89M.R. RUDGE

Ecology Division, Department of Scientific and Industrial Research, Private Bag, Lower Hutt, New Zealand

THE DECLINE AND INCREASE OF FERAL SHEEP (Ovis aries L.)ON CAMPBELL ISLANDSummary: Sheep introduced to Campbell Island in 1895 for farming reached numbers of over 8000 in 1916 thendeclined to 1000 by 1961 (exponential growth rate r = -0.05 p.a.). Numbers increased to around 3000 from 1961to 1969 (r = 0.14 p.a.). The island was divided into halves by a fence in 1970, and all sheep north of it werekilled. The southern population continued to grow from 1970 to 1984 (r = 0.053 p.a.). The southern half of theisland was cleared of sheep in 1984 except for about 800 fenced off on a peninsula.

Censuses in 1961 and 1969, and the sample shot in 1970, showed that population growth rate, lambing rate,sex ratio, and time of breeding varied in different places on the island, and from year to year. The averagenumber of lambs present per 100 females ranged from 30 to 40 in the farming years up to 1931, and from 60 to80 by 1970. The age structure in 1970 was consistent with a stable age-distribution in an expanding population.

During the farming years the southern hemisphere was in a cold climatic phase. From the mid 1930s therewas a general warming, especially during the 1950s. These better conditions, together with falling grazingpressure and absence of burning, allowed an increase in the island's woody and herbaceous vegetation. Thesheep developed traits for self shedding of the unshorn fleece and for breeding at one year old. These changesare discussed in relation to the decline and subsequent increase of the population.

Keywords: Feral; sheep; Ovis aries; Campbell Island; subantarctic; population history.

Introduction

Campbell Island is a subantarctic island some 500 kmsouth of New Zealand at 52∞S 169'E (Fig. 1). It is theremnant eastern rim of an eroded volcanic cone andhas an area of about 10.9 km2. The western side,facing the prevailing seas, is fringed by high, sheercliffs and rocky promontories. The eastern side slopesmore gently to sea level and is indented by narrowinlets, the largest of which, Perseverance Harbour,runs 8 km inland and nearly bisects the island.

The terrain is generally hilly with five peaks over450 m, of which the highest is Mount Honey (569 m).Rock is exposed in places but the slopes are generallycovered in deep peat.

The dominant woody plants are Dracophyllumscoparium, D. longifolium, Coprosma spp., andMyrsine divaricata. These occur as scrub associationsup to an altitude of 200 m and as dwarf trees up to 5m tall at lower levels and where there is shelter. Boththe original woody and herbaceous vegetation wasgreatly altered by burning and grazing. The originaltussock grassland (Chionochloa antarctica) and largeendemic herbs have been replaced as dominants by theless palatable Bulbinella ross, Poa litorosa tussock,and other unpalatable native herbs. European pasturegrasses were sown which, when grazed, producedextensive open swards (see Meurk, 1975, 1977, 1982).General accounts of the history and natural history ofthe island have been given by Bailey and Sorenson(1962) and Kerr (1976).

Figure 1: Location of Campbell Island, and the place

names used in the text. Col corresponds to North Col; Faye

to Faye + Fizeau; and Lyall to Lyall + Mowbray of

Wilson and Orwin (1964). All sheep were removed from

north of the fence in 1970. A study population was

restricted to the area round Mt Paris when animals in the

southern half were exterminated in 1984.

Domestic sheep (Ovis aries L.) were introduced in1895, but pastoral farming met with indifferentsuccess. Stock numbers were highest in 1916 (over8000), after which there was a steady decline (Wilson

90 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 9, 1986

and Orwin, 1964). When farming was discontinued in1931, over 4000 sheep and about 20 cattle (Bos taurosL.) were abandoned. Estimates made by coastwatchers during the second world war (1940-45)indicated that sheep numbers continued to decline.

