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1'-:i M ,I M

PROCEEQINGS OF THE

SECOND INTERNATIONAL REINDEER/CARIBOU SYMPOSIUM

17.-21. SEPTEMBER 1979

R(2)ROS, NORWAY

EDITED BY:

EIGIL REIMERS ELDAR GAARE

SVEN SKJENNEBERG

DIREKTORATET FOR VILT OG FERSKVANNSFISK TRONDHEIM 1979

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Reimers, E., Gaare, E. and Skjenneberg, S. (eds.) 1980. Proc. 2nd Int. Reindeer/Caribou Symp., R(l)ros, Norway, 1979. Direktoratet for vilt og ferskvannsfisk, Trondheim.

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POPULATION ECOLOGY OF REINDEER ON SOUTH GEORGIA

N. Leader-Williams

British Antarctic Survey, Madingley Road, Cambridge CB3 OET, U.K.

ABSTRACT

Data from an ecological study are used to evaluate differences in population trends shown by the introduced reindeer of South Georgia, and those on other islands. On St Paul and St Matthew Islands, the populations increased exponentially in number, but rapidly declined within 20 to 40 years of their introduction. The two discrete herds on South Georgia increased up to maximum densities (23 animals km2) ~imilar to those on other islands, but the increase and subsequent decline (to present densities of 3 to 13 animals km2 50 to 65 years after their introduction) has been gradual, and reindeer from one herd emigrated to colonise a new area. There is selective grazing on preferred food plants in summer (with elimination of some species), but range quality, and thus body growth, are insufficient to allow conceptions in calves, and condition fluctuates widely between seasons. In winter reindeer depend exclusively on the resilient coastal tussock grass, Poa f/abe/lata, but grazing pressure has reduced winter range carrying capacity, probably ensuring a continuing slow decline in numbers.

1. INTRODUCTION

Reindeer (Rangifer tarandus L.) have been introduced to several small arctic islands which lacked predators and on which the opportunity for migration was resticted, sometimes with dramatic results. On St Paul Island (Pribilof group) and ~t Matthew Island (NE Central Bering Sea) the original small populations reached high densities, but then cr~shed to very low numbers within 40 and 20 years, respectively, of their i,ntroductions

,<

(Scheffer 1951; Klein 1968). The results of these introductions are of great interest as they did riot follow the general pattern of most ungulate eruptions occurring after liberation or introduction (Caughley 1970):

Norwegian reindeer were introduced to two separate areas- of the subantarctic island of South Georgia (53°30' to 55°S and 35°30' to 38°30'W) by whalers in the early part of this century. Permanent ice and.inow cover c. 60% of its surface and large sea-entering glaciers act as barriers to movement. Snow lies seasonally down to sea-level, usually from June to September. The vegetation is typical of subantarctic tundra and comprises twenty-four native vascular species, c. 200 bryophytes and c. fifty lichens together with c. fifty largely non­persistent aliens (Lewis Smith and Walton 1975). It is dominated by the coastal tussock grass Poa f/abe/lata, and is substantially different to that encountered by reindeer in the northern hemisphere.

Ten reindeer were introduced to the Barff Peninsula (Fig. J) in 1911. This herd was originally confined to the Barff area, but in 1961-65, an unknown number of reindeer first spread into the area to the north of Royal Bay (Lindsay 1973) and formed a second herd. A third herd came from a separate import of seven rein­deer to the Busen area in 1925. Reindeer calves are born in November and the rut occurs in late March (Olstad 1930; Bonner 1958). There are no native land mammals and no further interspecific competitors (except the brown rat, Rattus norvegicus) have been introduced. The reindeer have never been husbanded or actively mana­ged. Results from the natural experiment which their introduction has created are evaluated in the present paper. Attention is focused upon two main points of interest; firstly, the ways in which reindeer have responded to this unusual habitat, and secondly, the differences between subantarctic and arctic island populations of introduced reindeer.

