Evolution research proposal essay(~1500 words, 3 page single space, APA format)

Afr. J . Ecol. 1983, Volume 2 1, pages 269-283

The giraffe and its food resource in the Serengeti. 11. Response of the giraffe population to changes in the food supply*

R . A . P E L L E W The Mary M arshall& Arthur Walton Laboratory, Physiologi- cal Laboratory* University of’Cambridge, Downing Street, Cambridge CB2 3EG

Summary The dynamics of the giraffe population of the Serengeti were quantified to ascer- tain the response of the population t o the increasing biomass of the available browse resource. Aerial counts conducted in 1971 and 1976 together with analyses of fecundity and mortality data suggest a rate of population increase of some 5-6Yo per annum. Compared with the dynamics of a stable giraffe popu- lation in Nairobi National Park, the sub-adult components of the Serengeti regional populations are larger and juvenile growth rates marginally faster: calving intervals are shorter and the age of first conception younger, resulting in higher fecundities per adult female: calf survival rates are higher and adult mortalities lower. These criteria suggest that the Serengeti giraffe population is expanding as a consequence of its increasing food availability, implying that the population may previously have been food-limited.

Resume

La dynamique de la population de girafe d u Serengeti fut quantifiee pour verifier la reponse de la population a la biomasse croissante des ressources disponibles en pdturage. Des comptages aeriens effectues en 197 1 et I976 combines a des ana- lyses de donnees de fecondite et d e mortalite suggerent une augmentation d u taux de population de quelque 5-6’10 par an. Comparke a la dynamique d’une popu- lation stable de girafe au Parc National de Nairobi, les composantes subadultes des populations regionales d u Serengeti sont plus importantes et les taux d e crois- sance des juveniles plus rapides; les intervalles de mises-bas sont plus courts et l’dge de primiparite plus bas, ce qui amene une fecondite plus elevee par femelle adulte; le taux de survie des jeunes est plus eleve et la mortalite des adultes plus faible. Ces criteres suggerent que la population de girafes du Serengeti est en expansion grdce a I’augmentation de nourriture disponible, ce qui implique que cette population peut avoir ete limitee en nourriture auparavant.

*Serengeti Wildlife Research Institute Contribution No. 312.

Correspondencc: T h e Edinburgh Building, Cambridge University Press, Shaftesbury Road, Cambridge CB2 2 R U .

0141-6707/83/12004269 $02.00 0 1983 Blackwell Scientific Publications

270 R. A. Pellew

Introduction The survival, growth and reproduction of an ungulate are ultimately determined by its rate of nutrient intake, and thus by the quantity and quality of its food supply. Interpretation of population data thus necessitates the evaluation of the food resource, for a population’s dynamics reflect the physiological condition and reproductive success of its constituent members. The effect of changes in resource availability in plant-herbivore systems have been modelled by Caughley ( 1976) (the intrinsic eruption hypothesis), who showed that as the food supply increases, the herbivore responds by changes in its population density. These changes in- volve an initial increase as a result of the discrepancy between the carrying capacity of the habitat and the animal numbers present, a levelling off in numbers at a density exceeding the carrying capacity (the ‘over-shoot’ stage), followed by a steady decline of numbers. T h e process of readjustment terminates in a phase of relative stability in which the population density is higher than the original density, but lower than the peak density. As noted by Leader-Williams (1980), these stages are manifest by differences in the rates of fecundity and mortality, and are ultimately governed by the food availability.

T h e principal environmental change affecting the giraffe (Girufa camelopar- dalis tippelskirchi Matschie) of the Serengeti National Park during the last 15 years has been the substantial increase in the availability of browse (Pellew, 1983). The progressive conversion of mature Acacia woodland t o a more open regeneration-grassland phase as a result of elephant activity combined with the introduction of fire protection, has increased the productivity and biomass of browse available to giraffe throughout the year. This paper investigates the response of the giraffe population to this change in its food supply. T h e hypothesis is proposed that the population is now experiencing a phase of expansion, repre- senting the first stage of Caughley’s model, and implying that it was previously food-limited. The evidence presented to test this hypothesis is of two types: com- parisons of aerial sample counts conducted in 197 1 and 1976 which show gross population trends, and analyses of the dynamics of sample populations in different areas of the Serengeti which show the response of individuals. Klein (1970) sug- gested that deer populations limited by food availability, compared to those with adequate resources throughout the year, show low fecundities, decreased calf sur- vival, an increased age of attainment of sexual maturity, an increased proportion of old animals in the age structure, and an increased differential mortality of females. By inference, expansion of the food supply can be expected to reverse these parameters, resulting in an upswing in population density.