Campbell Island is the main breeding ground forthe southern royal albatross. When the island becamea reserve for the protection of flora and fauna in1954, a case was made to eradicate the sheep becausethey were thought to threaten nesting sites, thevegetation pattern, and some plant taxa (Westerskov,1959, 1963). A census in 1961 showed that theremaining cattle ranged over a small area of limestonecountry, that the sheep had declined at 5% per yearsince 1916, and that the number of breedingalbatrosses had apparently not changed for decades(Wilson and Orwin, 1964). Wilson and Orwintherefore suggested erecting a fence across the 'waist'of the island, and eradicating the sheep to the northso that vegetation and albatrosses could be studiedwith and without sheep, and plant taxa could beprotected. This proposal was not adopted.

Figure 2: Exponential decline in sheep numbers on

Campbell Island from 1916 to 1961 extrapolated to A.D.

2000. Modified from Wilson and Orwin (1964) with the

actual count in 1969 added.

A count in 1969 showed that the sheeppopulation had not continued to decline as predicted(Fig. 2). Instead, numbers had increased by a factor

of about three, and the royal albatross numbers haddoubled since 1958 (Taylor, Bell and Wilson, 1970). Inview of the increase of sheep and their uncertainrelationship to albatross numbers, the 1964 proposalto build a fence was adopted.

The fence was erected in January-March 1970.Most of the sheep north of it were shot (Bell andTaylor, 1970) and the remnant was shot over the nextyear. From the sheep shot in 1970 hunters collectedjawbones so that an age distribution could beprepared which might indicate the beginning ofpopulation growth. Vegetation quadrats andphotopoints were established on both sides of thefence.

From 1970 to 1984 the remaining sheephave been counted in most years, and in 1984the first winter study was conducted (Ballance,1985). Vegetation changes with and withoutsheep have been compared (Meurk, 1982). In1975 10 sheep were taken to a research farmfor studies of reproduction and physiology(Cockrem, 1976). Thus, since 1970 ourknowledge of the sheep themselves and theirnumbers on the island has greatly increasedcompared with the flimsy record before 1961. Inthis paper the 1970 age structure and thecharacteristics of the sheep and their islandenvironment are described. The synthesis is usedto account for both the decline and theincrease in numbers.

MethodsHunters removed and labelled lower jaws, andrecorded the locality (as Faye, Color Lyall, see Fig.1), sex, fleece colour, and occurence of horns. A tallywas kept of all sheep shot and not recovered. Jawswere boiled and scraped clean, and a few were lostwhen Norway rats (Rattus norvegicus) dragged themaway at night. Of the 1281 sheep shot, 1066 (83%)were eventually represented by a usable lower jaw andfield sheet.

The sequence of eruption of the permanentdentition was recorded and any abnormalities in theteeth and supporting bone were described. The firstpermanent incisors were extracted, the root wasground transversely, and growth lines in the cementumwere counted.

Age classes up to 4 years old were separated usinga combination of relative jaw size, replacement of thedeciduous dentition, and lines in the cementum of thefirst permanent incisor. Beyond 4 years (jaws withpermanent teeth), relative wear on cheek teeth

RUDGE: SHEEP ON CAMPBELL ISLAND 91

augmented the cementum line counts. Cementum linesin 70% of known-age domestic sheep were accurate towithin I year (Rudge, 1976). Subsequent studies offeral goat teeth showed that errors in reading lineswere not biased in anyone direction, so over a largesample they should cancel out. To minimise errorsthree assessments of the tooth replacement sequencewere made several months apart, and two people readcementum lines in incisor I and then repeated theprocedure using the second incisor of the II pair.

Some of the females were dissected forexamination of the uterus and ovaries. Ovarianfollicles were counted and corpora lutea exposed byslicing each ovary into a 'book' of leaves 2 mm thick.

ResultsThe results described are from the 1970 shot sample.AppearanceThe majority (98.7%) of the sheep were white, andthe remaining 1.3% were equally divided amongwholly pigmented animals and those with pigmentedpatches. Double fleeces were common, but mostfemales were shedding fleece from the neck, belly andflanks.