2. METHODS

Historical data on reindeer numbers, size and feeding habits are derived variously from Olstad (1930), Bonner (1958) and Leader-Williams (1978). The method ot.collection and numbers of reindeer shot between 1973 and 1976 for the present study are detailed in Leader~Williams (1980a); selected results from part of this sample are used in the present paper. The ages of reindeer {n,ere determined by a combination of tooth eruption patterns

\ and annulations in incisor cementum (Leader-Williams 1979). The total weight of unbled reindeer (exc;:epting

'

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Fig. 1. Map of reindeer areas of South Georgia. Stippled areas represent permanent ice and snow.

loss from the shot) was taken on a 200 kg spring balance; the total length was measured from the nose, along the spine to the ba~e on the tail with the animal lying on its right side with neck extended; hind foot length was measured from the 'point of the hock to the horn line of the hoof in a straight line. Rump fat qepth and kidney fat weight of each reindeer were measured as described by Dauphine (1976) and the! percentage 0f femur marrow fat was determined from a c. 2 g sample using the method of Neiland (1970). The diet of reindeer was assessed by quantitative analy~es of rumen contents (Leader-Williams et al. 1980).

Vegetated areas within the reindeer ranges were traced from monochrome aerial photographs taken in November 1973 at a height of 1600 m. These tracings were transferred to the largest available maps (scale of 1 :50000 (Barff Peninsula and Busen area) or 1:25000 (Royal Bay area)) with cross-reference to landmarks visible on both aerial photographs and maps. Both total and vegetated areas available to reindeer were measured with a planimeter, the former probably to within.± 5% and the latter to within.± 10%. Corrections were not made for sloping terrain.

3. RESULTS AND DISCUSSION

3.1. Numerical change

Ungulates introduced into previously unoccupied areas normally adjust to their new environment with a single eruptive oscillation (Riney 1964). Historical estimates of total reindeer numbers on South Georgia (Leader­Williams 1978) and accurate counts of adult and yearling reindeer numbers made from 1972 to 1976 (Leader­Williams 1980a) are shown in detail in Leader-Williams and Payne (1980). These data are summarised in Fig. 2, together with a summary of the available population estimates for St Paul Island (Scheffer 1951) and St Matthew Island (Klein 1959, 1968). Data are available forSt George Island (Scheffer 1951) but are not shown for reasons of scale; this herd reached a maximum of c. 220 reindeer in 1921 and fluctuated slowly thereafter between seventy and ten animals. Several estimates lack accuracy, at least for South Georgia (Leader-Williams 1978). However, Fig. 2 shows that the eruptive oscillations on each island differed (a) in the maximum number attained (the Barff and Busen herds also differeQ in this respect), (b) in the number of years after introduction that these peaks occurred, and most especially (c~ in their declines.

Initially introduced ungulates increase in number in response to the discreRancy between carrying capacity of the environment and the numbers actually present (Riney 1964). The total areas available to introduced

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Tab. 1. Areas of reindeer range on different islands and for each herd on South Georgia

Island/Herd

Barff Peninsula Royal Bay area Busen area St Matthew Island St Paul Island St George Island

Total area (km2)

131 58

124 332* 107* 91*

Vegetated area occupied by reindeer (km2 )

30 9

11

*Converted ~rom areas (miles2) in Klein (1968) and Scheffer (1951), with 1 mile = 2.59 km2

6000

5000

4000

BUS EN

BARFF --- BARFF AND ROYAL BAY e e e e e ST PAUL

ST MATTHEW

6000

l .. .. I~ . . . . . .

:Q 3000 w

3000

lD ::; :::> z

2000

1000

1910 1920

2000

• •• •• • • • • • • • •

. . . . . . . . . .

: . . : . . . .

'· ., ' , ___ _

800

~­.~-· ., ,.... . .... .. , .. ··-··-··-··-·· . ..!·

...... ,. .. . 1930 1940 1950 1960 1970 1980

Fig. 2. Summary of reindeer numbers on South Georgia, St Paul Island (from Scheffer 1951) and St Matthew Island (from Klein 1968)

reindeer, which for present purposes can be substituted for carrying capacity, are shown in Tab. 1; the areas of actual vegetated range used by each herd are known only for South Georgia. If the eruptive oscillation of each herd is described in terms of density (Fig. 3), the maxima attained were similar (18 to 23 animals km2 of total area), except forSt George Island (2.4 animals km2, not shown) and the Busen herd. Scheffer (1951) could advance no reason for the differences between St Paul and St George Islands. In the Busen area reindeer have not negotiated four mountain passes leading into potentially available, but as yet ungrazed, range within their overall area (Leader-Williams 1978). If density is calculated using only occupied and grazed areas (Tab. 2), the peak densities in the Barff and Busen herds :fe similar. These were also similar to 1976 densities in the Royal Bay herd, which at that time had probably declined slightly from maximum numbers attained after emi­gration .to this new area (Leader-Williams 1980a). Thus miximum numbers attaihed by introduced reindeer

I

generally have been proportional to the area available. 1

" I E "' ..

