Introduced mammals coexist with seabirds at New Island, Falkland Islands: abundance, habitat preferences, and stable isotope analysis of diet

Polar Biol (2008) 31:333–349

ORIGINAL PAPER

Introduced mammals coexist with seabirds at New Island, Falkland Islands: abundance, habitat preferences, and stable isotope analysis of diet

Petra Quillfeldt · Ingrid Schenk · Rona A. R. McGill · Ian J. Strange · Juan F. Masello · Anja Gladbach · Verena Roesch · Robert W. Furness

Received: 26 April 2007 / Revised: 29 August 2007 / Accepted: 4 September 2007 / Published online: 16 October 2007© Springer-Verlag 2007

Abstract The largest known colony of Thin-billed prionsPachyptila belcheri has been coexisting with introducedmammals for more than 100 years. Three of the introducedmammals are potential predators of adults, eggs and chicks,namely ship rats Rattus rattus, house mice Mus musculusand feral cats Felis catus. We here determine habitat prefer-ences over three seasons and dietary patterns of the uniqueset of introduced predators at New Island, Falkland Islands,with emphasis on the ship rats. Our study highlights spatialand temporal diVerences in the levels of interactionbetween predators and native seabirds. Rats and mice had apreference for areas providing cover in the form of thenative tussac grass Parodiochloa Xabellata or introducedgorse Ulex europaeus. Their diet diVered markedlybetween areas, over the season and between age groups in

rats. During the incubation period of the prions in Novem-ber–December, ship rats had mixed diets, composed mainlyof plants and mammals, while only 3% of rats had ingestedbirds. The proportion of ingested birds, including scav-enged, increased in the prion chick-rearing period, when60% of the rats consumed prions. We used �13C and �15Nto compare the importance of marine-derived food betweenmammal species and individuals, and found that rats in allbut one area took diet of partly marine origin, prions beingthe most frequently encountered marine food. Most housemice at New Island mainly had terrestrial diet. The stableisotope analysis of tissues with diVerent turnover timesindicated that individual rats and mice were consistent intheir diet over weeks, but opportunistic in the short term.Some individuals (12% of rats and 7% of mice) were highlyspecialized in marine-derived food. According to the iso-tope ratios in a small sample of cat faeces, rodents and rab-bits were the chief prey of cats at New Island. Althoughsome individuals of all three predators supplement their ter-restrial diet with marine-derived food, the current impact ofpredation by mammals on the large population of Thin-billed prions at New Island appears small due to a numberof factors, including the small size of rodent populationsand restriction mainly to small areas providing cover.

Keywords Ship rat · House mouse · Invasive species · Stable isotopes

Introduction

Small ground-nesting birds on islands rarely survive intro-ductions of mammalian predators such as rats (e.g. Atkin-son 1985), because their dispersal opportunities are limitedand because often such insular populations evolved in the

Ingrid Schenk: deceased

Electronic supplementary material The online version of this article (doi:10.1007/s00300-007-0363-2) contains supplementary material, which is available to authorized users.

P. Quillfeldt (&) · J. F. Masello · A. Gladbach · V. RoeschMax-Planck Institut für Ornithologie, Vogelwarte Radolfzell, Schlossallee 2, 78315 Radolfzell, Germanye-mail: [email protected]

I. Schenk · I. J. StrangeNew Island Conservation Trust, The Dolphins, Stanley, Falkland Islands

R. A. R. McGillScottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, UK

R. W. FurnessInstitute of Biomedical and Life Sciences, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, UK

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absence of natural enemies. Thus, many populationdeclines and extinctions of nesting colonies on islands havebeen observed after accidental or deliberate introduction ofexotic predators, notably feral cats and rats, but also foxesand wekas (e.g. Moors and Atkinson 1984; Brothers 1984),and studies of the eVects of introduced species havebecome a key issue for conservation biology.

However, New Island is an example where a populationof around two million pairs of Thin-billed prions breedssuccessfully at a site with more than 100 years’ history ofintroduced ship rats, house mice and feral cats. Understand-ing the mechanisms of such coexistence is important, as itis important to establish a baseline for future monitoring.New Island is an Important Bird Area (IBA), hosting thelargest seabird colony in the Falkland Islands, and signiW-cant populations of several globally threatened species.Seabirds breeding here include black-browed albatrossThalassarche melanophrys, white-chinned petrel Procellariaaequinoctialis, rockhopper penguin Eudyptes chrysocome,gentoo penguin Pygoscelis papua, Magellanic penguinSpheniscus magellanicus, Falkland skua Stercorarius ant-arctica, dolphin gull Larus scoresbii, kelp gull Larus domi-nicanus, imperial (blue-eyed) shag Phalacrocorax (atriceps)albiventer and rock shag Phalacrocorax magellanicus.

New Island also has a unique set of introduced mam-mals, not found elsewhere in the Falkland Islands. NewIsland is the only island in the Falkland archipelago wherethe black or ship rat Rattus rattus has been found. NewIsland has no evidence of the larger Norway rat Rattus nor-vegicus, which is common elsewhere in the Islands. WithNew Island’s long history of occupation by man, commenc-ing around 1774 by American whalers, and the start of per-manent settlement with sheep farming in 1860, the absenceof Norway rats is fortuitous. There is little evidence ofwhen the ship or black rat may have been introduced,although possibly around 1906 when New Island became asite for a new whaling operation employing a factory shipthe Admiralen. However, introductions were more probablein 1908 when a land-based whaling factory was establishedin South Harbour. This latter operation was a relativelylarge facility employing some 80 men, had a jetty to receivefairly large vessels and a slipway for the repair of shipsused in whaling. (Strange 1995). Such a facility with itsconsiderable ship traYc presented very favourable condi-tions for the introduction of rats and mice. A resident at thetime of the 1908 whaling station, wrote of the station’s clo-sure in 1916, that “Rats of all shapes, sizes and colours—without food from the station—started to roam the island,so our cats had to soon earn their keep” (D. McRae)—astrong inference that rats were not present before the estab-lishment of the station.

Cats were also present in the whaling station, and in the1920s about 30 cats were brought to New Island in order to

control rats. Cottontail rabbits Sylvilagus sp. were deliber-ately introduced by whalers as a source of food. The identi-Wcation of the rabbit species, including the possibility of thepresence of European rabbits Oryctolagus cuniculus, is cur-rently underway (I.J. Strange).

Thus, the alien fauna of New Island today comprisesfour species including three predators or omnivorous spe-cies, and represents a potential threat to the seabirds, espe-cially to the small burrowing thin-billed prions. Since 1972,eVorts have been directed at reducing rats and mice by gen-eral island husbandry. This has involved a strict control onthe disposal of household waste, burning rubbish, installa-tion of better sewage disposal, plus disposal of animalremains such as old sheep and cattle carcasses. Sheep andcattle were completely removed from New Island South in1975. For the last 18 years, a programme of rodent controlusing bait and traps has been in operation.

After preliminary studies in the season 2000–2001, aprogramme of systematic trapping was set up by the lateIngrid Schenk and Ian Strange and carried out by them andsubsequent Weld assistants in three seasons (2001–02,2002–03 and 2003–04). The present paper reviews and syn-thesizes the work carried out so far. We report the results ofthe trapping study (previously unpublished), including ananalysis of diet of ship rats during the incubation period(previously unpublished) and additional new diet data as wellas data published in reports only locally available (MacKayet al. 2001) on diet during the chick-rearing period of thethin-billed prions. The latter aspect is given the most attention,and is complemented with a stable isotope analysis.

A stable isotope approach has been used by previoussuccessful studies of diets of introduced predators (Hobsonet al. 1999; Stapp 2002; Major et al. 2007), using carbon(13C/12C) and nitrogen (15N/14N) isotope ratios. DiVerencesin nitrogen isotope ratios are frequently used to determinetrophic level and diet composition (e.g. Dahl et al. 2003;Morrison and Hobson 2004; Quillfeldt et al. 2005). In con-trast to nitrogen, carbon isotope ratios diVer more betweenterrestrial versus marine, inshore versus oVshore, andpelagic versus benthic food webs than by trophic level.Carbon can therefore be used to assess foraging location(reviewed in Hobson 1999; Rubenstein and Hobson 2004).Carbon and nitrogen isotope analysis gives an integratedpicture of what is assimilated into tissue from diet unlikeconventional dietary methods, which may provide a partialor biased snapshot of diet. Study of stomach contents orfaeces, for example, may under-represent food items whichleave little or no visual trace in samples and prone to over-estimation of less digestible materials which are highly vis-ible. Stable isotope data of tissues of diVerent turnover ratesalso allow detection of diet switches (e.g. Tieszen et al.1983) and patterns of individual diet specialization (e.g.Bearhop et al. 2006).