Horns were visible on 67% of males and 20% offemales. An additional 22% of males and 29%females had scurs - small horny growths on the frontalbones. Horned animals could be sexed because age forage, male horns had about twice the basalcircumference and length of female horns (AppendixI) and were curled, whereas those of females wereerect and curved. As the sex ratio was 43:57, thehornless sheep would be distributed 24:76 in a livepopulation census.Tooth eruption sequenceThe frequencies of various combinations of teeth werescored as a rapid indicator of relative ages, especiallyover the first 4 years. Eruption did not follow apredictable, linear sequence (Table 1). For example,molar 3, incisor 2 and premolars 2, 3 and 4 tended toerupt almost simultaneously, and chances were aboutequal as to which appeared first. The sequenceresembled that for domestic sheep (Habermehl, 1961),except that the appearance of incisor 2 and thepremolars was commonly reversed in the islandsample.Sex ratio

Within the total sample there were 461 males and 605females (1:1.3) (X2 for female:male = 24.16, 1 d.f.,P < 0.005). In Faye and Col samples the ratio

favoured males. Lyall samples differed in that the sexratio was equal (Table 2). No great biologicalsignificance is given to these differences becauseanimals may have moved around during hunting.

Although the overall sex ratio favoured females,it varied between age-classes (Table 3, and Fig. 3). Inage-classes 73-84 months and 97-132 months the biastowards males was significant (p < 0.005).

Table 2: Distribution of sexes between three localities on thenorth of Campbell Island. Overall contingency X2 = 5.85, d.f.= 2, p<0.05. The value for Lyall is significantly different at p= 0.05 because the ratio is the reverse of the others.

Age structure

The age structure formed a broad-based triangle,consistent with but not necessarily implying a stableage distribution and an expanding population (Fig.3).There were no inflated age classes to indicateaugmented cohorts in the 1960s. A sharp waist in 1966suggested a drop in recuitment or survival of thatcohort, but there was continuing strong recruitment inthe following three years.

Reproductive condition and lambing

During January-March 1970 no new-born lambs orpregnant females were seen, and none of 15 females

Table 1: Number of jaws in which different combinations ofpermanent teeth occurred in Campbell Island sheep.Sequence: M1> (M2 > I1) >(M3, P2,3,4) _ I2, > I3, >C1,.Habermehl (/962) gives: M1,>M2,>I1,>M3, (I2, PM2,3,4)>I3,>C1.

_ indicates that the sequence may be reversed.

Males Females Ratio X2

I1 I2 I3 Cl P2 P3 P4 M1 M2 M3 N

* 164

* * 47* * 3* C * 72* * * * 4* * * * * * 4* * * * * * * 8* * * * 2* * * * * * * 3* *. * * * * * * 79* * * * * * * * 4* * * * * * * * * 90* *. * * * * * * * * 333

Col 54 63 46:54 0.41Faye 325 463 41:59 1.29Lyall 82 79 51:49 4.15

92 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 9, 1986

Table 3: The observed number of animals of each sex in each age class, and their calculated number in cohorts of 1000 in the threelocalities and in the total sample (1067) of sheep shot on Campbell Island in 1970. The age-class sex ratio differs from thepopulation ratio of 43:57 at p < 0.025 (*) or p < 0.005( +). Å plus one of unspecified sex = 1067

presumably nursing. The two not lactating may havebeen in earliest pregnancy without visible embryos.Because females could breed soon after they were 12months old, the definition of a 'breeding' female and

dissected was visibly pregnant. Ten of the 15 less than9 months old had ripe ovarian follicles, so were aboutto ovulate. Five others had corpora lutea. Three ofthese five were also lactating a little and were

RUDGE: SHEEP ON CAMPBELL ISLAND 93

Figure 3: Distribution of ages in each sex (cohorts of

1000) in the aged sample of sheep on Campbell Island in

1970 (N = 1066 or 83% of animals shot. * sex ratio

departs from population expectation of 43:57 at

p<0.05).

of 'lamb' used below is greater or less than 12 monthsold respectively. This definition allows for theextended breeding period.

The overall proportion of Iambs to adults was350/0, and did not differ significantly between thethree localities. These ratios were similar to thoserecorded in 1969 by Taylor et al.,(1970) except thatlambing on Faye was 10% higher in 1970 (Table 4).

The proportion of Iambs younger than 6 months old(Table 5) was significantly greater on Faye(X2 = 10.23, 2 d.f., p < 0.01); that is, lambing waslater there than on Col and Lyall.