24

20

~ 16 ·;: ~

12

8

4

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• • •• •• •• •• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

. . ! . . . . . . . . .. . . . . . . . . . . . . .

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./\ .· ./· ·., ·. .· ~· ·. . . ,. . . ,. . .·. "'.· : . .. ,·· . o L_~~---,,_~~~··T-~·~·~-~·~·-~··r-~·~·~:~.~~··_-__ ··---.--~·L---,------.--

1910 1920 1930 1940 1950 1960 1970 1980

Fig. 3. Summary of reindeer densities on South Georgia, St Paul Island and St Matthew Island. Legend as shown in Fig. 2. Barff densities omitted during formation of Royal Bay herd.

Tab. 2. Densities of reindeer on South Georgia, with compara7ive data from Svalbard (animals-km2)

Herd Total area Occupied and grazed area

Peak 1976 Peak 1976

Barff 22.9 10.1 100.0 44.0 Royal Bay 13.3 84.8 Bus en 6.3 3.8 71.4 42.3

Svalbard* (1.4 to 2. 3) (4. 0 to 5.6)

* Data from Reimers (1977)

;~;f >·

..

In each herd the increase appears to have been exponential (Fig. 2), though there relatively few estimates for St Matthew Island and South Georgia. The length of the period between introduction and attainment of maximum numbers will have depended on (a) carrying capacity (above), (b) initial number introduced, and (c) rate of increase. An exponential curve was fitted for each island and herd by linear regression of the logarithm of reindeer numbers against years after introduction; the slope of each line was the instantaneous rate of increase of each population (Fig. 4). Both St Paul and St Matthew Islands had a larger number of reindeer at introduction than the Barff and Busen herds (25 and 29 vs. 1-.0 and 7). Analysis of variance indicated that there was no signi­ficant difference (P > 0,1) between the rate of i~crease for St Paul Island, the Barff and Busen herds, but that of the St Matthew Island herd was significantly gr~ater than the others (F3, 13 = 4,55, P < 0.025).

\ After increasing in number to the full carrying capacity of the range, introduced ungulates usual.ly follow

rn a: w ID ::! ::I z a: w w c z iii a: IL 0 .. "' E

9·0

8·0

7·0

6·0

5·0

4·0

0 3·0

• 2·0

1910

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0 •• • • •

•

0

m=0·14 .• , =0·97 ••

o.• • • •• • • •

0 •• 0

• • •

6 ·" ,. .. "

•

.. ,·· " " ..

1920 1930 1940

. . . . .• m =0·28

• r =0·99 .. . !' . . . . .

.. ,·· " . . . . . .

,·· ,·· ~··'·· :':~:~~ . ..

• " 6 _.-.,··

1950 1960 1970 1980

Fig. 4. Rates of population increase (m) of rein­deer on South Georgia, St Paul Island and St Matthew Island.

t::.. = Busen herd; .&. = Barff herd; o = St Paul Island • = St Matthew Island

m slope (=instantaneous rate of increase); r correlation coefficient

a sequence of a levelling-off in numbers, a decline, and then a phase of relative stability in which--population density remains lower than peak density (Riney 1964; Caughley 1970). So far, reindeer on South Georgia have not deviated from this pattern, though the recent formation of a new herd has complicated the picture. In con­trast reindeer on St P~ul and St Matthew Islands have been notable exceptions to the general patt!lrn, for they both showed an exaggerated decline (a crash on the latter; an exponential decreas~ on the for~er with m =

-0.25, r = -0.99). The upswing of an eruptive oscillation in ungulates appears to be initiated by an increased availability of

food and is terminated by overgrazing (Caughley 1970). In high latitudes there are two aspects of forage_.avai­lability which must be considered; winter range carrying capacity limits the size of the population, whereas summer range quality determines the size of the individual (Klein 1968). These hypotheses will be used as the basis of the ensuing discussion on the three aspects of numerical change described above.

