Population dynamics of the virunga gorillas: 1959–1978

Biological Conservation 26 (1983) 341-366

Population Dynamics of the Virunga Gorillas: 1959-1978 A. W. Weber Institute for Environmental Studies, Science Hall, University of Wisconsin Madison, Madison, W1 53706, USA

& A. Vedder Department of Zoology, University of Wisconsin--Madison, Madison, WI 53706, USA A BS TRA C T The Virunga gorilla population declined from 400-500 individuals in 1959-60 to 260-290 in 1971 73. Most of this decline is attributed to heat'>' hunting pressure in the Zairian sector. Large-scale habitat destruction and cattle disturbance in the Rwandan and Ugandan sectors are held responsible for further gorilla losses. Changes are documented in group size and composition, as well as in total numbers. Between 1971-73 and 1976 78, the population stabilised in the range of 252-285 gorillas. A tendency toward larger group sizes was observed in certain sectors, and the overall percentage of infants increased. Comparison of the 1976-78 group compositions with a simulated stable age structure model indicates a more healthy population than in 1971- 73. Potential for population growth is shown to be greater than previously predicted. Failure to increase in the inter-census period is attributed to continued poaching pressure. Current conservation efforts are described.

INTRODUCTION The species Gorilla gorilla occupies an extensive yet discontinuous range, stretching from Nigeria to R w a n d a across equatorial Africa. The 341 Biol. Conserv. 0006-3207/83/$03"00 '~ Applied Science Publishers Ltd, England, 1983. Printed in Great Britain

342

A. W. Weber, A. Vedder

subspecies known as the western gorilla G.g. gorilla occurs between the Atlantic coast and the interior Cross, Ubangui and Zaire river systems (Coolidge, 1929; Groves, 1970; Gartlan, 1980). Beyond a 1000 km gap to the east, a second subspecies is distributed in scattered pockets of forest in the highlands region of Zaire. These populations are currently classified as Grauer's gorilla G.g. graueri (Groves & Stott, 1979; Stott, 1981). Still farther east, across the Albertine (Great Lakes) Rift, only two gorilla populations exist. The classification of the Kayonza (Uganda) population remains in question, but that found in the Virunga mountains where Rwanda, Zaire and Uganda meet is recognised as a third subspecies, the mountain gorilla G.g. beringei. Though the first African national park was established in the Virungas in 1925 to assure their protection, mountain gorillas today are the least numerous and most endangered of the three subspecies. Numbering only 400 500 in 1960 (Emlen & Schaller, 1960; Schaller, 1963), the Virunga population has since declined dramatically to fewer than 290 (Harcourt & Fossey, 1981; Weber, 1981). This reduction has resulted from a combination of factors, including poaching, habitat loss, and disturbance by humans and their cattle. It is the intention of this study to examine the decline, current status, and longer-term survival prospects of the Virunga population. These subjects are addressed by analyses of census results and computer simulation of population growth. THE VIRUNGA E N V I R O N M E N T The Virunga region is characterised by its cool, humid climate and its rugged topography of eroded volcanic peaks, rising to more than 4500 m. The overall ecosystem comprises multiple vegetation zones typical of the East African montane phytogeographical region (Hedberg, 1951; Spinage, 1972; White, 1981). Two zones are most important for gorilla use;

(1) Hagenia Hypericum zone (2600 3200m): Stands of Hagenia abyssinica are concentrated in the western Mikeno-KarisimbiVisoke sector, while Hypericum lanceolatum predominates in the east; neither forms a closed canopy, thus allowing the establishment of associated patches of rich herbaceous growth; this zone provides abundant food for gorillas throughout the year. (2) Bamboo zone (2300- 3000 m): A rundinaria alpina dominates, with

343

Gorilla population dynamics I\ I I \ I

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Fig. I. Virungaconservation areas. Bounded regions include the Parc National des Volcans(Rwanda), Parc des Virunga (Zaire), and the Kigezi Gorilla Sanctuary (Uganda). little understorey growth where the canopy is complete; this zone can only support gorillas on a seasonal basis or at very low densities, since their preferred diet of bamboo shoots is available for less than five months each year. The Virunga range is protected within a multi-national conservation area which covers approximately 420 km 2. This area total is at variance with 375 km 2, quoted by Harcourt & Fossey (1981). Our figure is based on maps by C. Marius and 1974 aerial photographs of the Soci6t6 G6ographique Franqaise, and will be used for the purposes of this paper. Administrative responsibility for the area is divided among Zaire's Parc National Des Virungas, Rwanda's Parc National des Volcans, and the Kigezi Gorilla Sanctuary in Uganda (Fig. 1).