Clearly, reproduction extended over a longperiod. According to Mr C. Clark (pers. comm.), ameteorological observer who had spent several yearson the island, Iambs were born throughout the year

but most between August and December.

Table 5: Number of Iambs alive, and percentage born within 6

months of the sample date (early February 1970) in 3 localities

on the northern half of Campbell Island.

FayeCol Lyall Total

Total lambs 29 204 44 277

% < 6 months old 76 91 77 87

Sheep present 117 789 161 2067

Area (ha) 604 2580 2112 5296

Density/l00 ha 19.3 30.6 12.1 20.15

DiscussionCensuses and studies of these sheep were generallycarried out during brief summer expeditions untilBallance (1985) recently studied them in winter. Therehave been no year-round studies. The main populationhas now been exterminated from the south of theisland except for a study flock of about 800.

Table 4: Comparison of lambing percentages expressed as lambs/100 adults (and as lambs/100 ewes) during and after the farming

period on various parts of Campbell Island.

Year Col Faye Lyall All

northern

half

All

southern

half

Whole

island

Ref

1909 24 (37) Wilson & Orwin, 1964

1916 23- Wilson & Orwin, 1964

1928 20 (31) Wilson & Orwin, 1964

1960/61 15 (23) Wilson & Orwin, 1964

1968/69 33 25 37 28 26 26.5 Taylor et al. 1970

1970 33 (58) 35 (59) 38 (81) 35 (62) (this paper)

1971/72 25 Dilks & Wilson, 1979

1975/76 22 Dilks & Wilson, 1979

1976/77 30 Dilks & Wilson, 1979

1977/78 29 Wilson & Elliott, unpublished

1979/80 33 ç

1980/81 24 ç

1982/83 20 Dilks & Grindell, unpublished

1984 20 (35) Balance, 1985

94 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 9, 1986

Variation in time and place of population growth and

recruitment

From 1916 to 1961 the population declined at anaverage exponential rate of r = -0.05 each year (Fig.2). By 1969 this trend had become an average increaseof r = 0.14 but with marked local differences. In thenorthern half the population increased by a factor of2.9, and in the southern half by a factor of 3.5(Taylor et al., 1970). Even between smallersubdivisions of the island, population growth wasmarkedly different (Tables 4, 6). Mount Honey, thearea with the lowest population density in 1961,showed the greatest growth by 1969 (Table 5).Movement of animals from areas of locally highrecruitment may have precipitated this growth butclearly some areas were, or became, capable ofholding many more sheep than they had in 1961.Taylor et al., (1970) found that in 1969 there was anegative correlation between density and lambing rate(r = -0.68, p<0.05). From 1970 to 1984 thepopulation on the southern half of the islandcontinued to grow but at a slower rate (r = 0.053,Table 7).

During the farming period, lambing lasted at leastsix months (Wilson and Orwin, 1964). Lambingpercentages were obtained at the main docking andshearing muster, which could extend from October toMay depending on the weather (Timms, 1978). Thevalues thus contain a varying error because ofdiffering opportunities for mortality. Lambing

Table 7: Number of sheep on the southern half of Campbell

/slandfrom 1961 to 1982. From 1960/61 to 1970/71 (10 years)

exponential growth rate r = 0.144; from 1970/71 to 1984

(13 years) r = 0.053. For the whole period 1960/61 to 1984

(23 years) r = 0.089.

Year counted Total Sheep

596

2088

2521

2400

2861

3175

3561

3341

3540

4700

1960/61

1968/69

1970/71

1975/76

1976/77

1977/78

1979/80

1980/81

1982/83

1984

estimates from later censuses have come mostly frombetween December and February (Table 4).

Lambing percentage is a useful guide to theperformance of the population, but the various figureshave been derived in three different ways. In thefarming period they came from musters in which thecount of ewes and lambs was sound but was spreadover 6 months. In live censuses the adult sex ratio wasuncertain because hornless animals could not beinfallibly sexed; and the 'lamb of the year' designationwas suspect because of the extended breeding period.In the 1970 sample both sex ratio and age were

Table 6: % increase in the number of sheep in 7 localities on Campbell/sland between 1961 and 1969 compared with the density in

1961. r = yearly exponential growth rate between 1961 and 1969.