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In this paper we consider carbon and nitrogen stable iso-tope data from the introduced mammals of New Island, anddiscuss them in the context of mammal abundances and incomparison with dietary data obtained by visual analysis ofstomach contents, to assess the impact of introduced mam-mals on the native bird populations, in particular the thin-billed prion. SpeciWcally, we will:

1. Describe patterns of abundance and habitat preferenceof Rattus rattus and Mus musculus.

2. Summarize dietary data obtained by visual analysis ofstomach contents of Rattus rattus

3. Compare C and N stable isotope ratios of mammal tis-sues and faeces with potential marine prey (seabirds,marine invertebrates, marine algae) and terrestrial prey(rabbits, rodents, terrestrial plants), in order to analysediets of Rattus rattus, Felis catus and Mus musculus.

4. Investigate intra-speciWc dietary diVerences in rodentsfrom diVerent areas (open areas with low abundance ofrodents and high prion numbers vs. sheltered areas withhigh abundance of rodents)

5. Investigate inter-individual variability in patterns ofdiet of the two rodent species.

Materials and methods

Study site

New Island, West Falkland (51°43�S, 61°17�W), is 13 kmlong and on average 0.75 km wide, with a total of 2,362 ha.Until recently, it was divided in two properties, and all datapresented were collected in New Island South (1,181 ha).The management of New Island South as a wildlife reservestarted in 1972 with all sheep being removed in 1975.

In section, the island is wedge shaped, with cliVs form-ing the western and northern coasts, while the easterncoasts are lower lying and gently sloped, comprising rockyshores and sandy bays. Upland areas are rocky or coveredwith short heath, Diddle dee Empetrum rubrum, MountainBerry Pernettya pumila and cushion plants (feldmark),while the slopes are mainly covered with short (oceanicheath) vegetation, in many places dominated by Small FernBlechnum penna-marina and the introduced grass York-shire fog Holcus lanatus. Some areas, in particular twoshallow valleys (South End Tussac area and the southernslopes of Rookery Hill), are covered with dense or looseformations of tussac grass Parodiochloa Xabellata andBlue Couch grass Agropyron magellanicum interspersedwith Wild Celery Apium australe. Some areas, in particulartwo shallow valleys, are covered with tussac grass (SouthEnd tussac area and Settlement Rookery tussac area; seemap and photos in Electronic appendix 1).

New Island is an Important Bird Area (IBA, BirdLife),and by far the most numerous seabird species breeding hereis the thin-billed prion. The prions arrive at the breedinggrounds in September to October, lay their single egg inNovember and after incubation for 46–48 days the chickshatch in the Wrst half of January (Strange 1980; Quillfeldtet al. 2003). Chicks are brooded for few days, and duringthe remainder of the chick-feeding period of 48 to 56 daysthey are usually only attended by the parents for short peri-ods at night (e.g. Quillfeldt et al. 2007a).

Sample collection 2000–2005: trapping programmes

Trapping was carried out in Wve seasons in total (australsummers 2000–01 to 2004–05). In the season 2000–2001,rodents were caught to establish whether Norway ratswere present, as they would represent a signiWcant threatto the thin-billed prions on the island. In October andNovember 2000, a total of 43 bait and trap stations wereset out in two areas of tussac grass. In addition, rats weretrapped around three study areas of thin-billed prions toreduce rat numbers and the potential level of predation(MacKay et al. 2001). The rodents caught in the prelimi-nary studies were identiWed as the ship rat and the housemouse (Derek Brown, New Zealand Department of Con-servation, Prof Tom Berry and Dr Scobie Pye, on siteidentiWcation).

Following this, systematic trapping across the southernhalf of New Island was conducted in the three seasons2001–02 to 2003–04. In the other years, less extense trap-ping focused on the impact of rats on seabirds, conducted inor adjacent to prion nesting areas and close to the Settle-ment Rookery, a mixed colony of rockhopper penguinsimperial (blue-eyed) shags and black-browed albatross.

Snap traps (Victor E-Z Set, model M206, WoodstreamCorporation, PA, USA) were set in the Weld, baited with ateaspoon of peanut butter, and attached by thin plastic cov-ered wire to a pin fashioned from No 8 or No 10 fence wire(4.0–3.5 mm). The pin is then secured Wrmly into theground to prevent possible removal by scavengers (striatedcaracara Phalcoboenus australis or cats). A cost-eYcientcover was developed to protect the traps from incidentalby-catch (like birds and small rabbits), but still allow easyvisual inspection of traps (see Electronic appendix 2).

Traps were placed in lines consisting of 10 to 25 trapsplaced in line with a distance of about 10–25 m andchecked daily during daylight. Lines were opened forperiods of several days, and some were reopened one ortwo more times during the season, with an interval ofabout 10 days to allow rodents to move into any vacantterritories.

Captured dead rodents were determined, and measure-ments were taken for all intact rats. A stopped rule was

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used to measure body length (from the tip of the nose to theend of the fur at the base of the tail), tail length (end of furto end of tail), foot length (from heel of right hind foot totip of toe, excluding the claw) and ear length (from the low-est point of the basal notch to the furthest extremity,excluding any hairs), to nearest millimetre. A Pesola springbalance was used to determine body weight, in grams, ifanimals were intact (i.e. not partly eaten by scavengingrodents). Sex was determined by examination of externalgenitalia.

Rats of three seasons (2000–01 to 2002–03) wereclassed in three colour morphs according to Corbet andSouthern (1977): all black (rattus type), grey with greybelly (alexandrinus type) and grey with cream belly (fru-givorous type). A small number of specimens were notdetermined because observers did not agree on their classi-Wcation. The stomach contents of a sub-sample of capturedrats of the periods 19 to 27 February 2001 (N = 15) and 14November 2002–17 January 2003 (N = 187) were checkedvisually, as a stereomicroscope was not available duringthat time. Of the latter sample, the contents of 17 rats wereundistinguishable, such that 161 stomach contents wereused for the analysis.

Sample collection 2005–2006: stable isotope samples

The stable isotope study includes samples collected from19 to 27 February 2005 and 10 February to 7 March 2006,when prion chicks were in the second half of their nestlingstage, close to Xedging. The data from both years are com-bined in the present analysis.

When captured, rats were dissected to obtain stomachcontents, faecal matter from the terminal part of the gut, aswell as liver and muscle tissue. The samples were stored in80% ethanol. Prior to preservation in ethanol, the stomachcontents of all intact captured rats of the period 19 to 27February 2005 and 14 February to 7 March 2006 (n = 28)were checked using a stereomicroscope at 20£ magniWca-tion. Captured mice (n = 12) were dissected to obtain gutcontents, as well as liver and muscle tissue, and sampleswere stored in 80% ethanol. For cats, we only obtained fae-cal pellets (scats). However, such scats contained a highproportion of undigested material such as small bones, furand feathers and thus were the best samples available torepresent cat diets, as isotopic depletion should be minimalin such little digested samples. Faecal pellets of cats(n = 15) were collected across the island to avoid replica-tion of individuals, and were stored at ¡20°C.

Rabbit fur was collected opportunistically around thesettlement, and most likely originates from rabbits predatedby cats or birds of prey.

Feathers and blood cell samples from seabird chickswere collected opportunistically from dead chicks or as part

of ongoing projects. Feathers of Falkland thrushes Turdusfalcklandii and upland geese Chloephaga picta were col-lected opportunistically in the Weld (moulted feathers).Plants and algae were collected and dried, and terrestrialinvertebrates (undetermined beetle larvae and spiders) werecollected and stored in 80% ethanol.

Sample preparation for stable isotope analysis

Before isotopic analysis, the lipids of all rodent tissues,stomach contents and faeces were extracted in a Soxhletapparatus for 6 h using chloroform and methanol mixture at2:1. Following extraction, the samples were dried under afume hood for at least 12 h and ground to a homogeneousWne powder.

Feathers were cut into small fragments, and red bloodcells were freeze-dried and ground. Plant samples, rabbitfur and cat faeces were ground to a homogeneous Wne pow-der at liquid nitrogen temperature in a ball mill. Carbon andnitrogen isotope assays were carried out on aliquots ofhomogenized powder and weighed into tin cups.