FIELD METHODS Direct sightings, repeated nest counts and regular monitoring of certain study groups were all employed to determine the number and composition of the Virunga gorilla population in 1978. Sixty-eight days and

344

A. W. Weber, A. Vedder

501 field hours were spent moving progressively from west to east across the range (excepting Mt Mikeno; see below). All gorilla signs were noted and fresh trails were followed until contact was made with the group. Once a group was sighted, high power binoculars permitted the observer to make drawings of noseprints--the unique set of markings above each gorilla's nostrils--for a total of 50 individuals. Such noseprints allow identification of individuals from past censuses, provide one of several checks against counting the same group twice, and they should facilitate follow-up studies of group movements and changes in group composition. In addition, visual contacts permitted direct counts and some age/sex determinations of individuals seen. Except as noted below, all individuals in the census were divided into six age-sex classes, similar to those established by Fossey (1974): infants (0-3½); juveniles (3½-6); young adults (6-8); adult females (8+); immature males, known as blackbacks (8-12); and fully mature males, or silverbacks (12 + years old). Field evidence indicates that the upper age boundaries of 3½for infants and 12 for blackbacks are more accurate than the 3 and 11 year ages conventionally employed. With the exception of those known from study groups, blackbacks and adult females without infants were grouped together, since census methods do not allow accurate sex distinctions between them (see below). For comparisons with the 1971-73 censuses, the number of non-sexed adults is divided into blackback and adult female classes using the same methods employed by Harcourt & Fossey (1981). With the exception of 58 gorillas in six groups that were studied on a regular basis, limitations imposed by thick vegetation and the reclusive nature of wild gorillas severely restricted visual counts. The most effective means of determining the number and composition of non-study groups was therefore to backtrack on their trail and make night nest counts. The use of nesting sites to estimate populations deserves elaboration. As described by Schaller (1963), Harcourt & Groom (1972) and Groom (1973), census efforts are greatly facilitated by the fact that each gorilla above the age of 3½ constructs a new nest to sleep in each night, within which it usually defecates. Variation in the dung size provides a relatively accurate key to the age class of individuals until they reach adulthood. Fully mature males are identifiable by their large dung as well as by their unique silvered hairs left behind in the nest. The existence and approximate age of infants can be determined by the presence of their very small dung in the same nest with that of their

Gorilla population dynamics

345

mothers, with whom they sleep. The presence of infant dung in the nest of an adult thus also allows identification of the latter as a female. Infants under 6 months generally leave no dung sign at all, as they eat little if any solid food before that age. Long-term data from the Karisoke Research Centre indicate that for every four infants identified through nest counts, approximately one additional infant under six months of age will be missed. In addition, solitary males which travel and sleep independently of groups leave little sign of their presence. Comparisons with data for known populations (Schaller, 1963; Harcourt et al., 1981) indicate an undercount of 54 ~ for this age class in the current census. These limitations, plus the fact that 7.5 ~o of the Virunga habitat could not be censused, means that our actual findings represent a conservative count of the population's lower limit. Allowing for the effect of this undercounting in several age classes and the area not covered, an upper limit is extrapolated and the resulting range, or its mean value, is used for comparison with specific published figures.

DATA BASES Analysis and discussion in this paper are based on three principal sets of data, each representing a different period of time. The first is drawn from Schaller's 1959-60 study (Schaller, 1963) and is identified for comparative purposes as 1960 data. The second set includes results of three censuses carried out in different sectors of the Virungas between 197 li and 1973, as well as related data from concurrent studies at the Karisoke Research Centre (Harcourt & Groom, 1972; Groom, 1973; Harcourt & Fossey, 1981; A. Harcourt, pers. comm.). Finally, the third data base consists of the results from the authors' 1978 census in combination with 1976 totals for Mt Mikeno. The latter count was made by A. and D. Lucas, who have kindly permitted us to use their unpublished data.