RUDGE: SHEEP ON CAMPBELL ISLAND 95

unambiguous. In farming convention, lambingpercentage is expressed as a ratio of lambs to females.Wilson and Orwin (1964) derived 37% and 31 % fortwo years of the farming era, (using an assumed sexratio of 35% males) and 23% for their 1961 census(Table 4). Their estimate for lambs was obtained byhalving the number of immature animals which wouldhave made some allowance for year-round breeding,but would have underestimated lamb production.

Despite the different methods, the estimatedproductivity in 1970 was so much greater than inearlier years that there must have been a genuineincrease. For example, lambing on the northern halfof the island as a whole increased from 28% in 1969to 35%, and on Faye from 25% to 35% (Table 4).Clearly, large variations can occur from year to year,so that no single estimate necessarily reflects theprevailing average performance of the population.

As well as year-to-year variations, localconditions induced local differences in breedingperformance and time of lambing. In both 1969 and1970, there were higher lambing percentages on Lyallthan elsewhere in the north. In 1970 there were 81lambs/100 ewes on Lyall. For an unmanagedpopulation after perinatal mortality had taken its tollthis was remarkably high given that the long-termaverage on New Zealand hill country farms is about100%. Lambing times too, may have varied aroundthe island as there was some indication of laterlambing on Faye in 1970 (Table 5). These differencesin lambing percentage and timing do not simply reflectmovements of early-lambing ewes from Faye to Lyall,because the sex ratio favoured males on Lyall andtotal numbers there were low. During the farmingyears Lyall ("The Peaks") was consideredun favourable for sheep and the shepherds nevermustered many from it (Wilson and Orwin, 1964). In1969 it had the lowest density of sheep on the island.

These annual and local differences inreproduction suggest that the growth of thepopulation as a whole was variable in time and place.However, there also seems to have been a consistentincrease in lambing % from an average of 20.5 up to1961 to one of 28.0 thereafter (Table 4).The population age structure in 1970

Despite precautions and checks, two unavoidableproblems remained in producing an accurate agedistribution from jaws. They are particularly pertinentto this study, which seeks specific years showingespecially good or bad recruitment.

Firstly, Campbell Island has an oceanic climatewhich minimises seasonal alternations of growth rate

(and hence of cementum lines) compared withcontinental conditions (Klevezal, 1973). Thus therewill be some errors in absolute age, even though therelative distribution is probably real. Secondly,lambing was not tightly pulsed although more than75% were born in a 5-month period of winter andspring. At the time of sampling (midsummer) over75% of animals aged by cementum lines would havebeen in the correct age-class even though their actualages spanned 9 months. The remainder would havebeen in the next highest class.

Clearly there are constraints on what can bededuced from this age distribution. But there wouldhave to be greater distortions than are accounted forby the known sources of error to materially alter theoverall shape of it. In this discussion therefore the agestructure in 1970 is taken to be biologically real (Table3, Fig. 3). Each age-class represents net recruitment,that is, the indistinguishable combination of birthsand subsequent survival.

By 1970 the population had been growing rapidlyfor at least part of the preceding decade. Consistentwith this, the age distribution showed no greatlyexpanded age-class that reflected a single pulse ofrecruitment within the 1960s. Either there was asudden and sustained change in recruitment ormortality around 1960, or one or both graduallychanged throughout the 1960s.Changes in the sheep and their environment(a) Meteorological changesClimate and weather have two general classes ofeffects. Firstly, long-term climatic trends influenceplant productivity and accumulated biomass, thebalance of woody to herbaceous species, andaltitudinal zonation. For the sheep these combine todetermine carrying capacity and shelter. Secondly, theseverity and frequency of short term climatic events(the weather) at crucial times can affect survival,particularly of lambs.