A problem in the interpretation of such population data is the absence of any control population for comparisons. Foster & Dagg (1972) and Dagg & Foster (1976) described a population structure for the giraffe of Nairobi National Park which they considered to be ‘characteristic of a stable giraffe population. . . close to the carrying capacity of the Park under present conditions of climate and absence of fire’. This conclusion they based upon the monthly ground counts of the Park which showed a relatively unchanged mean population structure and density during the 7 years preceding their study (Foster & Coe, 1968), and which has continued stable in more recent years (Hillman & Hillman, 1977), although individuals do show considerable movement into and out of the Park area. Wyatt (1969) also suggested that the Nairobi population was stable at or near its carrying

Population Dynamics of Girafle 27 1

capacity. Although direct comparisons between different environments can be misleading, the Nairobi data provide a useful reference for the analyses of the Serengeti dynamics.

Methods

Population estimates Estimates of the giraffe population ofthe Serengeti were derived from two sources:

Repeated aerial census of non-migratory ungulates. Sinclair ( 1972) estimated the I97 1 Serengeti populations of seven non-migratory ungulate species, includ- ing giraffe, a census that was repeated in 1976 (Grimsdell, 1979) to determine broad trends in animal numbers during the 5-year period. T h e sample design and technique of data analysis are described by Sinclair (1 972).

Total counts of’sample woodland areas. Three sample areas were established in each of Sinclair’s (1972) woodland strata. Their locations were selected to include woodland types representative of each stratum (Herlocker, 1976), com- bined with relative ease of access for regular monthly ground counting, and of the maximum size possible to census accurately from the ground in one day. Each sample area comprised an individual land facet in the Serengeti Landscape Classification (Gerresheim, 1974) so as to maximize its environmental homo- geneity.

T h e three areas can be briefly described as follows: i. The Kirawira study area (2 10 km*) lies in the western stratum to the East of

Kirawira Hill and North of the Grumeti River, in Acacia melli/era open grass- land habitat with A. senegal/A. rohusta scrub woodland typical of much of the Western Corridor.

ii. T h e Bologonja study area ( 1 75 km2) lies in the northern stratum between the Bologonja River and the Park boundary, in A. gerrardii/A. drepanolohium/ open grassland habitat typical of the northern extension.

iii. T h e Seronera study area (240 km2) lies in the central stratum and comprises the catchment area of the Upper Seronera River, in A . tor(ilis/ Commiphora rrothae woodland that covers much of the Central Woodlands. Regular censuses ofthese areas were conducted for a minimum of I year ( 1976)

at intervals of 2 weeks, 1 month, and 2 months in the Seronera, Kirawira, and Bologonja areas respectively. Total aerial counts were conducted with same-day ground checks of groups located from the air. Flight-lines were flown at I-km intervals w i t h the observer counting a strip of approximately 500 m on one side of the aircraft only. By back-tracking down the same flight-line, a total strip width of I km was counted in two halves with all observations through the same window. The flying height was 300 A with a ground speed of about 100 km h-1, producing a search rate ofabout halfthe maximum figure of 240 km2 h-I proposed by Norton-Griffiths (1978). Giraffe are relatively inactive during the middle of the day, so by conducting the aerial counts mid-morning (08.00-10.00 h ) most of the groups located from the air could be ground checked before their compositions changed.

272 R. A . Pellew

Population dynamics The collection of population data, other than basic population structures, was based on individual animal recognition using identification photographs (Foster & Dagg, 1972). This programme was confined to the Seronera study area, where during 3 years of fieldwork (1975-77) a total of 71 1 giraffe were individually recorded. These included 267 adult females, of which 142 were relocated at least ten times, with 153 attendant calves. This same study area (referred to as the Seronera woodlands) was also the site for the quantification of the available browse (Pellew, 1983), so the food supply and population parameters can be direct 1 y related.