P O P U L A T I O N G R O W T H SIMULATION The model

Projections of growth of the Virunga population were carried out via computer simulation, using a matrix model of population growth largely

346

A . W . Weber, A. Vedder

developed by Leslie (1945). According to this model, the number of females aged 0 to 1 alive at the end of one year is equal to the sum of the number o f females born to each yearly age class that survive that period. This can be represented as: no.1 =

~ Fxnx.o

x=0

where

nx,t = the number of live females aged x to x + 1 at time t F x = the number of female offspring born in the interval t to t + 1 per female aged x to x + 1 at time t that will be alive in the age class Oto 1 one year later (assuming all age-class specific mortality occurs midway through the year)

The number of females in each remaining age classat time t + 1 year is equal to the number o f females in the preceding age class (aged x to x + 1) at time t multiplied by the probability that a female of that age class will survive to the next age class at time t + 1 (Px): hi, 1 =

no.oP o

n2.1 =

nx.oP 1

nn. 1 = nn-

l.oPn - 1

This series of n + 1 linear equations can then be represented in matrix form. When this matrix is post-multiplied by a column vector of the distribution of individuals in age classes at time 0, a column vector representing the age-class specific population after one year results. This can then be continued for the desired number of years to simulate population growth over longer periods of time. The finite rate of increase of the population is found to be equal to the dominant latent root of the fertility/survival matrix described above (see Poole, 1974, for further explanation of the matrix model). Based on this model, projections for females were made from the 1971-73 Virunga gorilla censuses to the years in which the more recent counts were conducted. The data were divided into two simulations: the 1971 72 census results (all regions except Mt Mikeno) projected to the year 1978, and the 1973 count (Mikeno) projected to 1976. Assuming an equal sex ratio through 12 years of age, a number of males equivalent to that of females was then added to the results for those age classes. Since the numbers of lone silverbacks are highly variable among censuses, only the number of those expected to be counted within groups was added to

Gorilla population dynamics

347

the projected population number. This expected value is calculated by multiplying the mean number of silverbacks per group found in each region of the first censuses (1971-73) by the number of groups found in each of the latter counts (1976, 1978). To project populations into the future, where number of groups is unknown, the number of silverbacks was calculated to equal the number of females older than 12 years multiplied by a factor of 0.70, since the mean number of silverbacks per female within groups for all censuses combined since 1973 is 0.70. The simulation also gives a calculation of stable age structure (SAS), i.e., the proportion of the population in each age class once they remain constant. This assumes constant rates of fertility and mortality, which are not altered by factors extrinsic or intrinsic to the population. Thus, comparisons made to a SAS indicate degree of disturbance imposed upon the population, normally resulting in increased age-specific mortality.

Survival schedule Calculation of survival rates was based on long-term records from gorilla groups studied from the Karisoke Research Centre, between Mts Karisimbi and Visoke, during the period September 1967-December 1980 (Fossey, 1974, 1981). Each immature individual of known history for yearly age class intervals was included in the sample, giving a survival rate of 0.792 for the age interval 0 to 1 year (n = 24), 0.90 for 1 to 2 years (n = 20) and 1.00 for the remaining yearly age classes (n = 2i, 21, 18, 16, 13 and 13). These rates are higher than those reported by Harcourt et al. (1981), but since they are calculated from larger sample sizes their reliability is presumed to be greater. Undoubtedly some accidental mortality takes place between the ages of 2 and 8, so a survival rate of 0-99 is used for those years instead of 1.00. Adult survival rates have been reported by Harcourt et al. (1981), and their conservative figure of 0-95 is used.

Fertility schedule Fertility rates were derived from the same Karisoke Research Centre data base as above. Thirty-four births were recorded among 27 females above 8 years of age, representing a total of 145.9 adult female years. Thus a value of 0.233 births per adult female per year was calculated.

348

A. W. Weber, A. Vedder

Harcourt et al. (1981) discounted fertility rates similarly derived as being greater than expected outside of the more protected Karisoke research area, and thus unrepresentative of the total Virunga population. This does not seem to be the case, however. The number of individuals counted in each age class can be broken down into yearly age class estimates, given the presumed survival rates previously described. A birthrate can then be extrapolated again using age-specific survival rates. By so doing, the 1971-73 census results give a rate of 0.216 births per adult female per year (0.224 on Mt Mikeno and 0-199 in sectors excluding Mikeno--including the Karisoke study area). Infant survival has been argued to be lower in areas outside the richly vegetated Karisoke region (Harcourt et al., 1981). If this were the case, it would mean the birthrate indicated by census results would in fact be higher. In addition, the census results include documented effects of poaching on immature age classes, as described below, which give a lower actual survival rate and therefore also a higher birth rate calculation than the 0.216 value cited. The use of 0.233 as a fertility rate therefore appears justified. RESULTS A N D DISCUSSION