Carbon and Nitrogen isotope ratios were measured bycontinuous-Xow isotope ratio mass spectrometry (CF-IRMS) using a Costech Elemental Analyser (EA) linked toa Thermo Finnigan Delta Plus XP Mass Spectrometer.Approximately 0.7 mg of each sample of animal tissue and1–2 mg of plant tissues were combusted in a tin cup for thesimultaneous determination of carbon and nitrogen isotoperatios. Two laboratory standards were analysed for every 10unknown samples, allowing any instrument drift over a typ-ical 16 h run to be corrected. Stable isotope ratios wereexpressed in � notation as parts per thousand (‰) deviationfrom the international standards V-Pee dee belemnite (car-bon) and AIR (nitrogen), according to the following equa-tion � X = [(Rsample/Rstandard) ¡ 1] £ 1,000 where X is 15N or13C and R is the corresponding ratio 15N/14N or 13C/12C.Measurement precision of both �15N and �13C was esti-mated to be less than 0.3‰.

Data analysis

Trapping eVort was calculated as total number of trapnights. Trapping eYciency was calculated for each line, asthe number of rats or mice per 100 trap nights following themethod described by Cunningham and Moors (1996). Toanalyse diVerences in distribution and abundance, trap linescovering one of four distinct habitat areas (open areas,gorse Ulex europaeus close to the settlement, tussac grass-land around the Settlement Rookery and tussac grassland atthe South End Tussac area) were distinguished, and meanvalues for all lines of a particular habitat area are given.Some lines did not enter in any these categories, and weretherefore not included in this analysis.

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Due to diVerent metabolic routing, diVerent tissues havediVerent isotopic signatures. To compare and interpret iso-topic diVerences between tissues, we took these diVerencesinto account as follows. In order to correct for the biochem-ical components of food change when incorporated into thetissues of a consumer, we applied a discrimination factor of1–2‰ for carbon and 3% for nitrogen to the d13C andd15N values (Tieszen et al. 1983; Minagawa and Wada1984; Major et al. 2007). To account for the biochemicaldiVerences between the sampled liver and muscle tissueand feathers in the samples of birds, we used discriminationfactors found by Mizutani and Wada (1991).

In the statistical comparisons of groups, we compared�15N and �13C values of similar tissues among rodents ofdiVerent species, area, and year using one-way ANOVA.

For tests of dietary consistency we correlated the diVer-ent sample types of each individual using General LinearModels that controlled for the eVect of the trapping area byincluding area as a categorical independent variable (‘fac-tor’). In these tests we assumed that discrimination factorsvary between tissues, but much less between individuals.SigniWcance was assumed at P < 0.05.

Results

Abundance and habitat preference

Ship rats and house mice were caught in all seasons. Thedistribution of rats and mice diVered between habitats (�2

tests for each season and species, all P < 0.001). The dataindicated that open areas, which are the most extensivehabitat on the island, have a very low density of rats andmice (Fig. 1). In contrast, areas providing cover in theform of the native tussac grass or introduced gorse maycontain considerable numbers of both species of rodents(Fig. 1). The density was consistently higher in one of thetwo tussac areas, close to the Settlement Rookery (pairedt-test of data of four seasons; t = ¡3.264, df = 3,P = 0.047, Fig. 1).

Inter-annual variability in abundance

Two index lines were used identically in 2001–02 and2003–04 (Table 1), and one survey line has been used overthe whole 6-year period. The data from the index lines sug-gest that the abundance of mice diVered between years inthe two lines (Diddle dee line: �2 = 42.2, df = 1, P < 0.001;South End tussac area line: �2 = 105.6, df = 1, P < 0.001),but that of rats was similar (Diddle dee line: no rats, SouthEnd tussac area line: �2 = 0.129, df = 1, P = 0.720). Thedata from the survey line (Settlement gorse, Table 2) indi-cated seasonal changes both in rats (�2 = 25.7, df = 4,

P < 0.001), and, more strongly, in mice (�2 = 103.2, df = 4,P < 0.001).

Colour morphs, sexes and measurements of ship rats

Of the trapped rats, frugivorous was the most common typewith 67–75% over the years (Table 3), followed by alexan-drinus with 25–33%. Rattus was uncommon, as only threespecimens were recorded in total, two of which were caughtat the settlement. The diVerence in the occurrence of eachtype was highly signiWcant in all years (Table 3).

The sex ratio of trapped animals was even in the Wrst twoseasons 2000–01 (31 females: 41 males, Binomial test,P = 0.289) and 2001–02 (34 females: 25 males, Binomialtest, P = 0.298). In the last two seasons, however, it becamestrongly male-skewed with a ratio of about 1:10 in both2002–03 (17 females: 167 males, Binomial test, P < 0.001)and 2003–04 (5 females: 65 males, Binomial test,P < 0.001).

The mean size and weight of trapped male and femalerats did not diVer (Table 4), except for the slightly largerfoot length of females (diVerence of less than 1 mm). Thesize distribution of trapped ship rats, however, diVeredbetween years (Fig. 2). Initially, the distribution was unimodal,

Fig. 1 Rodent abundance during three seasons of systematic trappingat New Island, Falkland Islands

Rattus rattus

2001-2002 2002-2003 2003-2004

sthgin part 001 / staR

0

2

4

6

8

10

12

14

16 (3700 trap nights) (3910) (3405)

Mus musculus

Season

2001-2002 2002-2003 2003-2004

1 / eciM

sth gi n part 0 0

0

10

20

30

40

Settlement gorseOpen areasSouth End tussacRookery tussac

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with the strongest class of 150–170 mm body length foundin the middle of the distribution and comprising 40% of thecases in 2000–01. This changed progressively to a bimodaldistribution with the strongest classes found to comprisemore than 32% of the trapped rats each, at 110–130 and170 to 190 mm in 2003–04. The initially strong size classof 150–170 mm body length had completely disappeared inthe sample of 2003–04 (Fig. 2), and this change in distributionwas found in both males and females.

Visual determination of ship rat diet

Rats sampled during the incubation period of the prions hada mixed diet (Table 5), with plants occurring in 128 of 161stomach contents (80%) and animal items in 82 of 161stomach contents (51%). The stomach contents were classedas 20% pure animal matter, 49% pure plant matter, and 31%of mixed origin. Of vegetation, tussac grass Wbres were themost important food source by occurrence, present in 74 of128 stomach contents of vegetal content (58%). Vegetalcontent also included the Xowers of gorse. Items were iden-tiWed in 58 samples (i.e. stomach contents of one individual)of animal origin. Of these, 43 (74%) contained mammals, asdetermined by the presence of hairs, of which at least 5 werescavenged as indicated by the presence of maggots in thestomach contents. 10 samples (17%) contained insects andinsect larvae, and 5 samples (8% of animal items or 3% oftotal samples) contained traces of ingested birds, threefeathers and two yolks. The time and place of the samplescollected with yolk suggest that they originated from dis-placed rockhopper penguin eggs (F. Zuñiga, personal com-munication). The overall distribution of animal versusvegetal diet did not diVer between rats trapped in three areas(Table 5: vegetal diet: �2 = 0.04, df = 2, P = 0.97, Animaldiet: �2 = 0.20, df = 2, P = 0.91). However, there was a

Table 1 Numbers of ship rats trapped in lines used repeatedly in two seasons

Trap nights Number of rats Trap eYciency Number mice Trap eYciency

Open area (50 traps in open diddle dee heath)

Season 2001–02 450 0 0 27 6.0

Season 2003–04 900 0 0 2 0.2

South End tussac area line (50 traps, mainly in dense tussac, with some grass patches)

Season 2001–02 550 11 2.0 111 20.2

Season 2003–04 600 15 2.7 1 0.2

Table 2 Numbers of ship rats trapped in Settlement Gorse Survey lines in six seasons

Trap nights Number of rats Trap eYciency Number mice Trap eYciency

Season 2000–01 1378 56 4.1 71 5.2

Season 2001–02 320 23 7.2 63 19.8

Season 2002–03 220 1 0.5 3 1.4

Season 2003–04 460 5 1.1 10 2.2

Season 2004–05 84 4 4.8 3 3.6

Table 3 Distribution of colour morphs of ship rats at New Island

The percentage of occurrence in the sample of trapped animals is given, and chi-square tests were carried out on raw data

Season 2000–01 (N = 56) Season 2001–02 (N = 61) Season 2002–03 (N = 180) Season 2003–04 (N = 21)

Alexandrinus 26.8% 24.6% 29.4% 33.3%

Frugivorous 69.6% 75.4% 70.0% 66.7%

Rattus 3.6% 0% 0.6% 0%

Test (df = 2) �2 = 28.0, P < 0.001 �2 = 39.9, P < 0.001 �2 = 101.0, P < 0.001 �2 = 11.2, P = 0.004

Table 4 Mean size and weight of trapped male and female ship ratsRattus rattus at New Island 2000–01 to 2003–04 (Mean § SE, all sea-sons and ages combined)

Parameter Males (N = 231)

Females (N = 75)

Mann–Whitney U test

Weight (g) 122.8 § 4.1 130.6 § 7.6 U = 8070.5, P = 0.374

Body length (mm) 154.4 § 1.9 159.0 § 3.4 U = 7845.0, P = 0.219

Tail length (mm) 175.4 § 2.0 178.9 § 3.8 U = 8027.5, P = 0.340

Foot length (mm) 31.3 § 0.2 32.0 § 0.4 U = 7302.0, P = 0.040

Ear length (mm) 21.6 § 0.2 21.8 § 0.3 U = 7907.0, P = 0.252

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diVerence in the diet between young and adult rats. Youngrats fed largely on vegetation, as only 18 of 64 stomachsfrom young rats (28%) contained any animal material. Incomparison, adult rats had a signiWcantly higher proportionof ingested animals (52 of 82 stomachs, or 63%, Chi-squaretest of young vs. adults �2 = 5.1, df = 1, P = 0.016). Themost pronounced diVerence in diet between adults and

young rats was found in the South End Tussac area (Fig. 3)where more than 80% of the samples of adults were of ani-mal origin. Sampled adults were mainly males, but therewas no sex diVerence in the proportion of animal diet (63%in 68 samples of males, 64% in 14 samples of females,�2 = 0.001, df = 1, P = 0.971).