A g e estimation of immature girafe. The ages of sub-adult giraffe up to 42 months were estimated from age-height correlations of animals of known age. Heights were measured using superimposed stereoscopic paired photographs according to the technique described by Foster & Dagg ( 1972).

Population structures. After each aerial count, groups located from the air were ground checked, each animal being sexed and the sub-adults aged. Mean monthly population structures were calculated, based upon the total counts in 1 calendar year ( 1 976).

Natality. Estimates of natality and calf survival derived from mean population structures are unreliable because of the high proportion of neonates that are omit- ted, because either they are concealed or killed by predators during the isolation phase (Langman, 1976). The reproductive performance of individual females and the survival of their calves was monitored by photographic recognition of cow-calf pairs with regular resightings. A total of 89 newborn giraffe (< 4 weeks old) were recorded and aged to the nearest week. Twenty-four adult females were recorded as producing second offspring, the approximate birth-dates of all forty-eight calves being known. T h e mean calving interval was calculated, from which was deter- mined the mean calving rate per adult female per annum. The mean annual nata- lity (the mean number of calves born per annum a s a percentage of the total population) was then estimated, where

Mean annual natality = mean calving rate x O/o adult females in the of population adult female-' annum-1 total population

Mortality. T h e age-specific mortality rates up to 36 months were determined by monitoring the survival of the 89 calves of known age. No significant sexual differentiation in juvenile mortality was detected, so a combined male and female survivorship curve was drawn. The neonate sample located at the time of birth is small ( n = 5 ) , but by 1 week has more than doubled: the sample size at 36 months is also small ( n = 5 ) .

Adult mortality was estimated from the location of carcasses during the regular bimonthly systematic air and ground counts of the Seronera study area, combined with reports from other scientists, tourist guides, and Park personnel. Hall-Martin (1976) described a technique for ageing adult giraffe based on maxillary first

Population Dynamics of Girafle 273

Table 1 . Changes in the giraffe population estimates during the period 1971-76 for the three woodland strata of the Serengeti National Park (from Sinclair, 1972; and Grimsdell, 1979)

Western stratum Central stratum Northern stratum 1971 1976 1971 1976 1971 I976

Population estimate 2855 95% confidence limits 430 95"/0 c.1. as O/o of estimate 15%

Mean density km-2 0.74 95% confidence limits 0.1 1

Stratum area ( k d ) 3862 Mean transect width ( m ) 384 Area sampled (km2) 646 Sample fraction 16.7%

*Increase significant at P <0.05.

2240 929

4 I Yo 0.58 0.24

353 367

9.5%

2780 4970* 634 855

23% 17% 0.80 1.42* 0.18 0.24

382 400 478 479

13.7% 13.7%

3487

1672 not 644 counted

38% 0.9 I 0.34

384 227

I844

12.3%

molar wear patterns. However, tooth-wear of the Serengeti giraffe, even of old bulls, never exceeded that of medium-aged (k 12 years) animals illustrated by Hall-Martin. The small number of adult carcasses located in the Seronera area (n = 27 over a 3-year period) was allocated into three general categories of young, middle-aged and old adults on the basis of tooth wear and cranial ossification.

Results Population estimates

Repeated aerial census of non-migratory ungulates (Table 1). Although the population density of the central stratum shows a significant increase ( P < 0 . 0 3 , this is not confirmed by the western stratum, which in fact suggests a marginal decline. It is considered that the 1976 figure for the western stratum is a n under- estimate. The giraffe of the western woodlands are strongly aggregated, with a mean group size of 17, and a maximum recorded size o f 2 3 9 (Table 2). T h e 1976 sampling intensity of the western stratum is inadequate for reliable population estimation in view of the aggregated giraffe distribution, a conclusion shared by Grimsdell ( 1 979). Multi-species aerial counts typically result in under-estimates because of the diversity of retinal images necessary to pick out each species, particularly when the transect is so large that the observer has to actively scan the ground (Watson, Freeman & Jolly, 1969).

Total counts of sample woodland areas (Table 2). T h e mean monthly population density of the western (Kirawira) area (2.6 k 0.4 km-2) is significantly greater than that of the central (Seronera) and northern (Bologonja) areas (P<0.05), which have similar densities (1.5f0.5 km-2 and 1.4k0.4 km-2). This density estimate of the central woodlands is similar to that of the 1976 aerial count (Table I ) , but the figure of the Kirawira area is more than four times greater than that of the western stratum (Table I ) , further suggesting that this 1976 figure is an underestimate. Seasonal differences in monthly population densities within each study area are not significant, implying that giraffe respond to seasonal changes in the biomass and distribution of their food resource by local small-scale movements and not by large-scale migrations (Pellew, in press (a)).