3.2. Fecundity rates, growth and summer forage

The age of first breeding in female reindeer can vary (e.g. Reimers 1972). and Cole (1954) noted its importance in determining the intrinsic rate of increase of a population. Age of first breeding and fecundity rates of reindeer on South Georgia are known only for the years 1973 to 1976; however, the Royal Bay herd represented an earlier stage of an eruptive oscillation than the Barff and Busen herds (Leader-Williams 1980a), thus providing a wider range of data. Pregnancy rates were assessed in shot reindeer either directly (presence or absence of foetus) from May to November or indirectly (lactational status). from November to February. Thirty-three fe­males aged from 6 months (age at first possible conception) to 15 months (end of first possible lactation) were examined, including eleven females from the Royal Bay herd. None showed any sign of pregnancy or of having given birth at 1 year of age. Amongst 114 females over 18 months of age from the Barff herd, thirty-three from the Royal Bay herd and forty-six from the Busen herd, there were pregnancy rates of 90, 91 and 93%, respec­tively (Leader-Williams 1980a). Furthermore, pregnancy rates of the age-class calving for the first time at 2 years did not differ (P > 0,1, x2-test) from older animals in any herd, though the sample size for each was rela­tively small. Age-specific pregnancy rates were thus combined for all herds and are shown in Tab. 3. ·There were no differences (P > 0.1) in pregnancy rates even f9r combined age-classes (reindeer calving at 2+3 years vs. 4+5 years, etc.). It was thus concluded that female calves did not reach the size threshold for puberty, that fecundity rates were similar in all herds and that there was 1 no difference in the oerformance of age-classes calving at 2 .. \

years and over.

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Busen reindeer are consistently larger than those' in both other herds on South Georgia, but as their an­cestors derived from different stock to the Barff introduction {Leader-Williams 1978), it is not possible to sepa­rate phenotypic from genotypic differences in body size. There are few early data for the Barff herd. The age of three reindeer shot by Olstad {1930) could be determined as they had erupting dentition {Leader-Williams 1979); comparative data for 1928 and 1975 are shown in Tab. 4. The age of seven males shot by Bonner {1958) could only be determined approximately as 3+ years and only measurements of length are available. In January 1955 total length of males > 3 years was 162 ± 4.1 em {n = 7) compared with 166 ± 4.1 em {n = 3) {including

Tab. 3. Age-specific rates of pregnancy and calf loss

Age at Pregnancy Calf loss calving (years) Total Number Pregnancy Number Number Calf loss

number non- rate (%) which with (%) pregnant calved lost

calf

2 44 5 89

21 3 10

3 41 4 17 1

- - - -4 23 2

96 8 0

5 29 0 17 4 16

- - - - - -

6 30 3 92

9 3 49. 7 9 0 1 1

- -8 8 2 2 1 ,-d 9 8 0 2 l2 .}'

10 0 82

0 75

n 1 1 0 ..

Total 193 17 91 77 15 19

Tab. 4. Comparison of means and SE of morphometric data for Barff reindeer between 1928 and 1975

Sex Age 13 February 1928 10-21 February 1975 (year class) n Total wt Total length n Total wt Total length

(kg) (em) (kg) (em)*

M 2-3 1 120 169 6 85±3.4 161±2.6 F 2-3 2 71±3.0 155±0.5 4 66±0.8 149±1.8

i * Addition of 10 em for tail length to measurements made to base of tail only

\

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Tab. 5. Comparison between the Royal Bay and Barff herds of slopes and elevations of regressions of morphometric data against the logarithm of age (months). Differences in slopes and intercepts were compared with an anlysis of variance and co-variance (Snedecor and Cochran 1967)

Sex, body character and herd

Males

Total weight

Royal Bay Barff

Hindfoot length

Royal Bay Barff

Total length

Royal Bay Barff

Females

n

42 25

42 25

42 25

Total weight

Royal Bay. 58 Barff ~ 39

Hindfoot length

Royal Bay 58 Barff 39

Total length

Royal Bay Barff

57 39

Correlation Intercept coefficient

0.9418 0.9258

0.9198 0.9705

0.9633 0.9692

0.9343 0.9341

0.9070 0.9458

0.9500 0. 9364

13.687 17. 151

32.942 32.249

97.469 96.005

22.282 19.046

32.930 32.221

103.197 95.469

Slope Probability levels

Inter- Slope cept

23.313 0.001 15.884

2.733 3.028

NS

16.965 0.00-1 14.399

11.954 0.005 11.384

1. 945 NS 2.282

11.040 0.05 12.812

0.001

NS

0.05

NS

NS

0.05

..

10 em for tail length) in January 1975. From these albeit limited data it was concluded that the size of males was large when the herd was expanding rapidly in 1928, but that males were of similar size in 1955 and 1975. In contrast, the size of females differed by only a small amount even between 1928 and 1975.