A small data set was collected in February 2001, and isconsistent with data from 2005–06 (Table 5). During thenestling period of Thin-billed prions, ship rats took a mixeddiet of rats, with prions being an important component.Prion feathers occurred in the stomach contents of somerats from all habitats, although in low incidence in the Set-tlement Rookery tussac where prions are uncommon. Ofother components of the diet, plants were particularlyimportant in the Settlement Rookery tussac, mammals wereconsumed in all areas and some marine food (Wsh and crus-taceans) was also consumed. The presence of maggots indi-cates that a part of the animal diet was obtained byscavenging.

Isotopic background data and comparison of mammal species and prions

We found that the carbon isotope ratios varied between ter-restrial and marine organisms (Fig. 4). However, the threehabitat types overlapped considerably in �15N, where themain diVerence was found in highly elevated values for tus-sac grass sampled around seabird colonies (Fig. 4).

The �13C measured in the fur of rabbits and the diet ofrats, mice and cats is shown in Fig. 5. Rabbits, although nota potential predator, are included as reference for a terres-trial herbivore. Among the other mammals, �13C values ofthe diet of rats, mice and cats diVered (H = 12.7, df = 2,P = 0.002). Dunn’s post hoc tests indicated a signiWcantdiVerence between the diets of rats and the cats only(P < 0.05), the �13C in cat faeces being lower (i.e. indicat-ing more terrestrial diet) than that of rats. The �13C mea-sured in the diet of all three potential predators of prions(rats, mice and cats) diVered from the �13C of prion chickred blood cells (H = 44.7, df = 3, P < 0.001, Dunn’s posthoc tests for prions vs. each mammal P < 0.05). Thus, allthree mammals had a �13C indicative of a mixture of preyderived from marine and terrestrial sources.

Stable isotope analysis of rat diet

Rats captured in the four areas are analysed separately inorder to gain a deeper insight into the variability of foragingstrategies. Consistent with the observed mixed diets in ratstomach contents, the carbon isotope ratios of stomach con-tents of rats showed a wide range (Fig. 5), but also consid-erable overlap with the �13C found in prion chick red bloodcells.

Fig. 2 Distribution of body sizes of ship rats trapped at New Island infour seasons. a Both sexes combined and b separate for each sex

0

10

20

30

40

50

a

100120

140160

180200

220

20012002

20032004

Obs

erva

tions

%

Body

leng

th(m

m)

Rats (sexes combined)

0

10

20

30

40

50

60

100120

140160

180200

220

20012002

20032004

Obs

erva

tions

%

Boyd

ltgne

h(m

m)

Rats (females)

0

10

20

30

40

100120

140160

180200

220

20012002

20032004

Obs

erva

toins

%

Bdoy

len

tgh

(mm

)

Rats (males)

b

123

340 Polar Biol (2008) 31:333–349

The isotope ratios of all sample types (stomach contents,faeces, liver and muscle tissue) diVered between the areas(Table 6). When carbon and nitrogen were analysed sepa-rately, nitrogen diVered between areas for all sample types(ANOVAs, all tissues P < 0.001). There was no diVerencein �13C of samples of stomach contents, faeces and liver(ANOVAs, all P > 0.05). However, the rats from diVerentareas had distinct �13C values in muscle tissue (ANOVA,F3,30 = 10.4, P < 0.001). Post hoc tests revealed that the ratsof the Settlement Rookery tussac were distinguished fromall other areas by low �13C (all P < 0.01), whereas rats fromthe South End tussac, the gorse areas and the open areas didnot diVer in �13C.

The liver and muscle were between 1 and 4‰ enrichedin �13C compared with stomach contents and faeces(Table 6), and between 3 and 5‰ enriched in �15N. Meanvalues found in liver and muscle were similar within areas

(Table 6). Within-individual correlation of isotope ratios ofthe four sample types of ship rats indicated some degree ofindividual consistency of diet, especially a strong correla-tion between liver and muscle samples. The evidence for aconsistency of tissues with the last few meals (stomachcontents and faeces) was more mixed (Table 7), indicatingsome day-to day variability in the diet.

Of all rats, those caught in the Settlement Rookery tus-sac showed the most terrestrial diet according to their mus-cle tissue and stomach contents (Fig. 6; Table 6), andelevated �15N values in all sample types indicated that tus-sac grass was an important part of the diet for rats in thisarea both in the long and short term. The data of the stom-ach contents are also consistent with predation or scaveng-ing on mice and rats, as these are not distinguished wellfrom the tussock-forming grasses. A single stomach thatcontained Wsh had an isotope signature well separated fromthe other values, and grouping with the marine isotopic data(Fig. 6).

In contrast to the rats from the Settlement Rookery tus-sac, the isotope ratios of rats from the South End tussacarea indicated a mixed diet with terrestrial and marine com-ponents (Fig. 7). The muscle and liver isotope ratios of ratsfrom the South End tussac area fell between the terrestrialand the marine background data, except for one rat, whichgrouped with the marine data, indicating consistent prefer-ence for marine-derived food over a period of severalweeks. The stomach of that latter rat contained feathers ofthin-billed prions as well as green plant material.

Rats caught in traps adjacent to gorse lines borderingprion areas also showed a mixed diet (Table 5), and theirisotope ratios spanned a wide range within and between themarine and the terrestrial data (Fig. 8). As in the South End

Fig. 3 DiVerence in the diet composition of adult and young ratstrapped in the South End tussac area at New Island

Young rats

AnimalsTussac grassundet. vegetal matter

Adult rats

Table 5 Summary of ship rat diet data obtained by visual stomachcontent analysis during the incubation period of Thin-billed prions(three areas in the period 14 November 2002 to 17 January 2003),

compared to the chick-rearing periods 2001 (19 to 26 February 2001,MacKay et al. 2001) and 2005–2006 (19 to 27 February 2005 and 14February to 7 March 2006)

Diet type Incubation 2003 Chick-rearing 2001 + 2006

Rookery Tussac (N = 96)

South End Tussac (N = 32)

Gorse areas (N = 13)

February 2001 (N = 15)

Rookery Tussac (N = 7)

South End Tussac area (N = 8)

Gorse areas (N = 11)

Open areas (N = 2)

All 2005–6 (N = 28)

Vegetal 78% 81% 85% 60% 86% 25% 18% 100% 43%

Tussac grass 56% 77% 18% - 86% 13% 0% 0% 25%

Animal 51% 56% 62% 93% 71% 100% 100% 100% 93%

Mammals (hairs) 71% 79% 86% 7% 14% 25% 27% 50% 25%

Birds (feathers) 0% 7% 0% 60% 14% 88% 64% 100% 61%

Birds (eggs) 6% 0% 0% 0% 0% 0% 0% 0% 0%

Fish – – – – 28% 0% 0% 0% 7%

Invertebrates 23% 14% 14% 0% – – – – –

Crustaceans – – – – 14% 0% 9% 0% 7%

Maggots – – – 13% 0% 25% 9% 0% 11%

Figures denote % occurrence; a line indicates a parameter not recorded in the dataset

123

Polar Biol (2008) 31:333–349 341

tussac area, one individual had high muscle and liver �13C,indicating consistent preference for marine-derived foodover a period of several weeks. The stomach of that latterrat contained a mash of crustaceans, including legs of krillEuphausia sp., but no traces of prion feathers or tissues.