Taylor et al., (1970) found no tendency towardsbetter weather from 1957 to 1970, but if a longerperiod is taken there has been a steady trend towarmer, less windy conditions since the farming days.The period 1900 to 1935, which included the farmingyears, was the coldest since records began in NewZealand and was termed a mini ice-age (Salinger andGunn, 1975; Salinger, 1980). The last shepherds sawicebergs, and ice washed up on the beaches (Spence,1968). Between 1935 and 1975 mean annualtemperatures rose by 1 °C over the whole New Zealandregion and the mid 1950s were much warmer thanaverage. Simultaneously, the duration and frequency

96 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 9.1986

of strong winds fell so that there was less evaporativecooling (Salinger and Gunn, 1975). For CampbellIsland this meant an increase in Growing Degree Daysfor plants (i.e. no. of degrees by which the daily meantemperature exceeded 0∞C x the number of days).From 1942 to 1955, nine of the 14 years fell below thelong-term mean, and the other five were only slightlyabove, but from 1956 to 1972 eight years were belowand nine substantially above the long-term mean.There was an especially warm period between 1968and 1971 (Fig. 4).

Figure 4: Growing Degree Days (GDD) per year above

base O∞C compared with the long term mean (LTM) on

Campbell Island from 1942 to 1969. From 1942 to 1955, 9

years were below the mean and 5 above; from 1956 to

1972, 8 years were below and 9 above.

Short-term, high-intensity events in the weatherare harder to discern in a general meteorologicalrecord and to relate to animal performance andbehaviour. An analysis of mean monthly temperature,rainfall, and hailstorm frequency in the four mainlambing months (August-November) showed noconsistent trends. Mean monthly temperatures rarelyexceeded the long-term mean from 1941 to 1955.From 1956 to 1962 that patten was reversed, but insucceeding years it was again erratic. From 1960 to1980, total rainfall rarely showed any consistency frommonth to month. A series of drier Septembers andOctobers ran from 1955 to 1962 but even so, 70-100"70of days were rain days (precipitation> 0.2 mm).Hailstorms correlate closely with a sudden drop intemperature and increasing wind speed above Beaufort8 (gale). Between 1959 and 1960 the lambing monthshad more hailstorms than in much of the 40-yearrecord.

In summary, general climatic conditions becameconsistently warmer and calmer from the mid 1930s topresent. Some of the main lambing months werewarmer and drier than average, but there was no

consistent improvement. Unpredictable stormsoccurred in these months in most years.

(b) Changes in the vegetationMeurk (1975, 1976, 1977, 1982) has summarised themodification of the island's vegetation and flora.During the late 19th century and throughout thefarming era, much of the vegetation was burnt. Atfirst the tussocks re-sprouted vigorously, but afterrepeated burning and grazing they died and theshepherds scattered seeds of introduced grasses tocreate new pasture. The large subantarctic herbs couldnot withstand any burning or grazing.

There were 4000-5500 sheep when farming endedin 1931, but only about 1000 by 1960 (Wilson andOrwin, 1964; Spence, 1968). Under this reducedgrazing pressure, and the improving climate, the sub-antarctic vegetation would be expected to recover,providing more scrub for shelter, and herbs andtussock grasses for food. Since 1930, the scrub linehas spread upwards to 200 m and become thicker (Belland Taylor, 1970; Meurk, 1977). This would havebeen assisted by the warming climate, and also by theexistence of open swards because scrub seedlings areshade intolerant and slow to invade Chionochloa(Zotov, 1965). Although the scrub species are largelyunpalatable they do provide shelter.

The capacity of the vegetation to respond toreduced grazing was shown after the sheep wereremoved from the northern half of the island in 1970.By 1980, Chionochloa, introduced grasses (Poapratensis, P. annua, Festuca rubra and Agrostistenuis) grew dense with massed seedheads up to 0.5 mtall (Dilks and Wilson, 1979; Meurk, 1982). Palatableendemic herbs, Pleurophyllum speciosum, P.crini/erum, P. hookeri, Anisotome lati/olia,A. antipoda and Stilbocarpa polaris became common.Such changes could well have occurred in the lightlypopulated parts of the island between 1931 and 1960.Wilson and Orwin (1964) and P.R. Wilson (pers.comm.) described the areas with few sheep in 1961 asbeing covered in rank vegetation. By 1969 many ofthese places carried large numbers of sheep (Taylor etal., 1970) and continued to do so up till 1984. Asnumbers built up the sheep created lanes through thescrub (Dilks and Wilson, 1979; Meurk, 1982).