Further evidence is available from a consideration of differences between reindeer in the Royal Bay and Barff herds. Comparative data were collected in December and January; those for the Royal Bay were from 1973/74 (n =50) and 1975/76 (n =50) and those foLthe Barff from 1974/75 (n = 64). At this time reindeer were not at their peak weights, which occurred at the rut in March and April. Significant differences in slope (Tab. 5) reflected different growth rates in each herd; differences in intercept indicated that size differences are consistent throughout each age class. These data support historical data by showing more pronounced differ­ences in the size of males than of females, especially in total weights. This is amplified further by comparing the total weights of reindeer> 3 years of age from that sample (males - 110.0 ± 0.63 kg (n = 20) vs. 84.3 ± 5.63 kg (n = 4); females - 68.5 ± 0.40 kg (n = 21 )'"V~· 63.5 ± 0.82 kg (n = 20)). Thus even recently colonised range offers only a small advantage in growth rate to f~male reindeer.

Data in Tab. 5 are comparable with the small array forSt Matthew Island in 1957 (expanding in number) and 1963 (immediately prior to the crash). In contrast to South Georgia, St Matthe~ females showed a marked

!

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increase in total weight (P < 0.025 for intercept, P < 0.001 for slope), total length (P < 0.001 and P < 0.05, respectively) and even showed a difference for h indfoot length (P < 0.025 and P < 0.1, respectively) (Klein 1968). No data were available on age of first breeding or on pregnancy rates, though Klein suspected that calves became pregnant when the herd was expanding rapidly. Relatively small increases in female growth rates on South Georgia have not resulted in calf pregnancies. However, the large increases on St Matthew Island most probably would have allowed calves to reach the size threshold for puberty, thereby explaining the higher rate of increase of this population (Fig. 4).

Post-natal growth rates of reindeer and caribou are highest in early summer (Krebs and Cowan 1962; McEwan 1968), when their nitrogen intake increases (McEwan and Whitehead 1970). The quality of summer forage available to reindeer thus governs growth rates within limits set by the genotype of the individual. On South Georgia, reindeer encountered a species-poor, subantarctic flora which had never previously been grazed owing to the absence of native ungulates. Ten years after spreading to the Royal Bay area, reindeer had virtually eliminated lichens and some stands of the grass Deschampsia antarctica, and overgrazed closed swards of the burnet, Acaena magellanica (Lindsay 1973). Forty-five years after the Barff introduction, reindeer had eliminated completely closed A. magellanica swards and reduced considerably its size when growing in other communities (Bonner 1958); similar observations were made in the Busen area (Kightley and Lewis Smith 1976). It appeared that A. magellanica and D. antarctica were selected preferentially in summer, for they do not grow in communi­ties which are available in winter months (Leader-Williams, et al. 1980). Lichens have never been considered an important forage on South Georgia in either summer or winter (Olstad 1930; Bonner 1958; Leader-Williams et al. 1980) as their standing crop even on ungrazed areas is small.

Further evidence is available from quantitative analyses of rumen· contents using an adapted line-intercept technique (Leader-Williams et al. 1980). In 1974/75, Barff reindeer selected rushes of the Juncus family (mean of 25-30% in each sample) in early summer (December, January), the alien grass Poa annua (25%) in mid­summer (February, March), and D. antarctica (30%1 in late summer (April); Poa f/abellata remained a major component (1 0-30%) of their diet throughout the summer. Only small amounts of Acaena spp. (maximum of 10% in January) were found. However, there were significantly (P < 0.001 and 0.002, respectively) larger amounts of Acaena spp~ (24%) and D. antarctica (12%) in Royal Bay samples collected in January 1974 than in those from the Barff in January 1975. Deer can select the most nutritious herbage available to them, especi­ally that which is nitrogen- and phosphorus-rich (Klein 1970). In early summer on recently colonised range,

1 ~

Tab. 6. Chemical composition of important forage ~pecies 6n South Georgia, expressed as % dry weight

Forage species Element Month

November January April

Acaena N 4.00 2.50 1.42 magellanica p 0.59 0.42 0.27

Juncus N 2.90 2.50 1. 70 scheuchzerioides p 0.58 0.30 0.20

*Poa N 4.09 3.95 annua p 0.61 0.47

Deschampsia N 3.50 4.60 4.90 antarctica p 0.39 0.47 0.43

Poa N 1.80 1.40 2.00 flabellata p 0.19 0.20 0.44 .•.

Data from Lewis Smith and W~lton (1975), exceptin~ * from Pratt

et al. (in press).