Finally, two rats were caught in open areas with shortvegetation, close to prion burrows. The low sample sizehere is explained by the scarcity of rats in this habitat, seeabove. The stable isotope ratios of stomach contents andfaeces of the two individuals (Fig. 9) again suggested amixed marine and terrestrial diet, consistent with theirstomach contents (containing prion feathers in both cases,one mixed with plants, one with hairs). The liver and mus-cle isotope ratios suggested that prions were a regular com-ponent of the diet of both rats caught in open areas.

Thus, we found individuals that specialized on marine-derived food during the time of our analyses in two of threeareas (in total 4 of 34 rats or 12%).

Stable isotope analysis of mouse diet

Feral house mice at New Island mainly had a terrestrial diet(Fig. 10). None of their gut contents overlapped with prionred blood cells (Fig. 5), but one mouse from an open areagrouped with the marine data for muscle isotope ratios(Fig. 10), indicating a specialisation of this latter mouse inmarine-derived food. In muscle tissue, several mice hadisotope ratios in a position intermediate between terrestrialand marine values, indicative of a mixed diet, while mostindividuals grouped best with a terrestrial diet (Fig. 10).Mice of the diVerent areas diVered in their isotope ratios(Table 8), mainly due to elevated nitrogen isotope ratios inthe vegetation of the tussac areas, particularly in the Settle-ment Rookery Tussac area.

Within-individual correlation of isotope ratios of thethree sample types of mice indicated a high degree of indi-vidual consistency of diet, with a strong correlationbetween gut contents, liver and muscle samples (Table 9).

Stable isotope analysis of cat diet

We consider four main prey species observed: rabbits, rats,mice and prions (Figs. 5, 11). Of 15 cat scats analysed, 2(13%) were separated in �13C values and matched withprion chick red blood cells (Fig. 5), but also overlappedwith the higher values within rat and mice muscle tissues;thus providing only a maximum estimate for prion preda-tion by cats. For the remaining samples, potential prey con-sists of rabbits, mice and rats. The lowest �13C valuesobserved in mice muscle tissue and rat muscle tissue were

Fig. 4 Carbon and nitrogen stable isotope ratios for animals andplants from diVerent areas and habitat types at New Island, Falkland Is-lands. White symbols mark plants, white grey symbols mark animals.For thin-billed prions, three means were included, which had similar

�15N, but diVered in their and �13C (from left to right: chick feathers,chick down, egg membranes). Prion diet consisted of a mixture of crus-taceans (mainly euphausids, amphipods and copepods, Quillfeldtunpublished data)

Terrestrial

Marine

Coastal

δ13C

-30 -28 -26 -24 -22 -20 -18 -16 -14 -12 -10

δ51N

0

5

10

15

20

25

30

35

40Prion chicks Rabbits (fur)Diddle dee (Empetrum rubrum)Tussac grass (South Bay)Tussac grass (Burnt Island)Tussac grass (Rookery tussac)Upland geeseFalkland thrushYorkshire fog (Holcus lanatus)Coastal algaePrion dietGorseShag chicksGentoo chicksAlbatross chicksTerrestrial invertebrates

Fig. 5 Carbon stable isotope ratio of diet and tissue samples of fourintroduced mammals at New Island. Terrestrial plants and red bloodcells (RBC) of prion chicks are included for reference. For house mice,open symbols are used for gut contents, Wlled symbols for muscle sam-ples. Rat stomach contents are subdivided in categories according tothe visual identiWcation of contents

Carbon isotope ratioTerrestrial Marine

-30 -28 -26 -24 -22 -20 -18 -16 -14

Terrestrial plants

Rabbits (fur)

Mice (gut content+muscle)

Cat prey (feces)

Rats (stomach contents)

Prions (chick RBC)

Mainly plantsMainly mammalsMarine (fish, krill)containing prion

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342 Polar Biol (2008) 31:333–349

¡24.03 and ¡22.81‰, respectively, whereas the highestvalue observed for rabbit fur was ¡25.5%; and fur is likelyto be slightly elevated compared with rabbit muscle. Thus,scats consisting mainly of rabbits may be separated using acut-oV point of ¡26‰ (e.g. Fig. 4), mixed samples ofrodent and rabbit origin are expected in the range of ¡26 to¡24‰, and rodent prey would be expected above ¡24‰.According to this, a total of 3 samples (20%) containedmainly rabbit tissue, while 3 samples (20%) containedmainly rodents, and the remaining 7 samples were mostlikely of mixed origin (rabbits and rodents). In summary,our estimate for the occurrence of prey is: prions in 13% ofscats (maximum estimate), rodents in 67% of scats and rab-bits in 67% of scats.

Discussion

In the present study, the trophic relationships betweenintroduced mammals and seabirds in a remote subantarcticisland are investigated. The present study highlights spatialand temporal diVerences in the levels of interactionbetween predators and native seabirds.

Presence and abundance of rodents in diVerent habitats

The abundance of both rats and mice diVered stronglybetween habitats. Open areas, which are by far the mostextensive habitat on New Island, had very low densities of

Table 6 Stable isotope signa-tures (‰) of ship rats at New Island (mean and standard error), and results from Multivariate ANOVA tests for diVerences between areas

Rookery Tussac (n = 7)

South End Tussac area (n = 8)

Gorse areas (n = 11)

Open areas (n = 2)

All (n = 28)

Stomach content

�13C ¡22.4 § 0.7 ¡23.1 § 0.9 ¡23.1 § 0.9 ¡20.8 § 0.2 Wilk’s � = 0.229, P < 0.001

�15N 26.8 § 2.4 17.3 § 1.3 12.2 § 0.4 14.3 § 1.3

Feces

�13C ¡23.6 § 0.3 ¡23.9 § 0.3 ¡23.2 § 0.9 ¡22.6 Wilk’s � = 0.107, P < 0.001

�15N 27.3 § 0.4 17.9 § 1.7 12.6 § 0.5 14.8

Liver

�13C ¡21.2 § 0.4 ¡19.9 § 0.4 ¡20.0 § 0.4 ¡18.6 § 0.2 Wilk’s � = 0.086, P < 0.001

�15N 32.2 § 1.2 22.1 § 1.0 16.9 § 0.2 17.1 § 1.3

Muscle

�13C ¡21.8 § 0.2 ¡19.5 § 0.3 ¡20.2 § 0.4 ¡18.0 § 0.2 Wilk’s � = 0.061, P < 0.001

�15N 32.6 § 0.9 21.0 § 1.0 16.8 § 0.2 17.9 § 0.5

Table 7 Within-individual correlation of isotope ratios of four sampletypes of ship rats (Rattus rattus) at New Island

GLM were used to control for the eVect of area, but for readability wegive only the eVect size (eta squared value) of pair-wise correlationsbetween sample types

* denotes signiWcance values of P < 0.05, while ** denotes signiW-cance values of P < 0.01 and *** P < 0.001

Sample type Faeces Liver Muscle

Carbon

Stomach contents 0.153 0.249** 0.142*

Faeces – 0.568** 0.603**

Liver – – 0.726***

Nitrogen

Stomach contents 0.341* 0.123 0.102

Faeces – 0.009 0.004

Liver – – 0.699***

Fig. 6 Carbon and nitrogen stable isotope ratios for ship rats from theRookery tussac area, New Island. The background data, indicated ingrey shades, correspond to the data given in Fig. 4

13C

-30 -28 -26 -24 -22 -20 -18 -16 -14 -12 -10

51N

0

5

10

15

20

25

30

35

40

Terrestrial

Marine Coastal

Stomach contentsFecesLiverMuscleMice (muscle)

Rats –Rookery tussac

stomach containing fish

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Polar Biol (2008) 31:333–349 343

rats and mice. In contrast, areas providing cover in the formof the native tussac grass or introduced gorse containedhigher numbers of both species of rodents. New Island hasa small number of introduced feral cats, which prey mainlyon rats, mice and rabbits (Matias 2005). Predators are alsofound among the birds of New Island, such as the Falklandskua, short-eared owl Asio Xammeus and red-backed hawkButeo polyosoma. Other potential predators are the striatedcaracara and the crested caracara. Areas of cover maytherefore be preferred because they oVer protection for therodents from predation. Choice of a habitat providing ref-uge is an integral component of predator-avoidance behav-iour, and if a habitat has few refuges or a low complexityand animals feed only near refuge (e.g. Orrock et al. 2004),

then they will have access to less food. In such habitats,rodents may suVer non-lethal eVects of the perceived preda-tion risk such as lower growth rates and delayed reproduc-tion, as has been shown experimentally for house mice(Arthur et al. 2004).