274 R. A. Pellew

Table 2. Giraffe population estimates and densities of the three sample areas censused by total counts from ground and air, 1975-76

Sample area

Seronera Kirawira Bologonja

Mean monthly population estimate 95% confidence limits

95% c.1. as % of mean Mean monthly population density km-2

Mean wet-season population density (km-')

Mean dry-season population density (km-2)

Sample area (km2) Woodland stratum (Sinclair, 1972) in which sample

Yo of total stratum area Population estimate of stratum 95% confidence limits

95% confidence limits

95% confidence limits

95% confidence limits

areas lie

352 129

3 6 6 % I .47 0.54 I .32 0.42 1.71 0.58 2 4 0

554 Y O

16.2% 2.64 0.43 2.69 0.34 2.57 0.5 I 2 10

248 72

29 .O% I .42 0.4 I I .45 0.39 I .34 0.49 I 7 5

Central Western Northern 6.9% 5.4% 9.5% 5 1 I 4 10188 2613 I 8 7 4 1655 759

Number of total counts from ground 36 II 6 Number of total counts from air 18 7 4 Mean % of ground total seen from air 9 3% 88% 8 I l k 95% confidence limits 4 % 6% I 2%

Mean herd size 9 17 7 Maximum herd size 77 239 62

With repeated aerial counts using the same pilot and observer, high counting efficiencies were achieved (lower half of Table 2). In the open woodlands of the Seronera and Kirawira areas, some 90% of the ground total was recorded from the air. Ground checks showed a 100% counting efficiency for groups standing or lying in open grassland in all three areas. All herds (two or more individuals) were also located from the air. Undercounting errors arose from large groups feed- ing in the woodlands, particularly in the more broken terrain of the northern study area, where the mean underestimate increased to 19% with larger confidence limits. Groups located in the woodlands and not ground checked were corrected by a factor specific to each study area to compensate for undercounting.

Population dynamics Age estimation qf immature girafe. The mean growth curves for male ( n = 7)

and female ( n = 8 ) immature giraffe u p to 42 months (Fig. 1 ) show a marginally greater rate of height growth than the comparable rates suggested by Foster & Dagg (1972) for six giraffe calves in Nairobi National Park, and by Backhaus ( 1 96 1 ) for two captive giraffe.

The mean heights at birth ( < 1 week old) were 1.90, s.d. k 0.07 m (n = 8) and 1.82+0.08 m ( n = 5 ) for male and female calves respectively. The mean height of adult bulls (8 + years) to the horn tips with the head and neck held in the up- right alert posture was 5.06fO.17 m ( n = 16), and of cows ( 6 + years) was

Population Dynamics of Girafe 275

Fig. 1 . Mean growth rates of known- aged immature giraffe (with maximum-

* ,, s * minimum ranges and individual heights of juveniles older than 42 months).

0 l 8 Age lyearsl

4.22+0.13 m ( n = 14). These figures agree with other published data (Dagg & Foster, 1976; Skinner & Hall-Martin, 1975).

Population structures (Table 3, top half). Because of the range of heights for a given age (Fig. I ) , animals above 5 years old are included as adults. T h e Nairobi National Park data (Foster & Dagg, 1972) are mean figures determined from only two counts conducted in September 1967 and November 1970. T h e represen- tation of juveniles in the 36-60 month category in the 1970 count is assumed to be the same as in 1967 (12O/o), and these animals were deducted from the adult category in which they had been included in proportion t o the adult sex ratio. Foster & Dagg (1972) show that September and November are months with above average calving in Nairobi National Park (Fig. 2), so the proportion of calves ( I 1.5%) is exaggerated compared with the mean monthly data of the Serengeti populations.

Although not significantly greater than the Nairobi figure, the proportion of sub-adults, particularly in the Seronera and Kirawira areas, is very high, especially for an animal with a potential longevity of nearly 30 years. This high juvenile component is partly the result of the disparity in the adult sex ratio. In the Bolongonja area where the adult sex ratio is near parity, the sub-adult proportion is lower.