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more highly nutritious herbage (A. magel/anica and D. antarctica, as shown in Tab. 6) was available and thus selected. The reason for the higher growth rates of Royal Bay reindeer compared with those from the Barff, and for the early onset of overgrazing of these forage species becomes apparent. Barff reindeer clearly selected alternative species throughout the summer, at times when each was most nutritious (Tab. 6). The dominant vascular species, P. flabellata, is readily available and it has always been common in summer rumen samples (Olstad 1930, Bonner 1958), even though its nutritive value is low (Tab. 6). Busen samples taken in early summer were also low in Acaena spp. In spite of the reduction in quality of forage ingested (cf. Juncus spp. with Acaena spp. in Tab. 6), reasonable growth and high conception rates were still achieved by Barff and Busen reindeer during the present study.

Among the large number of forage species available to reindeer on St Matthew Island in summer, several had similar nitrogen contents (Klein 1968) to those originally available on South Georgia. However, the relative abundance of sedges, Carex spp., and other high quality forage in the vegetation cover was high and grazing stimulated further production (Klein 1968). On South Georgia, a species of low nutritive quality predominates and only P. annua responds favourably to grazing. This fundamental difference most probably explains the contrasts in growth rates that have been observed in females on the two islands, and hence the different rates of population increase.

3.3. Density, carrying capacity, winter forage and condition

Introduced reindeer populations have attained very high peak densities (Fig. 3). Moreover, densities found pre­sently on South Georgia remain much greater than the highest previously recorded for a naturally-occurring island population (Tab. 2) and therefore even greater than those of mainland reindeer (Reimers 1977). Unfor­tunately, consideration of areas (and densities) alone takes no account of the quantity of forage available within that area (i.e. standing crop, productivitY' when grazed, or the extent of limitation in winter) in relation to re­quirements of the population. Thus, the carrying capacity of 1 km2 of reindeer range will vary in contrasting geographical localities. In the absence of such data, a more subjective discussion of winter forage must suffice.

Though no data are available before 1974, Olstad (1930) and Bonner (1958) both thought that Poa fla­bellata was of greatest importance in the winter diet of reindeer on South Georgia. This species is wintergreen, grows in dense stands and has a large growth-form (up to 2 m in height in nitrogen-enriched areas, and c. 0.5 m elsewhere); though its productivity when grazed has not been assessed, it is highly productive in rion-grazed areas (Lewis Smith and Walton 1975). In July and August of a mild winter (1975) and additionally in September of a severe winter (197!1-), tussock grassland was the only community on which snow depths did not exceed 60 em, the critical depth through which reindeer and caribou do not dig (e.g., Pruitt 1959). It always r.emained the most accessible community throughout other winter months (June, October and sometimes Nov~mber). due in equal part to the height of P. flabel/ata and the lesser snow depths covering it (Leader-Williams et al 1980).

As expected, P. flabellata was found to be the almost exclusive component of rumen samples from June to September in both 1974 and 1975, and a major component (60-80%) in autumn (May) and spring (Oct~ber and November). Its digestability is not known, but P. flabel/ata has similarities to lichens as winter forage in being of poor nutritive quality (Tab. 6). but in having a high energy content; up to 70% of its above-ground dry weight is comprised of soluble carbohydrate, chiefly fructose (Gunn 1976). With its ready availability in winter and high standing crop relative to other species, there seems no obvious reason why P. flabel/ata has not always been the principal winter forage.

The importance of Poa flabellata for reindeer may be assessed by comparing their condition at the start and end of winter. Such data are only available for Barff and Busen reindeer in 1974 and 1975; these could well have differed when these herds were expanding in number (presumably when greater quantities of forage were available). The largest array is for Barff females, and data on three fat measurements are shown in Tab. 7. Fe­males shot in May 1975 and June 1974 and 1975 were in early pregnancy (the earliest months in which it could be macroscopically detected). and forage had not yet become limited by snow cover; those shot in December 1974 and January 1975 had all given birth, and condition of females was at its poorest. A loss in fat reserves over the winter is normally expected in reindeer but condition of Barff females showed a very marked seasonal fluctuation; a similar loss was seen in Busen females. There are comparable data for a barren-ground caribou population (Dauphine 1976) dependent on lichens as winter forage (Miller 1976), but none for other island populations. The condition of caribou did not fluctuate as widely as reindeer on South Georgia. Femur marrow fat, for example, the first fat reserve to be replenished in summer and the last to be repleted in winter, fluctuated only from c. 75 to 50% in adult females (Dauphine 1976). As this population of caribou was also decreasing in number when sampled (Parker 1972), it may well be that tussock grass is of a lesser quality as winter forage than lichens. l,