The abundance of rats in areas of cover at New Island,on the other hand, was not related to the presence of smallburrow-nesting seabirds, as one area of relatively high ratdensity (Settlement Rookery Tussac) has no or very fewnests of prions (personal observation). Moreover, althoughthe present study included the time of rockhopper penguinand imperial shag incubation and hatching, very little evi-dence was found in the rat diet for an interaction betweenthe rats and penguins. The present data therefore suggest

Fig. 7 Carbon and nitrogen stable isotope ratios for ship rats from theSouth end tussac area, New Island. The background data, indicated ingrey shades, correspond to the data given in Fig. 4

13C

-30 -28 -26 -24 -22 -20 -18 -16 -14 -12 -10

51N

0

5

10

15

20

25

30

35

40

Terrestrial

Marine Coastal

Stomach contentsFecesLiverMuscleMice (muscle)

Rats – South End tussac area

Fig. 8 Carbon and nitrogen stable isotope ratios for ship rats from the“Prion house” gorse line, adjacent to dense prion nesting areas. Thebackground data, indicated in grey shades, correspond to the datagiven in Fig. 4

13C

-30

51N

0

5

10

15

20

25

30

35

40

Terrestrial

Marine Coastal

Rats - Prion House gorse

Stomach contentsFecesLiverMuscleMice (muscle)

-10-12-14-16-18-20-22-24-26-28

Fig. 9 Carbon and nitrogen stable isotope ratios for ship rats fromopen areas of New Island. The background data, indicated in greyshades, correspond to the data given in Fig. 4

13C

-30 -28 -26 -24 -22 -20 -18 -16 -14 -12 -10

51N

0

5

10

15

20

25

30

35

40

Terrestrial

Marine Coastal

Rats - Open areas

Stomach contentsFecesLiverMuscleMice (muscle)

Fig. 10 Carbon and nitrogen stable isotope ratios for muscle samplesof house mice from diVerent areas of New Island. The background da-ta, indicated in grey shades, correspond to the data given in Fig. 4

δ13C

-30 -28 -26 -24 -22 -20 -18 -16 -14 -12 -10

δ51N

0

5

10

15

20

25

30

35

40

Terrestrial

Marine Coastal

Mice (muscle) - all areas

Open areaPrion HouseSouth End tussac areaRookery tussac

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344 Polar Biol (2008) 31:333–349

that the rats prefer the Settlement Rookery Tussac for coverand as a food source, and not for the proximity to seabirds.In line with this, Igual et al. (2006) found that Cory’s shear-waters breeding in burrows of vegetated slopes were morevulnerable to predation by ship rats than birds breeding inopen, rocky habitat.

This has consequences when it comes to managementactions, as an important part of the cover is provided bygorse, introduced as an ornamental plant in the settlement,but which is now also found in remote sites of the island. Itis thus important to remove this vegetation, and a programaimed at 50% reduction is currently underway.

Inter-annual variability and population responses of rats

During the Wrst two seasons of trapping, more than 130 ratswere trapped and killed, approximately half of themfemales. This appeared to cause a signiWcant demographicimpact. We observed a change in the size distribution fromnormal to bimodal and a change from equal sex ratio to

strongly male-biased sex ratio over the study period. Theobserved change may be a population response to adecrease in density. Mice and rats have the potential to self-regulate their density through social interactions, termedspacing behaviours, that include territoriality, pre-satura-tion dispersal, breeding inhibition and various forms ofsocial mortality (e.g. Singleton and Hay 1983). After aninduced sudden drop in density, such as imposed control,rodent populations have an acute ability to recover rapidly(Drummond 1970). Population responses to decreased den-sity may be the result of compensatory mechanisms such asdensity-dependent mortality and fecundity, i.e. changes inbreeding, survival and recruitment. A decrease in densitythrough removal of individuals may result in a reduction ininter-speciWc or intra-speciWc competition for foodresources, and may promote a higher breeding capacity ofthe remaining animals, with increased pregnancy rates and

Table 8 Carbon and nitrogen stable isotope analysis of stom-ach and contents as well as liver and muscle tissue, of house mice at New Island

Isotope data (‰)

Rookery Tussac (n = 4)

South End Tussac area (n = 7)

Gorse areas (n = 2)

Open areas (n = 1)

Gut content

�13C ¡23.7 § 0.8 ¡23.3 § 0.4 Wilk’s � = 0.910, P = 0.685

�15N 23.0 § 0.8 24.4 § 1.3

Liver

�13C ¡22.0 § 0.2 ¡22.3 § 0.5 ¡19.1 Wilk’s � = 0.192, P = 0.007

�15N 29.2 § 0.6 26.7 § 1.1 13.8

Muscle

�13C ¡23.1 § 0.1 ¡22.2 § 0.4 ¡22.3 § 0.5 ¡19.9 Wilk’s � = 0.078, P < 0.001

�15N 29.0 § 0.5 26.4 § 1.0 17.4 § 1.0 13.4

Stable isotope data are given (mean and standard error), and the Wnal column describes re-sults from Multivariate ANOVA tests for diVerences between areas

Table 9 Within-individual correlation of isotope ratios of four sampletypes of house mice (Mus musculus) at New Island

GLM were used to control for the eVect of area, but for readability wegive only the eVect size (eta squared value) of pairwise correlationsbetween sample types

* denotes signiWcance values of P < 0.05, while ** denotes signiW-cance values of P · 0.01 and *** P · 0.001

Sample type Liver Muscle

Carbon

Gut contents 0.418* 0.167

Liver – 0.782***

Nitrogen

Stomach contents 0.851*** 0.699***

Liver – 0.667**

Fig. 11 Carbon and nitrogen stable isotope ratios for cat faecal sam-ples from diVerent areas of New Island. The background data, indi-cated in grey shades, correspond to the data given in Fig. 4

13C

-30 -28 -26 -24 -22 -20 -18 -16 -14 -12 -10

51N

0

5

10

15

20

25

30

35

40

Terrestrial

Marine Coastal

Cats (feces) - all areas

2005Prion HouseAbove SettlementGorseline to rookeryMice (outside tussac)Rabbits

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Polar Biol (2008) 31:333–349 345

litter sizes (e.g. Davis and Christian 1958; Montgomery1981), and high rates of survival of adults or juveniles inthe population (Gliwicz 1981; Montgomery 1981; Gunder-sen et al. 2001). Further, density-dependent dispersal alsocan be important through immigration (e.g. Sullivan 1979;Montgomery et al. 1997). As a response to removal, studiesof rodents found population compensation through immi-gration of smaller, younger animals (e.g. Brown and Tuan2005) and by commencing reproduction earlier (e.g. Schi-eck and Millar 1987; Krebs et al. 1976). Further, somestudies show that more males colonized the removal areathan females (e.g. Schieck and Millar 1987).

Thus, the demographic change we observed is typical fora population response to a decrease in density, but verystrong compared to other studies. The delay in the response(detectable from the third season) suggests that the patternis caused by changes in breeding parameters rather than byimmigration. The fact that such a strong response wascaused by the removal of ca. 70 animals in the Wrst seasonand 60 animals in the second season suggests that the totalpopulation is probably relatively small.

Diet of rats

Similar to other studies on rats (e.g. Major et al. 2007), thepresent data indicate a high plasticity of rat diet. We foundtussac to be an important source of food and shelter for rats,similar to other sub-Antarctic islands (e.g. Pye et al. 1999).Diet analyses carried out between mid-November 2002 andmid-January 2003 showed that tussac grass was the single-most important food source, occurring in identiWable formin nearly half of the samples. In particular young rats fedextensively on tussac (Fig. 3), whereas older rats also tookmany mice and possibly young rats, as suggested by thepresence of hairs in over a third of the stomachs of all agescombined, and about half the stomachs of adult ship rats.The presence of maggots in a number of stomachs indicatesthat scavenging is also important.

One concern has been the inXuence of the rats on thepopulation of thin-billed prions at New Island, as this spe-cies probably has its most numerous breeding populationhere. During the incubation period of the thin-billed prions,rats had a mixed diet, with plants occurring in 80% of thesamples and animal items in 51%. Only Wve stomach con-tents (8% of animal items or 3% of total samples) containedtraces of ingested birds, three feathers and two yolks.

However, data collected during the chick-rearing periodof thin-billed prions in diVerent years consistently indicatedthat the amount of animal tissue taken strongly increasedlater in the season (Table 5) and dead and/or alive prionchicks may become an important part of the rat diet duringtheir nestling period. This was supported for some areasusing stable isotope methods (see below).

We compared the diet of rats trapped in diVerent areas,and found strong diVerences among areas. While rats inthree of four areas had a diet with marine as well as terres-trial content, rats of the Settlement Rookery tussac areaconsumed a high proportion of terrestrial food. Among allareas sampled at New Island, this area consistently had thehighest density of ship rats as well as house mice over threeseasons (Fig. 1), and thus the terrestrial food web appearsfully suYcient to maintain these populations. Rats wouldappear to take marine food opportunistically to supplementtheir diets.