In particular years, surges of plant growth occur.Meurk (1976) described the profuse growth andflowering of Chionochloa tussocks in the summer of1975/76 after the unusually warm summer of 1974/75,and again in 1980/81 (C.D. Meurk, pers. comm.).Something similar would have happened in theexceptionally warm years of the 1950s. Such bursts of

RUDGE: SHEEP ON CAMPBELL ISLAND97

growth may have had a flushing effect on both sexes,enhancing ovulation and spermatogenesis through anincrease in liveweight.

(c) Changes in the sheepThe shepherds observed that virtually every commonbreed could be recognised in the population (Spence,1968; Timms, 1978). Wilson and Orwin (1964) foundthat Merino, Lincoln, Corriedale and Romney hadbeen among the establishment flocks.

In 1953, three Cheviot rams were landed, the firstnew blood since 1923 (Wilson and Orwin, 1964).Presumably they were vigorous animals to justify theeffort but there is no record that they survived. TheCheviot breed is noted for its clean limbs andwillingness to forage into scrub, both of which arefeatures of the present sheep. However, no evidenceof Cheviot blood can be diagnosed in theconformation or wool character of the population(W.R. Regnault, pers. comm.). These three rams musttherefore be discounted as a trigger for the resurgenceof the population.

During the farming period management wasminimal. Mustering was so incomplete that in 1929/31there were, reputedly, 4500 managed sheep and about1000 'wild' ones (Spence, 1968). Sheep were not culledfor quality, and ewes were not assisted with lambing.Apparently, skuas (Stercorarius skua lonnbergi) usedto attack lambs (Spence, 1968), and in 1975 evenpenned adults had to be protected from them(Regnault, 1976). The sheep were not treated forinternal parasites, and hydatids were so common thata clean, edible liver was a rarity. Animals missed atshearing developed double fleeces that trailed on theground (Spence, 1968), and rams with long belly woolwere trimmed to increase their mobility in the muster(Timms, 1978). When animals were no longer shornand docked, multiple fleeces would have beencommonplace and all would have had long tails.

Most females now progressively shed their wholefleece and rams shed their belly fleece (Regnault,1976). Self-shedding is genetically linked topigmentation in other feral sheep populations. On Pittand Arapawa Islands, most animals now show bothcharacteristics although the populations there werenon-shedding white sheep 40-70 years ago (Rudge,1983; Orwin and Whitaker, 1984). However, theCampbell Island sample shot in 1970 was still 98.7%white, and later censuses up to 1984 have not revealedincreasing pigmentation.

Pigmented sheep are generally more fertile thanwhite ones (Adalsteinsson 1984), yet the expandingpopulation on Campbell Island has not shown an

increasing proportion of pigmented phenotype.Nevertheless, there has been a marked improvement inlamb production. During the farming years, lambingpercentage was only 30-40 (Spence, 1968), whereas itis now about 60-80. Regnault (1975) found thatdouble-fleeced females were almost invariably lamb lessand that the untidiest (Le. shedding) females usuallyhad the biggest, healthiest lambs. In captivity theCampbell Island yearling females went into oestrusearlier than Romneys under the same managementregime, and though they produced no twins, almost allbecame pregnant and their lambs had excellentsurvival rates (Bigham and Cockrem, 1982). Regnault(1976) therefore proposed a relationship betweenreproduction (increased lambing, early fertility andlonger reproductive life of females) and fieldobservations on lambing and fleece growth. Thickfleece on the belly of males and the perineum offemales, matted in both sexes with peat and faeces,could impede copulation. If females were inseminatedas yearlings when they still had their initial shortfleece, the combined stress of pregnancy and winterwould augment the normal fineness break in the fibre.Freed of her fleece, the female could continue tobreed each year. If she did not breed as a yearling shewould possibly never do so as fleece and tail became agreater impediment to copulation. In males theselection would be for clean bellies.