Current population Two hundred and forty-six gorillas in 28 groups plus six solitary males were counted in the 1976-78 censuses. This total of 252 individuals may be considered a minimal figure for the entire Virunga population, as some gorillas almost certainly went uncounted. Allowing for under-representation of young infants and solitary adult males, as well as those gorillas in the 7.5 ~ of available habitat that was not surveyed, an upper limit of 285 is reached, derived from base of 252 plus: 6 infants, 0 - 6 months old; 7 solitary males; and 20 additional individuals projected for uncensused areas (see Field Methods). Group composition figures are presented separately for the 1976 Mikeno count and the 1978 census of the remaining Virunga sectors (Tables 1,2). Group sizes range from 3 to 21 members, with a mean of 8-8 (median = 7). Of the 28 groups, one includes three silverbacks, ten (35.7 ~o) contain two silverbacks each, and all others (60.7 ~) have only one mature male per group, lmmatures (½--8 years) comprise 35.8 ~o of the total population, with infants (42) totalling slightly less than the number of juveniles (23) and young adults (25) combined.

349

Gorilla population dynamics

TABLE 1 1976 Census Results: Mt Mikeno Group label

SB a

BB

AF

UA

YA

J

I

Total

I

2

2

7

0

2

1

7

lI II1 IV V VI VII VIII IX

3

1 (2) b

2 (3)

2

0

2

3

21 13

1

0

2

0

1

1

0

5

1 2 1 1 2

0(1) 0 (1) 0 1 0 (2)

0(1) 0 (2) 0 (1) 2 3 (5)

2 3 1 0 4

0 0 1 1 3

0

0

3

1

0

6

0

0

3

l

0

6

3

0

15

I

0 (1)

2 (4)

3

0

l

2

9

Total

14

4 (10)

18 (27)

15

8

10

12

81

o/,, of total population

17.28

4-94

22.22

18-52

9-88

12.35

1 4 - 8 2 100.00

Mean number per group

1-68

0-44

2.00

1.67

0.89

1.11

1.33

9.0

SB, silverback; BB, blackback: AF, adult female; UA, adults of undetermined sex; YA, young adult; J, juvenile; 1, infant. b Values in parentheses are estimated numbers ofblackbacks and adult females if unsexed adults are redistributed into the two classes, as done by Harcourt & Fossey (1981). Distribution

o f the V i r u n g a

population

We have m a p p e d the a p p r o x i m a t e locations where the 28 gorilla g r o u p s were f o u n d in the m o s t recent censuses (Fig. 2). T w e n t y g r o u p s c o n t a i n i n g 186 individuals are located in the western sector, while o n l y 60 gorillas in 8 g r o u p s inhabit the east. Based o n m e a n values o f the p o p u l a t i o n range, a density o f 0.70 gorillas k m - 2 is c a l c u l a t e d for the western sector, versus 0.50 gorillas k m 2 for the east. In a d d i t i o n , western g r o u p s differ f r o m those in the east with respect to b o t h m e a n g r o u p size (9.3 vs 7.5) a n d m e a n n u m b e r o f silverbacks per g r o u p (1.55 vs 1-13). Several a u t h o r s have discussed the u n e v e n distribution o f the gorilla p o p u l a t i o n within the Virunga range. Differential h u m a n i m p a c t ( G r o o m , 1973; H a r c o u r t & C u r r y - L i n d a h l , 1979; H a r c o u r t & Fossey, 1981) as well as the m o r e restrictive shape o f the eastern sector (Schaller, 1963; H a r c o u r t & Fossey, 1981) have been cited as possible causal factors.