Natality (Table 3, bottom half). The mean calving interval of the sample of 24 adult cows that calved twice in the Seronera area was 1 8 4 (s.d. f 1.9) months, with a minimum-maximum range of 15-8-24.2 months. As suggested by Foster & Dagg ( 1 972), a correlation exists between the survival of the first calf and the subsequent calving interval ( r = +0.51, n = 2 4 , P=O.O1) (Table 4).

Calf survival rates (Fig. 3) are higher in the Serengeti than in Nairobi National Park, so a longer mean calving interval might be expected. In fact, the interval is shorter, although the difference is not significant. T h e calving rate per adult female per annum is thus marginally greater in the Serengeti, producing greater mean annual natality rates than in Nairobi.

Giraffe calve throughout the year in both the Serengeti and Nairobi National Parks (Fig. 2). The monthly calving frequencies suggest a bimodal distribution of

216 R. A . Pellew

Table 3. Mean monthly giraffe population structures (1975-76) of the three sample areas, compared with the giraffe population structure of Nairobi National Park

Sample area

Nairobi Seronera KirawiraBologonja National Parkt

Adult males (60+ months) 19.3% 21.59/0 27.5% 2 5%

Fourth year males & females (36-48 months) 7.5 7.4

Second year males & females (12-24 months) 9.6 8.8 7.6 I I

Adult females (60+ months) 32.4 31.6 29.8 31

I 2 6.8

9.5

Fifth year males & females (48-60 months) 6.7 7. I

Third year males & females (24-36 months) 8.4 8. I 7. I

First year (calves) (0-12 months) 16.1 15.5 14.6 11.5

6.6 l- Mean monthly population estimate 352 554 248 86 Sample size used for population structure

determination 9639 5376 1468 250 Total adult proportion 51.70/0 53.1% 57.3% 5 6% Total sub-adult proportion 48.3% 46.9% 42.7% 44% Ratio adult male : 1 adult female 0.60 0-68 0.92 0.8 I Ratio male calf: I female calf I .08 0.94 0.96 I .07 Mean calving interval (months) 18.8 18.8* 18.8* 20 s.d. I .9 I .9 I .9

Calving rate adult female-' y e a r ' 0.64 0.64* 0.64* 0.60 O/o Mean annual natality 20.7% 20.2% 19. I '/o 18.6%

*Not determined, but assumed the same as Seronera; tdata derived from Foster & Dagg (1972) and Dagg & Foster (1976).

'7

I Fig. 2. Monthly calving frequencies of giraffe in the Serengeti (n=89) (solid line) and Nairobi National Parks (n=43; Foster

2

J F M A M J J A S O N D

t&?~?-*%o?%~%n & Dagg, 1972) (broken line).

births with, in the Serengeti, a minor peak in December-January followed by a major peak in May-August. Although Foster & Dagg (1972) show that the Nairobi peaks coincide with the drier months of the year, in the Serengeti May is the month of maximum available browse biomass (Pellew, 1983). The peak period of parturition and post-partum lactation, when energy requirements are greatest, thus coincides with the time of rapidly declining browse availability at

Population Dynamics of Giraffe 277

0 I 2 0 24 28 32 3 6 0 4 8 12 16

Age lmonthsl

Fig. 3. Survivorship curves for giraffe calves in the Serengeti (1975-77) (solid line) and Nairobi National Parks (1966-68: Foster & Dagg. 1972) (broken line).

Table 4. Relationship between calf survival and subsequent calving interval for the sample of twenty-four individually recognizable adult female giraffe in the Seronera study area which calved twice during the study

Age of calf at Sample Calving interval in months time ofdeath size mean s.d.

il week 2 16.6 0.8 I week-I month 3 17.3 0.9 1-3 months 4 18.5 0.8 3-6 months 3 19.1 I .2 6-12 months I 18.7 Calf survived I I 19.7 2 . I Total 24 18.8 1.9

the beginning of the dry season. However, the presence of neonates in all months of the year suggests that adult females have developed a feeding strategy that enables them to attain the metabolic threshold for reproduction even during the driest months (Pellew, in press (b)).