Because body tissues diVer in their metabolic activity, itis possible to analyse multiple tissues to estimate an ani-mal’s diet over a range of time scales. Turnover times fordiVerent animals somewhat vary, and it is recommendedthat when turnover rates are unknown for the species infocus, then turnover rates obtained from another speciesclose in body size, taxonomy and ecology is possibly usedfor the measured species (Dalerum and Angerbjorn 2005).The small mammals best studied in this respect are gerbilsMeriones unguiculatus. In diet-switching experiments withgerbils Tieszen et al. (1983) found that liver had high turn-over rates of carbon stable isotopes, with a half-life ofabout 1 week, while they were much longer in muscle tis-sue (ca. 4 weeks).

We analysed individual consistency of diet in ship ratsusing stomach contents and faeces (representing the lastfew meals, but also representing a relatively depleted iso-tope signature in comparison with diet) and liver and mus-cle tissue (representing diet over a period of several weeks).Rats showed a high consistency between liver and muscleisotope ratios, and thus were specialized in prey types for atime of several weeks. The lower rates of correlationbetween the isotope ratios of the last few meals (stomachcontents, faeces) and longer-term ratios found in liver andmuscle underpins the opportunistic feeding behaviour ofrats in the short term. Consistent with this, we found a widerange of isotope ratios in rat diet (e.g. Fig. 5).

Most rats in our study showed a mixed diet, but in two ofthe three areas, we found individuals specialized on marine-derived food (in total 4 of 34 rats or 12%). Prions were themost frequently encountered item of marine food in thediets. Among the potential sources of marine foods, otherthan prions, are food items lost by imperial shags whenharassed by skuas. Fish are found occasionally in areaswith regular Xight routes of imperial shags, and this mayexplain the Wsh found in one stomach of a rat in the Settle-ment Rookery tussac, which is situated on the top of a verti-cal cliV. Skuas also chase adult prions, and dead adults withwounds or half eaten are found occasionally. In busy nightsat the colony, prions often collide with each other andregurgitated crustaceans are found on the ground in themorning. Further sources of marine food other than prions

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346 Polar Biol (2008) 31:333–349

may be regurgitated by Magellanic penguins, especially oflobster krill Munida gregaria. Along the shoreline, weobserved occasional beaching of krill, lobster krill, marineisopods or squid. Thus, there is a range of potentially avail-able marine food sources.

The present study also demonstrates that individual ratsuse diVerent dietary strategies both within and betweenareas and habitats. This may partly depend on their size andsex. For example, we found that during the austral spring,adult rats consumed a larger proportion of animal diet thanjuvenile rats. Further studies of rat diets could focus on thecauses of the individual diVerences observed during thechick-rearing period of prions.

Diet of mice

Feral house mice on sub-Antarctic islands typically feed onseeds and insects (Copson 1986; CraVord 1990; Chown andSmith 1993). Recent studies of house mice at Gough Island(Cuthbert and Hilton 2004), however, suggested that micemight have a larger inXuence on seabird populations than pre-viously recognized. Therefore, an analysis of marine versusterrestrial contents in the diet of mice at New Island seemeddesirable, and the stable isotope approach appropriate.

We found that dietary diVerences among individual micepersisted for several weeks, indicating some specialisation.Most mice had a terrestrial diet, a few had a mixed marineand terrestrial diet and one mouse from an open areagrouped with the marine data for muscle isotope ratios,indicating a specialisation of this latter mouse in marine-derived food. Unfortunately, the gut content of this mousewas not sampled. The gut contents of all other mice showedno overlap with prion tissue according to the isotope ratios,indicating that none had recently consumed prion tissue.

Diet of cats and indirect eVects of rabbits

The diet of cats inferred from the stable isotope ratios wasbroadly comparable to an analysis using visual contents(Matias 2005) which also found that other introduced mam-mals were the main prey of feral cats at New Island. Theisotope ratios in faeces were consistent with rodents andrabbits as the chief prey of cats at New Island, both appear-ing about equally important. Cat faeces overlapped in their�13C values with prion chick red blood cells for 13% ofscats, which we interpreted as the maximum frequency ofscats containing prions as their main item. In comparison,Matias (2005) found a similar number (12.8%) in the settle-ment area, but greater frequencies of occurrence in otherareas (up to 57% in a small sample from Rookery Hill, adense prion nesting area). Rabbits, mice and rats were thechief prey of cats identiWed by visual analysis of scats(Matias 2005). While not yielding the same detail in prey

identiWcation, the stable isotope method can be used totrace prey of which little remains in the scats would be visi-ble. For example, Matias (2005) found that the rabbitstaken by cats were small individuals. However, large indi-viduals would not be consumed whole and would thereforeleave traces with the isotope method, but not with the visualanalysis. On the other hand, Matias (2005) found 2–3 indi-vidual preys in each scat on average; thus our method mayhave overlooked some prion remains when they were minorin weight compared with other prey.

The inXuence of cats on the population of prions is esti-mated to be very small, due to the small number of catspresent on the island in comparison to the large populationof prions, and also due to the abundance of other prey likerabbits. However, recent studies have drawn attention to thepossibility of a hyperpredation eVect (Courchamps andCaut 2005). This occurs when one or several prey species(in this case, mice and rabbits) introduced into an environ-ment in which a predator has also been introduced (in thiscase cats) sustain high predator numbers, such that localprey, less adapted to high levels of predation, could suVer apopulation decline and possibly even extinction. Such aprocess has consequences when it comes to managementactions, as the introduced prey plays a key role that is notalways obvious. Predators are often perceived as having themost deleterious eVects on invaded ecosystems (Cour-champs and Caut 2005), and consequently control pro-grams more often target them, sometimes neglecting theintroduced prey. Yet, through a hyperpredation eVect, intro-duced prey may have an indirect impact on indigenousprey. In parts of New Island, rabbits also compete for spacewith prions for digging burrows. Thus, rabbits should alsobe taken into account when considering the possibility oferadicating introduced mammals.

Coexistence with seabirds and conclusions

New Island is the only place in the Falkland archipelagowhere the ship rat is found. All current eradication pro-grams in the Falkland Islands are directed against theBrown or Norway rat Rattus norvegicus, which is a larger,more ferocious species. We have no evidence of the latterspecies at New Island.

The breeding colony of thin-billed prions has coexistedwith introduced ship rats and feral house mice for about100 years according to available records. The present datasuggest that the coexistence has been possible due to the ratpopulation of New Island being relatively small andrestricted mainly to areas providing cover, and probablydue to the absence of Norway rats.

It has been pointed out that the coexistence of introducedrodents and seabirds at New Island may be due to predator

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swamping in this highly seasonal environment (Catry et al.2007), and this is supported by the small population sizesindicated by the present data. Over-winter survival may bean important factor regulating the rodent populations, as thefood may become scarce during that period, and NewIsland has no winter-nesting seabirds. The absence of sheepand cattle over the past 30 years may also be a contributingfactor. In several studies on introduced rodents on islands,winter-nesting seabirds were aVected much more than sum-mer-nesting birds (e.g. Brothers 1984; Cuthbert et al.2004). In addition, tussac as an important source of foodand shelter for rats is restricted to few areas on New Islandand thus, predator survival in winter may be low at present,but possible changes need to be taken into account. Sub-Antarctic ecosystems have been found to respond sensi-tively to variation in ambient temperature (e.g. Chapuiset al. 2004), which have increased across several parts ofthe Antarctic and sub-Antarctic for the past 50 years (e.g.Smith 2002; Turner et al. 2005). Climate change may leadto more favourable conditions for reproduction (e.g. Ferre-ira 2006) or over-winter survival of rodents or less favour-able conditions for thin-billed prions (see Quillfeldt et al.2007a, b), which may inXuence the relative numbers of pri-ons and rodents and thus may disrupt the apparent presentequilibrium.

The diet analyses also indicate that during the incubationperiod, the interaction between rodents and thin-billed pri-ons is minimal. However, an impact of rats has beendescribed on the survival of prion chicks in the South Endtussac area (Catry et al. 2007). Thin-billed prion chicksmay be vulnerable to predation by rats shortly after hatch-ing (MacKay et al. 2001), although in a study plot in anopen area, but close to the settlement gorse line where ratsoccur, no predation on hatchlings was observed (e.g. Quill-feldt et al. 2003). The reports of prion hatchling mortalityof MacKay et al. (2001) refer to the South End tussac area,where 1,660 trap nights resulted in a total of only 16 ratscaught. Further, the rats in the South End Tussac area thatwere found with down feathers in their stomach contentsweighed only 33–72 g; thus were immatures and possiblythus more likely to scavenge than to kill a pre-Xedgingprion weighing about 150 g. There have been reports ofsmall rodents killing surprisingly large prey, but only whenit is relatively immobile, as in the case of 35 g mice killingalbatross chicks weighing >10 kg (Cuthbert et al. 2004). Inthe second half of February, when the present samples werecollected, prion chicks are mobile except when poorly fedand hypothermic.