350

A. W. Weber, A. Vedder TABLE

2

1978 Census Results: Sectors Excluding Mikeno Group labeP

SB ~

BB

AF

UA

YA

0 (3)" ~1 1 2 I 0 0 (1) I (2) 0 0 (2)

2 (5) 1 5 0 4 4 1 (2) 1 (2) 2 4 (7)

6 0 0 0 0 0 2 2 0 5

0 2 0 0 I 2 1 1 0 1

1-K 2-K 3-K 4-V 5-V 6-V 7-V 8-V 9-V 10-V

1~ 2 1 1 2 2 2 2 1 2

11-V

1

1

4

0

12-Mu 13-Mu 14-T 15-S 16-S 17-S 18-S 19-M

1 1 2 1 1 1 I 1

1 1 (2) 1 (2) 0(1) 0 0 1 0

1 (2) 1 (2) 3 (5) 3(5) 2 2 (3) 1 (2) 2 (3)

1 2 3 3 0 1 1 1

11 (21)

43 (60)

27

J

1

Total

0 0 1 1 1 2 0 1 2 1

2 0 3 0 4 2 1 0 1 2

1

1

4

1 1 1 1 0 1 0 1

0 1 0 0 0 0 1 1

1 0 3 3 0 2 1 1

11 6 11 4 13 12 7 8 6 15 12 6 7 13 11 3 7 6 7

15

13

30

165

Total

26

~o of total population

15.76

6.67

26.06

16.36

9.09

7.88

18.18

100.00

Mean number per group

1.37

0.58

2-26

1"42

0.79

0.68

1.58

8.68

a Group labels: K, Karisimbi; V, Visoke; Mu, Muside; T, Tshanzu; S, Sabinyo; M. Muhavura. b Age/sex classes as in Table 1. c Values in parentheses as in Table 1. D i f f e r e n c e s in n a t u r a l v e g e t a t i o n , h o w e v e r , a r e p r o b a b l y o f m a j o r i m p o r t a n c e . L a r g e a r e a s o f the e a s t e r n s e c t o r a r e u n s u i t a b l e f o r e x p l o i t a t i o n b y g o r i l l a s o n a y e a r - r o u n d basis (see Fig. 2). T h e s e a r e a s i n c l u d e the p o o r l y v e g e t a t e d l a v a fields o n m u c h o f M u h a v u r a , t h e e x t r e m e l y r u g g e d c a n y o n walls o f S a b i n y o , a n d t h e n u t r i t i o n a l l y deficient e n v i r o n m e n t s in the s u b a l p i n e a n d a l p i n e z o n e s . T h e b a m b o o z o n e c o v e r s 37 ~o o f the e a s t e r n s e c t o r a n d r e p r e s e n t s a critically i m p o r t a n t g o r i l l a

Gorilla population dynamics

IT ~~[~'~ ~

~

351

4 1 ~ 11 Ir~ Potentialyear-round use by gorillas " "~'~\\ Habitableseasonallyor by extremely ~] Virtuallyunusableby gortllas

Fig. 2. Location of gorilla groups found in the 1976 and 1978 censuses. Range divided into three zones differing in potential for gorilla use. Arrows indicate our division of the regions into western and eastern sectors. Adapted from vegetation maps by C. Marius (unpublished).

food resource. This is true, however, only during the two seasons of bamboo shoot availability which total five months per year. This leaves only the middle altitude forest and associated zones as capable of satisfying gorilla nutritional needs throughout the year. Densities calculated on the basis of this suitable year-round habitat, rather than the total area, eliminate the gap between west and east, as 2.10 gorillas k m - 2 are found in each zone. These comments about vegetation are not intended to negate the importance of human impacts on the gorillas, but rather to stress ecological factors influencing their distribution as well.

Virunga population dynamics: 1959-1978 The Virunga gorilla population has declined dramatically since Schaller's 1960 estimate of roughly 450 individuals to no more than 285 in 1978. This decline, however, has been uneven both spatially and temporally, and has included changes in group size and composition as well as in total number. These changes, their possible causes, and their potential impacts on long-term survival of the population are discussed below.

352

A. IV. Weber, A. Vedder

Population size

The most striking element of change in the size of total populations since 1960 is the drastic drop in the Mikeno population from approximately 250 to 84, or 67~o, between 1960 and 1973 (Fig. 3). Although the K a r i s i m b i - V i s o k e - M u s i d e ( K - V - M ) population also declined 11 ~o over the same period, the loss of roughly 166 gorillas from the Mikeno sector alone accounts for the vast majority of the overall Virunga decline. 500d