Mortality. The survival of immature giraffe up to 3 6 months is shown in Fig. 3 . The first-year calf mortality in the Seronera study area is about 58%. Approximately 50% of calves die in the first 6 months of life, with mortality greatest during the first month (22%). In the second year the mortality rate drops to 12%, and then to about 8% in the third year. Above this age the rate drops to a few percent per annum. Differences in male and female mortality u p to 3 6 months were not significant.

278 R. A . Pellew

Table 5. Estimates of the Serengeti giraffe population

Estimated population Estimated population density size

Year ( N o . km-*) (95% C.L.) (No.) (95% C.L.) Authority

1968 0.6 6590 Kruuk (1972) 1971 0.76 0.10 8348 I 0 9 8 Sinclair (1972) I 9 7 6 0.98 0.28 10,764 3075 Grimsdell (1979) 1977* I .2 0.3 13,127 353 I Pellew (1981)

*A systematic aerial sample count with a very low sampling intensity (4.1%).

Independent confirmation of these rates can be obtained from the population structures of Table 3. In the Seronera area, a mean annual natality of 20.7% and a second-year juvenile proportion of 9.6% suggest a first-year mortality rate of 54%, declining to about 12% in the second year. The suggested mortality rates in the third year and above are exaggerated because of the emigration of young males, resulting in the biased adult sex ratios. The first-year mortality rate in Nairobi National Park was estimated at about 73% by Foster & Dagg (1972), falling to 7% in the second year. However, these estimates were not obtained by monitoring the survival of calves of known-age, but by comparing theoretical and observed calf populations at different ages.

Adult mortality in the Seronera study area was very low, averaging 5.lo/o, (s.d. + 1.3%) per annum of the mean adult population, compared with the estimate of 13% per annum in Nairobi National Park (Foster & Dagg, op. cii.). Twenty- seven carcasses of adult animals were located over a 3-year period, which were categorized into age classes as follows:

Young Middle-aged Old Total Bulls 14 2 4 20 cows 2 2 3 7

Approximately half the adult mortality comprises young bulls. Identification data suggest that most (57%) of these young males are non-resident, a conclusion supported by the observed tendency of known juvenile males of 3 years and older to disperse out of their natal area. With an estimated mean adult female popu- lation in the Seronera study area of some 114 animals (32.4% of 352, Table 3), the seven located carcasses suggest an annual mortality rate of adult cows of only 2%. The distribution of searching effort for carcasses throughout the area was obviously not uniform because of the layout of the road network. If it is assumed that all dead adult giraffe were located in the most intensively searched main tourist area and this density of carcasses is extrapolated to the whole study area, then the adult mortality rate increases to 8.2% per annum, with adult female mortality at 3.4% per annum. Adult mortalities would appear at the moment to be approximately compensated for by recruitment from the fifth-year age-class.

Population Dynamics of Giraffe 279

Table 6 . Population densities ofgiraffe in the Serengeti National Park compared with other African conservation areas

Giraffe density (No. k m ~ ? )

Area sampled

Location (km*) Authority

2.64

2.6

I .9

I .47

I .42

I .23

1.13

0.98

0.88

0.76

0.72 0.67

0.34

0.18

0.16 0.03

Western woodlands, Serengeti N.P. 1976

Timbavati Private Nature Reserve, S.A.

Longido Game Control Area, Tanzania

Central woodlands, Serengeti N.P. 1976

Northern woodlands, Serengeti N.P. 1976

Hans Merensky Nature Reserve, S.A. 1969

Tarangire N.P.. Tanzania,

Serengeti N.P. woodlands, I976

Akira Ranch, Rift Valley, Kenya, 1972

Serengeti N.P. woodlands, 197 I

Nairobi N.P., Kenya, 1966 Manyara N.P.. Tanzania,

1971 Ruaha N.P., Tanzania,

1973 Tsavo East N.P., Kenya-

N. of Voi River, 1971 Kruger N.P., S.A.

Tsavo East N.P.. Kenya- S. of Voi River, 1971

1958-6 I

2 10 This study

5 50 Hall-Martin (1974)*

I I05 Dagg & Foster ( I 976)*

240 This study

I75 This study

52 Oates ( I 970)*

c'a 100 Lamprey ( I 964)*

7349 Grimsdell ( I 979)

316 Blankenship & Field (1972)*

9576 Sinclair (1972)

Foster & Dagg (1972) 90 Douglas-Hamilton (pers. comm.)