I

Scheffer (1951) and Klein (1968) suggested I that, due to their heavy use and topographical situation, lichens were the principal winter forage on St Paul and St Matthew Islands. Reindeer attained high densities

. \ because of a high rate of increase, an initially abundant lichen supply and a period when winters werr mild

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Tab. 7. Fat reserves of female Barff reindeer at the start of winter and after calving

Months n Back fat depth Kidney fat Femur marrow weight fat

Mean Frequency (g) (%) (mm) of zero

readings (%)

May & 17 11. 0±2. 79 12 25.9±3.40 84.3±3.38 June

December & 17 0 100 6.5±0.38 18.1±3.17 January

(Klein 1968). Lichens, however, recover very slowly from grazing and trampling by reindeer (Palmer and Rouse 1945); these densities could not be maintained because no other forage appeared to have been available, even though it is known that various forage species remain wintergreen (Sorensen 1941). Interestingly, present high densities of reindeer found on Svalbard are maintained in the absence, due to overgrazing, of lichens (Reimers 1977) and where other forage species must presumably remain available in spite of the servere winters occurring at 80°N. In subantarctic tundra, the presence, and utilisation by reindeer, of tussock grass provides a turther fundamental difference between South Georgian and most other Rangifer populations. Even though not of ideal quality, its other properties apparently more than compensate to allow the high reindeer densities seen on South Georgia.

3.4. Mortality, population decline and stabilisation

As an introduced population approaches peak density, then declines and finally stabilises (Riney 1964), its rate of increase will, in turn, tend to zero, become negative and return to zero. Caughley (1970) att~ibuted differences in rates of increase in the eruptive oscillation of ungulates mainly to differences in mortality patterns. The onset of a population decline is initiated by overgrazing (Caughley 1970); the transition from this phase to that of stability occurs only if a balance between continued availability of forage on overgrazed range at critical seasons and the lower density of animals on that range is soon reached. Such a balance clearly was not achieved on St Paul and St Matthew Islands (Scheffer 1951; Klein 1968).

Though P. f/abellata has been more resilient under continuous grazing and trampling pressure than lichens, it too has responded unfavourably to grazing by reindeer. Extensive damage to tussock grass was not recorded up to 1957 (Bonner 1958), but by 1973 was apparent where it was most accessible in winter (ridgetops and coastal flats) (Kightley and Lewis Smith 1976). As all three herds have reached maximum numbers (Royal Bay) or declined (Barff and Busen), the overgrazing of the principal winter forage species, and consequent de­crease in winter range carrying capacity, have apparently had the effects predicted by Klein (1968) and Caughley (1970).

There are no data on mortality patterns before 1973. Permits to shoot Barff reindeer were first issued in 1916; small numbers, rising up to c. 100 per year in the 1950's, were shot from that herd (Leader-Williams 1978). This harvest, in relation to herd size and productivity, would have had a small effect on population trends. Poaching was suspected to have occurred in the Busen herd during World War II (Bonner 1958; Leader-Williams 1978). when the herd was in the early stages of expansion. This may have initially limited numbers (Fig. 2), but did not affect its overall rate of increase (Fig. 4). Apart from the sample shot for the present study, harvest has been negligible (up to 20 reindeer per year) in all herds since closure of the whaling stations in 1964/65 (Leader-Williams 1978). As there are no predators on South Georgia (Bonner 1958), all mortality since then, and much of it previously, must have occurred nati!ltally.

Calf mortality occurred either peri natally (wi~thin 1 month of birth) or in their first winter (Leader-Williams 1980a). From observational data, perinatal calf loss was estimated at 30, 15 and H5%, respectively, in the Barff, Royal Bay and Busen herds in November 1975. A combined total of seventy-seven females, which had calved

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that season, were collected whilst lactating from November to February from all herds. The ages of those which had lost their calf were not evenly distributed (Tab. 3). Interestingly, there was no difference in the incidence of calf-loss between the 2+3 and 4+5 year combined year classes (P > 0.1); this contrasts with other ungulates (Turner and Dolling 1965: sheep; Nievergelt 1966: ibex; Guinness et al. 1978: red deer) in which calf-loss is high in the youngest mothers. As expected, the difference between the younger (2 to 5) and older (6+) com­bined age classes was highly significant (X2 = 1 0.12, P < 0.005).