~ >

÷

200-

z I000 0 z 0

'60 '73 '76 MIKENO

'60 '71-'72 '78 K-V-M

l i--l-i

'60 '71-'72 '78 S-G-M

Fig. 3. Population change in three sectors of the Virungas: 1960-1978. Beginningwith a population sizeof 450, the 1960population is subdividedas describedby Schaller(1963). The latter two censuses are also divided into the sectors Mikeno, Karisimbi-VisokeMuside-Tshanzu (K V-M) and Sabinyo-Gahinga-Muhavura (S G-M) to accord with Schaller's data base. Vertical lines indicate estimated population ranges, bar values the means of those ranges. The primary cause of the decline in the Virunga gorilla population has previously been attributed to human conversion of habitat (Harcourt & Curry-Lindahl, 1979; Harcourt & Fossey, 1981). Nearly 100km 2 of Rwanda's Parc National des Volcans (PNV) were converted to agricultural use in 1968-69. Using a density figure of one gorilla per km 2, Harcourt & Fossey (1981) noted that 'habitat removal alone could account for 60 ~o of the drop in numbers'. This reduction in size of the Virunga conservation area must have had a serious impact on the gorillas, but it seems to have resulted in the loss of m a n y fewer gorillas than Harcourt & Fossey (1981) suggest. Its relationship to the principal population decline is more temporal than spatial, as nearly 85 ~o of the total Virunga decline between 1960 and 1973 occurred in the Mikeno sector of Zaire, where there was no concurrent habitat loss. Direct hunting is the only plausible cause of the Mikeno population crash. The fact that the people living around that area have no tradition of poaching gorillas, under normal conditions, would seem to contradict

Gorilla population dynamics

353

this (Verschuren, 1972; J .-P. Harroy, pers. comm.). We suspect, however, that the troubled conditions which prevailed in the mid- 1960s, when Zaire was engaged in a widespread civil war, contributed greatly to the decline in gorilla numbers. Specifically, the fighting that occurred around the Virungas in 1967 left in its wake a trail of burned and plundered fields, with the result that both local farmers and soldiers turned to the park's wildlife for food. While there is no certain evidence that great numbers of gorillas were hunted for meat and trophies, the fact that 26 Zairian guards were killed trying to defend the park during that period stands as ample testimony to the onslaught of armed hunters. Though not nearly as large as on Mikeno, a concurrent decline also occurred in the K - V - M sector. While some of this must also be attributed to poaching (it is known that at least one group of 7 gorillas was killed on the Rwandan side of Karisimbi during the pre-census period; Goodall, 1979), massive habitat losses are probably at least equally important. The elimination of 10 000 ha of parkland in 1968-69 was concentrated in the K - V - M sector, where the gorilla population declined from approximately 155 in 1960 to 139 in 1972. The loss of lower altitude forest and bamboo would have forced the gorillas to readjust their home ranges into a significantly reduced area. This smaller area would have consisted of the relatively narrow band between 2700 and 3300 m, as they could not expand on any long-term basis into the nutritionally impoverished subalpine and alpine zones (Harcourt & Fossey, 1981; Vedder, unpublished). Such forced crowding of a relatively large population would almost certainly have resulted in intensified inter-group conflicts. A study of gorilla vocalisations carried out soon after the habitat conversion found a high incidence of aggressive vocalisations among groups on MtVisoke (Fossey, 1972), and it is possible to interpret this as evidence of heightened conflict due to overcrowding. The gorilla population in the eastern sector also declined between 1960 and 1972. While the total numbers concerned are small, the drop from 45 to 36 individuals represents a loss of 20 ~o from an area with a critically low population to begin with. Although this area was not affected by the Rwandan parkland conversion scheme, habitat loss and disturbance are still the most likely causal factors for the decline. In 1950, nearly one-third of the Ugandan reserve was withdrawn for agricultural use (Spinage, 1972). Groom, in his 1972 census, found that even more of the Ugandan sector had been cleared by illegal settlers (Fig. 4). He also cited higher

354

A . W . Weber, A. Vedder

Fig. 4. Aerial photo of Mts Sabinyo, Gahinga, and Muhavura (L to R) demonstrating the restricted shape and extent of agricultural and human settlement impact on eastern sector of Virungas. Note irregular line of encroachment on the northern (Ugandan) border, in contrast to the static park boundary on the Rwandan side.

frequencies of human and cattle disturbance in the eastern region in general (Groom, 1973). By 1978, the gorilla population in the east had dropped further to 34. Most importantly, only one group of seven gorillas was found on Mt Muhavura, where there had been two groups with 12 individuals before. Whatever the cause, the almost total isolation of Muhavura's solitary group--neighbouring Mt Gahinga supports no gorillas on a year-round basis due to inadequate food resources--is an extremely serious problem. Lacking other groups with which to exchange individuals, inbreeding could pose long-term genetic hazards (Harcourt et al., 1976; Harcourt & Fossey, 1981). Even more serious, this last group