9640 Barnes & Douglas-Hamilton (1982)

Leuthold & Leuthold (1978)

Dagg & Foster ( I 976)* Leuthold & Leuthold (1978)

I I4

ca 100

19.000 ca 75

*Data from Dagg & Foster ( I 976).

Discussion Evidence of population increase The suggestion of an upward trend in giraffe numbers in the Serengeti can be inferred from the estimates in Table 5. With a total area of the Serengeti wood- lands of 10,984 km2 (Pellew, 1981), the 1976 mean density of the western and central strata of 0.98 giraffe km-* (Table I ) suggests a total population of some 10,750. Assuming a similar counting accuracy in 1971 and 1976 with the repli- cation of the census technique, comparison of these two estimates suggests an observed rate of population increase, f , of 0.058, equivalent to some 6% y e a r ' over the 5-year period.

T h e intrinsic rate of population increase, r, can be determined from the

280 R. A . Pellew

fecundity and mortality data (Wilson & Bossert, 1971). The individual birth rate, b,, equals 0.207 year-’ (Table 3 b i . e . 20-7 calves are born per year per 100 head of population. The individual death rate, do, can be calculated from the age- specific mortality data assuming annual mortality rates of 58%, 12% and 8% in the first, second, and third years, 2% in the fourth and fifth years, and 3.4% and 4.8% for adult cows and bulls; do equals 0. I59 y e a r - l 4 . e . 15-9 individuals die per year per 100 head of population, suggesting an intrinsic rate of increase of 0.048, or some 5% per annum. The difference between these two rates reflects the unreliability of the population estimates and does not imply that the rate of increase is declining. In 1976, the giraffe population of the Serengeti woodlands numbered some 10,750, and was expanding at a rate of 5-6% y e a r 1 .

Serengeti giraffe densities are high compared with other African conservation areas (Table 6). The Timbavati population in the eastern Transvaal Lowveld, the density of which was similar to that of the Kirawira study area, was probably above its carrying capacity, for Hall-Martin & Basson (1975) reported an increased mortality due to malnutrition, starvation and predation during the critical late dry-season period. Such mortality has not been experienced in the Serengeti.

That the Serengeti populations are expanding is also suggested by comparisons with the ‘stable population’ of Nairobi National Park. Regional Serengeti popu- lations have greater sub-adult proportions, shorter calving intervals, and higher fecundities per adult female: calf survival rates are higher and adult mortality lower. T h e records of individually recognized animals in the Seronera study area provide no evidence of any significant emigration, except for juvenile males of three years and older.

Two demographic parameters which have been shown in large mammals to be important mechanisms regulating the growth of herbivore populations are the duration of the inter-calf interval and the age of attainment of sexual maturity (Hanks & McIntosh, 1973; Laws, Parker & Johnstone, 1975; Croze, Hillman & Lang, I98 1). Fowler ( I 9 8 1) lists the species of large terrestrial mammals for which there is evidence of density dependence operating through the birthrate and the age at first reproduction, as well as through the rates ofjuvenile and adult survival. The mean calving interval and the age of reproductive maturation can be regarded as important indicators of the population status.

T h e Serengeti calving interval of 18.8 months is shorter than the estimates of 19.9 months for giraffe in the Timbavati Reserve (Hall-Martin & Skinner, 1978), and of 20 months in the Nairobi National Park (Foster & Dagg, 1972). Assuming a mean gestation period of 458 days (s.d.+3 days, n = 6 ) (Pellew, 198 I ) , the mean duration of the post-partum anoestrus in the Serengeti giraffe is 3.8 months, with a recorded range of 24 days when the calf died less than I week after birth to 9.2 months when the calf survived. With similar levels of calf mortality, the mean anoestrous period of the Timbavati giraffe was estimated at 7.1 months by Hall- Martin & Skinner (l978), who also stated that the shortest recorded reconception intervals of captive giraffe were 19, 23 and 27 days post-partum. The prolonging of the anoestrous period by lactation is implied by the significant increase in calving interval when the calf survives, although conception typically occurs whilst the female is still lactating (unless the calf dies). Sinclair (1977) suggests that a high level of energy intake resulting in good body condition before calving

Population Dynamics of Girafle 28 1

is the most important factor influencing the duration of the post-partum anoestrus in buffalo, a conclusion shared by Lamond (1970) for cattle. It is possible that the high quality of the Serengeti diet (Pellew, in press (a)) reduces the duration of anoestrus, increasing fecundity.