The hypothesis has not been tested experimentallyon Campbell Island or in commercial flocks, butfarming practice suggests that it has a sound basis.Each year at mating time all sheep farmers shear thebellies of rams and the perineum of ewes (crutching)to remove fleece and dags. In some circumstancesmales can even develop a suppurating infection of thepenis if encasing belly fleece is not removed. Longdaggy fleece and tails would have been an impedimentrather than a total barrier to copulations. But theindications are that it could have significantly reducedsuccessful mating and productivity until the selfshedding trait spread through the population.

In terms of fleece growth and distribution, andearly sexual maturation, the Campbell Island sheephave changed since they were abandoned in 1931. By1961 they had experienced 30 years of naturalselection in addition to that operating during thefarming era. The rate at which fleece characteristicshave changed in other feral populations (Orwin andWhitaker, 1984) confirms that this is long enough forthe pelage traits to have emerged. It is curious that onCampbell Island there is no phenotypic evidence ofincreased pigmentation to accompany the self-shedding and enhanced fertility, but it may yet emerge

98NEW ZEALAND JOURNAL OF ECOLOGY,VOL. 9, 1986

in the remnant flock left on the island.To summarise, this paper has drawn together

both historical facts and numerical information toexplain the decline and growth of the sheeppopulation and also the timing of events (Fig. 5). Theisland was probably overstocked in the first place, andthe vegetation was soon unable to withstandcontinuing burning and grazing. Coincidentally, thefarming years spanned a phase of climatic cooling. Awarming phase from the mid 1930s together withreducing grazing pressure, encouraged recovery of thevegetation. Sheep numbers ceased to decline by themid 1950s, to produce a stable and expandingpopulation by 1970. Fleecy tails and the matteddouble fleeces grown when shearing was discontinued,impeded copulation and reduced productivity. Thesheep gradually evolved a self-shedding trait in the

fleece, and early sexual maturation which together ledto improved productivity.

Home range behaviour may have also played apart in the increase. Shorn sheep always returned totheir own part of the island (Spence, 1948), andfemales and Iambs lived in very circumscribed ranges(pers. obs. Regnault, 1976). Such conservativebehaviour could account for the different localpopulation growth rates. Population growth wouldprobably not have begun at the same time in all partsof the island, and until pressure built up in thefamiliar ranges the animals did not need to move intonew areas. When the colonisation phase began therewould then have been a lag in population growth tothe stage where rank growth was broken down andfresh young growth was induced by grazing.

Figure 5: Diagrammatic synthesis of changes in the sheep population, the vegetation, and the climate (asGrowing Degree Days/year) from 1916 to the 1980's on Campbell Island; GDD/year based on atmospheric

warming, from Salinger and Gunn (1975).

RUDGE: SHEEP ON CAMPBELL ISLAND99

AcknowledgementsDuring the long gestation of this paper I havebenefitted from the results of later field surveys andmany discussions with colleagues. I thank in particularD.J. Campbell, R.H. Taylor and Dr P.R. Wilson ofEcology Division. Others who contributed withinformation or ciriticism were: Mansoor Kawaja(population analyst, Department of Statistics); S.Goulter (NZ Meteorological Service); Dr A.C.G.Heath and Dr D. Heath (Ministry of Agriculture andFisheries); Dr G. Caughley (CSIRO); Dr C.D. Meurk(DSIR Botany Division); W.R. Regnault (WoolScience, Massey University); Dr D. Orwin (WoolResearch Institute of NZ); A.H. Whitaker; the late DrF.M. Cockrem (Ruakura Agricultural ResearchCentre); Dr Stefan Adelsteinsson (AgriculturalResearch Institute, Reykjavik); and Dr J.A.McLennan (Ecology Division), P. Ross, R. Joblin, J.Clarke, R. Steele and Lynn Flannery assisted atdifferent times. The Journal Editor and referee madevaluable comments in the final stages of production.Thanks go to Brian Bell and his Wildlife Service teamwho helped to collect shot material in 1970 and whohave waited such a long time to find out why. JocelynBerney was the patient typist.

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Appendix I: Length (1) and circumference (c) of horns in cm, and eviscerated carcass weight (w) in kg, of Campbell

Island sheep.