Perinatal mortality of calves may be partly attributed to the poor condition of mothers at calving, already demonstrated on South Georgia. Most adult females also die in late winter and spring (Leader-Williams 1980a), when their condition is poorest and the physiological demands of late pregnancy and early lactation are highest. Annual mortality rates of Barff and Busen females in 1973 to 1976 were estimated to have been 32-34% from an analysis of the age structure of shot females (Leader-Williams 1980a); the sample shot in the Royal Bay herd was inadequate for such an estimate. By analogy with Caughley's (1970) study of introduced Himalayan thar (Hemitragus jemlahicus) mortality rates of both adults and calves will have increased since the herds were expanding in number, thereby effecting zero and negative population growth rates. From data on calf-loss, it appears that younger mothers hold their condition best over winter; with the continuing high pregnancy rates throughout life (Tab. 3) the direct effects of reduced winter forage are felt more strongly by older mothers.

In addition to the direct effects of decreased winter range carrying capacity, two mortality factors operate indirectly on South Georgia. Firstly, the most accessible tussock grass has been overgrazed since 1957 and rein­deer now have to forage in winter on less accessible and more dangerous areas. Deaths, due to falling from steep cliffs were thus more common in 1973-76 than in 1957 (cf. Bonner 1958 and Leader-Williams 1980b) compared with one probable observation in c. fifty carcasses in 1957 (Bonner 1958). Presently 9, 19 and 19%, respectively, of adult and yearling reindeer in the Barff, Royal Bay and Busen herds have large mandibular swellings, probably lumpy jaw; a further 11, 16 and 30%, respectively, have damaged molar teeth. Such serious dental abnormali­ties will have affected the foraging ability, and therefore condition, of a large proportion of reindeer.

No comparable data on patterns or rates of mortality are available from St Paul or St Matthew Islands. However, it is thought that these populations declined rapidly, rather than stabilised, because of food shortage in a period when winters were severe (Scheffer 1951; Klein 1 968). The differences between the causes of these

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Fig. 5. Climatic data from Grytviken (Fig. 1) from 1906 to present. Mean annual temp­eratures from Carter (1980); length of "winter" and "winter" precipitation from

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D.W.S. Limbert, B~itish Antarctic Survey. Data averaged for~consecutive five year

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declines and those on South Georgia appear to be threefold: (1) South Georgia reindeer rely upon tussock grass rather than delicate and slow-growing lichens as winter forage; (2) there has been a limited opportunity for emigration (spread of Barff reindeer to Royal Bay); (3) the prevalent climate on South Georgia. In Fig. 5 "win­ter" precipitation (both snow and rain) was measured in June, July and August, and length of "winter" was defined as the number of months during which the mean air temperature was below 0°C (data supplied by D.W.S. Limbert, British Antarctic Survey). It is clear that the maritime climate of South Georgia is less severe than that of St Matthew Island, at least, and that it has been very variable. Since the 1930's it has been warming gradually (an increase of c. 0.5°C in mean annual temperature and a shortening of "winter"). This has coincided with a period when winter food was initially plentiful and only latterly decreasing in quantity. Should this warming trend reverse, its effect on the reindeer would be more drastic than in the 1930's. A cold period coupled with a continued increase in "winter" precipitation (Fig. 5) would drastically limit the now overgrazed range.

The future of the South Georgia reindeer population therefore remains open to question. From present evidence, stabilisation or a continued slow decline in number seems the most likely in each herd. For the former to occur, P. f/abellata must achieve a balance with the lower densities of reindeer now found on South Georgia. Further spread may occur, most probably in the Busen herd where reindeer currently do not occupy the full extent of their range. Though the glacier boundaries to the other herd's ranges remain formidable barriers, and reindeer have never been observed to swim at South Georgia, the colonisation of other new areas such as Dart­mouth Point (Fig. 1) remains a possibility. In any event, future population trends should be observed with interest, especially in relation to the differences shown between arctic and subantarcti~ islands, and to the general theory of adjustment of introduced ungulates to a new environment (Riney 1964, Caughley 1970).

Acknowledgements

I thank Mr R.M. Pratt for assistance with fieldwork, Mr C. Gilbert and Mr A. Sylvester for preparing the illu­strations, Mrs F. Prince for preparing the typescript, Dr A. C. Clarke and Dr D.W.H. Walton for their helpful comments on the manuscript and Dr R.M. Laws for his advice and encouragement throughout the study.

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