In 1979, three adult females of known age (first photographed as neonates) were recorded with newborn calves. The calculated ages at first conception were 46, 51 and 54 months (mean 50.3, s.d.+4.0 months). Hall-Martin & Skinner (1978) suggested an age of sexual maturation of 5-6 years for female giraffe in the Timbavati Reserve, with more precise estimates of 46 months for captive giraffe ( n = 1 1 ) and 56 months for wild giraffe in southern Africa (n = 7). The early maturation of the Serengeti females is again probably due to superior nutrition, for the giraffe in Timbavati are subjected to seasonal nutritional depressions, and it has been shown that poor nutrition can delay puberty in mammals (Sadleir, 1969).

Food limitation as a regulating factor The demonstration of a positive response to an increase in its food supply can be interpreted as implying that the population was previously food-limited, if the other extrinsic and intrinsic factors of the environment remain constant. Increas- ing the food availability to giraffe has lifted this limitation, resulting in a population expansion manifest by increased fecundity. It is appreciated that populations may be regulated by factors other than food limitation (see Sinclair, I974a, for summary of theories of population regulation), particularly as direct evidence for densi ty-dependent regulation in ungulate populations is deficient. Sinclair ( 1 974b) demonstrated that the buffalo population of the Serengeti was regulated by adult mortality caused by under-nutrition as a result of food shortage, this shortage being induced by intra- and inter-specific competition. Laws, Parker & Johnstone (1975) described an elephant population declining slowly due to homeostatic mechanisms inducing a reduced fecundity as a result of food limitation.

The high fecundity and low mortality of the Serengeti giraffe population suggests that the homeostatic regulatory processes have been temporarily miti- gated by the increased food availability. Each of the characteristics proposed by Klein ( 1970) as indicative of a food-limited population has been demonstrated in reverse. The giraffe population is probably still within the upswing or first stage of Caughley’s (1976) model. Although the browsing impact is extreme, removing up to 85% of the primary production of certain Acacia species, there is no evi- dence of any decline in plant vigour, implying a reduction in productivity, that could be expected in the ‘over-shoot’ second stage.

One environmental change that has exerted a density-independent effect upon the giraffe population is the eradication in the early 1960s of rinderpest, a virus to which the giraffe show a moderate susceptibility (Plowright & McCulloch, 1967). It is possible that the expansion of the giraffe population throughout the 1970s represents an extension of a population recovery begun in the 1960s as a result of the disease elimination. Such a pattern of population growth would closely mirror that of the grazing ruminants, although Sinclair (1979) showed a 3-year period (1970-72) of zero population growth in buffalo at the end of the

282 R. A . Pellew

recovery phase and before the onset of the second stage of the expansion stimu- lated by increased food availability. Because the depression of the giraffe popu- lation by the disease will have been less severe than in the grazing ruminants, a period of resource limitation is likely to have been manifest around 1970 before the resurgence of the tree regeneration stimulated further population growth.

A sequence of ecological events in the Serengeti can now be traced. The eradi- cation of rinderpest in the early 1960s together with the increased dry season rain- fall throughout the 1970s, has resulted in an eruption of ruminant populations (Sinclair, 1979). These populations regularly reduce the biomass of grass left standing in the dry-season, so that the area of the Park that burns annually has steadily declined (Norton-Griffiths, 1979). This in turn has encouraged the regeneration of trees and shrubs, particularly in areas where the mature woodland canopy has been opened up by elephants. The giraffe population has responded to this increase in its food supply by raising its fecundity. With low adult and juvenile mortalities and no significant emigration, the high fecundity has resulted in a marked expansion of the giraffe population, which as yet shows no manifes- tation of density-dependent limitation.

Acknowledgements This study was funded by the Leverhulme Trust, the East African Wildlife Society, the University of London, and the Fauna Preservation Society: I am most grateful for their financial support. The aerial counts were piloted by Dr J . J. R. Grimsdell and financed by the African Wildlife Leadership Foundation. I am par- ticularly grateful to Professor P. A. Jewel1 for his comments on the manuscript, and for the use of his Department’s facilities at Cambridge University for the data analysis and writing-up.

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