizard. Bird counts showed that the lupe or Pacific imperial-pigeon population has recovered following a decline between 1994 and 2004 though the current hunting rate is considered unsustainable. Miti or Polynesian starling numbers have gradually declined over the period 19942012 which is a concern and hard to explain. Rat predation is a possible cause. There was also an indication that pekapeka or white-rumped swiftlet numbers have declined though more investigation is needed to confirm this. We were unable to find any hega or blue-crowned lory though a few individuals were reported to us. Numbers are clearly very low and the next step must be to gain a better understanding of where the species regularly occurs so that management can be applied.

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