While from the ship rats’ point of view live or dead pri-ons may be an important food source, in terms of the prionpopulation, the predation by rats, mice and cats seems tohave a negligible inXuence. One reason for this lack ofinXuence is that the number of introduced predators at

New Island is relatively low, but also because in such a largepopulation of prions (with 2 million pairs), even a smallpercentage of mortality would create a massive amount ofdead animal tissue to be scavenged.

Except for the cases when maggots were observed in thestomach contents, neither visual analysis of stomach con-tents nor the stable isotope data could reveal whether theprions observed in the stomach contents of rats or themarine foods traced in tissue isotope signatures werepreyed upon or scavenged, but we may extrapolate fromdata on chick mortality.

In total, we followed 213 Thin-billed prion chicks overfour breeding seasons 2003–2006 in a study plot in an openarea (close to the Settlement gorse line). The main causesof chick mortality were environmental factors, the mostimportant being starvation (18 chicks or 8.5%), followed bynest Xooding (2 chicks or 0.9%) and overheating (1 chickor 0.5%). Predation by striated caracaras was observed for8 chicks (3.8%), while we observed only one probable caseof predation by a rodent (Rattus rattus or Mus musculus) in2005. In both 2005 and 2006, prion chicks were not wellfed towards the end of the breeding season, and somechicks died of starvation when close to Xedging age in bothyears, coinciding with the time of sample collection (4 of74 chicks in 2005 (Quillfeldt et al. 2007b) and 1 of 39chicks in 2006 (P. Quillfeldt, unpublished data).

Thus, the number of chicks which died from causesother than predation in our study plot was considerablyhigher than the number of chicks preyed upon by rodents(21 vs. 1 case), and the number of chicks preyed upon bycaracaras was also higher than the number of chickspreyed upon by rodents (8 vs. 1 case). This indicates thatalthough prions appear to be an important food source forrats in certain areas, they may be obtained mostly byscavenging. This is supported by the fact that maggots arefound regularly with prion remains in stomach contentsof rats. Also, thin-billed prion chicks in the South Endtussock were observed to be vulnerable to predation byrats shortly after hatching (MacKay et al. 2001), butstomach samples in February 2001, when predation wasnot noted in that area, also contained prion feathers andmaggots.

At present, prions are virtually absent in the area of theRookery tussac where the highest density of rats isobserved, and in other tussac areas their breeding success islower than in open areas (Catry et al. 2007). However, thereis insuYcient evidence to show that rodents are responsiblefor the apparent lower breeding success of prions in tussachabitats on New Island. Other tussac islands, such as BirdIsland (SW Falklands), where rodents are absent and prionsbreed in large numbers, are an example where prions nestin much greater density in areas containing more friablesoils than in the dense tussac peat.

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348 Polar Biol (2008) 31:333–349

According to our data, the impact of predation by intro-duced mammals on the large population of thin-billed pri-ons at New Island appears to be small. We observed thechief cause of chick mortality to be starvation, followed bypredation by native predators, while predation by rodents isonly occasionally observed. The unique set of introducedmammals at New Island would appear to be mainly depen-dent on the terrestrial food web, while using marine-derivedfood as an opportunistic supplement to their terrestrial diet.

Since the removal of sheep, the vegetation cover of NewIsland has changed from a sparsely covered terrain to adense growth of vegetation. Ground nesting species such asFalkland pipits Anthus corredera grayi, long-tailed mead-owlarks Sturnella loyca falklandica and Falkland thrushesTurdus f. falcklandii are now common breeders. Black-throated Wnches Melanodera m. melanodera, tussac birdsCinclodes a. antarcticus and black-chinned siskins Cardu-elis barbata and grass wrens Cistothorus platensis falk-landicus breed in smaller numbers. Dark-face groundtyrants Muscisaxicola macloviana are also common breed-ers. The main inXuence on their increase and breeding is adirect result of removing stock, but is also an importantindicator that rodents have little impact on these species atNew Island.

One ground-nester, the Cobb’s wren Trogolodytes cobbiis absent from New Island in the breeding season, butsometimes observed in winter. However, this species is alsoabsent from several rat-free islands, two examples beingSouth Fur Island and South Jason Island. It is also veryuncommon on Beauchêne Island where it appears to berestricted to speciWc habitats. The absence of this speciesfrom New Island may therefore be due to unsuitable habi-tat, and care should be taken in the often generalizedassumption that rodents are solely the reason for its absencefrom some islands.

The present information therefore suggests:

1. there is no urgency for eradication of rodents at NewIsland; however, close monitoring of the present equi-librium is required,

2. short-term measures of control may not be eVective,because the populations have strong compensatoryresponses,

3. although the current population of thin-billed prions isnot at risk from predation in their main habitat (openareas), the eradication of rodents from the island wouldpotentially make more habitat available to the thin-billed prions and other ground nesting birds,

4. if any eradication is considered in the future, it mayneed to include all introduced mammals (rats, mice,rabbits and cats) because little information exists oninterrelationships and the possible eVects of partialremoval of invasive species.

Acknowledgments We are grateful to the New Island ConservationTrust for permission to work on the island and for providing accommo-dation and transport. We are grateful to Fabiana Zuñiga Olavarria,Matthew Strange, Monica Silva, Stuart McKay, Bart Groeneveld,Kathy Gunther, Wendy Gibble, Dan Birch, Georgina Strange, Riek vanNoordwijk and Paulo Catry for their contribution to the Weldwork.David Gladbach and Felix Weiss participated in the sample prepara-tion, and the Biology department of the University of Konstanz, Ger-many, facilitated access to laboratory facilities. This study was partlyfunded by grants provided by Falkland Islands Government (FIG), theOverseas Territories Environmental Programme (OTEP) and PQ re-ceived funding from Deutsche Forschungsgemeinschaft (Qu 148-1V).The study was carried out with permission of the FIG EnvironmentalPlanning OYce. All stable isotope analyses were carried out at theNERC Life Sciences Mass Spectrometry Facility (application numberEK93-7/06).

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South End tussac area

Grey C

hann

el

SOUTH ATLANTIC OCEAN

Prion house gorse

Settlement rookerytussac area

New Island

Map showing locations of the main trap sites. The trap lines in open areas were scattered over the upland sites.

View from Rookery Hill, New Island, showing habitat of the upland sites. Foto: Petra Quillfeldt

View towards Rookery Hill, New Island, showing habitat of the upland sites and the extension of the Settlement rookery tussac area (dark green). Foto: Petra Quillfeldt

tussac area

View towards Rookery Hill, New Island, showing the rookery surrounded by tussac grass. Foto: Petra Quillfeldt

Juan F. Masello and Hedrika (Riek) van Noordwijk setting a trap in the Settlement rookery tussac. Foto: Petra Quillfeldt

View from the prion study site towards Rookery Hill, showing prion nesting areas adjacent to gorse, with the Settlement rookery tussac in the background. Foto: Petra Quillfeldt

Prion house gorse

Prion burrows

Rookery HillSettlement rookery tussac area

Gorse flowering in and around the settlement, with prion nestingareas in the background. Foto: Petra Quillfeldt

Prion burrows

Rat trap set adjacent to prion nesting areas, and examples for burrow entrances (arrows). Note the bare soil around burrows caused by digging by the prions. Foto: Petra Quillfeldt

Prion burrows

trap

South end tussac area, New Island, and open areas in the background, looking south. Foto: Petra Quillfeldt

Tussac area

Open area

Trap design. A cost-efficient cover was developed to protect the traps from incidental by-

catch (birds, small rabbits), but still allow easy visual inspection of traps. Covers were made

from black polypropylene netting, normally used in airstrip construction. This netting is bent

to form a small tunnel of 30 cm length, supported by weaving three lengths of fence wire

through the netting. This wire protrudes from the bottom edges of the netting to allow the

tunnel to be pinned to the ground. In order to keep birds out and let rats and mice through,

traps were closed on one side with pieces of the polypropylene netting and fence wire pins.

The other side was directly towards vegetation or was partly closed with pieces of the

polypropylene netting and fence wire pins.

Trap set at the edge of tussac grassland, with polypropylen netting blocking parts of both

entrances:

Trap set in groundsel patch close to prion burrows, with open side towards vegetation

2