Gorilla population dynamics

355

could also be eliminated, leaving no gorillas at all on the two easternmost mountains of the Virunga range. The marked decline begun in the 1960s had come to a near-halt by the early 1970s (Fig. 3; Table 3). Identical counts of gorillas in groups (246) and similar population range estimates (260-290 vs 252-285) for the censuses of 1971-73 and 1976-78 indicate that this was a significant period of relative stasis across all sectors of the Virungas--with the noted exception of Muhavura. This finding lends support to the hypotheses presented above which cite the isolated phenomena of civil disturbancerelated hunting pressure in Zaire and large-scale habitat conversion in Rwanda as the primary causal factors in the gorillas' decline. The fact that those phenomena did not continue on a comparable scale caused predictions of an extended population crash to be in error. TABLE 3 Age/Sex Class Composition by Region: 1971-73, 1976-78

Region

Number of groups

SB a

AD

YA

J

I

39 37 6 10 34 40 7 7 3 7 10 14 6 3

13 8 2 2 14 7 1 2 4 1 3 2 1 1

9 10 1 0 11 10 0 1 1 0 4 1 1 1

8 12 2 2 17 17 1 1 2 3 1 6 2 1

Total

Mikeno 1973 Mikeno 1976 Karisimbi 1971 Karisimbi 1978 Visoke 1971 Visoke 1978 Muside 1972 Muside 1978 Tshanzu 1972 Tshanzu 1978 Sabinyo 1972 Sabinyo 1978 Muhavura 1972 Muhavura 1978

7 9 2 2 11 9 3 2 1 1 5 4 2 1

10 14 3 3 16 14 4 2 2 2 6 4 2 1

79 81 14 17 92 88 13 13 12 13 24 27 12 7

Total 1971-1973 Mean/group !!i, of population

31

43 1.4 17.5

105 3.4 42-7

38 1.2 15-4

27 0.9 11.0

33 1-1 13.4

246 7.9

Total 1976 1978 Mean/group 0;, of population

28

40 1-4 16.3

118 4.2 48.0

23 0.8 9-3

23 0.8 9.3

42 1.5 17.1

246 8.8

SB, silverback; AD, non-silverback adult: YA, young adult; J, juvenile; I, infant.

356

A. W. Weber, A. Vedder

Still, human impacts on the population did continue at a lower level through the entire period of this study. Their varied effects--along with attempts to mitigate them--are reflected in the census results. Beginning with an initial population structure as found in the 1971-73 censuses, a computer simulation of population change up to 1976-78 projects an increase in the number of gorillas living in groups to a total of 260. Since the comparable 1976-78 count was 246, human impact apparently continues to depress population growth. The problem of cattle grazing illegally in the Virungas has been present throughout most of the post-independence period (Schaller, 1963; CurryLindahl, 1969; Groom, 1973). Thousands of Ankole cattle were driven into the park by the Tutsi when their monarchy was overthrown by the Hutu majority in 1958-1960. Many remained there with their herds, even after calm was restored to the country, and Rwandan officials initially did little to enforce anti-grazing restrictions. The full impact of this on the gorillas cannot be determined, and it is known that cattle have been grazed intermittently in the Virunga forest for at least a century (Spinage, 1972). Still, there is no doubt that the presence of large herds on a permanent basis had some disruptive effect on gorilla ranging, feeding, and possibly even reproductive behaviour. The attack on illegal grazing launched by Rwandan authorities in 1974 was a positive development. Heavy fines of $10 per cow (in a region where per capita income averaged $72) generated park revenue which totalled several thousand dollars in 1976. Though some observers have continued to write about the cattle problem in the Parc National des Volcans, their reports are not based on current field experience: between 1978 and 1980 neither of us saw a single cow in any sector of the Virungas. Thus, the Rwandan cattle removal programme has been extremely effective and has certainly contributed to the restoration of a more healthy environment for the gorillas. In addition to their cattle, humans also had a more direct effect on gorilla numbers through renewed illegal hunting. Though they had no previous history of hunting gorillas, one or two bands of Twa pygmies began killing and capturing the animals at an alarmingly rapid rate in the mid-1970s. This change in behaviour resulted in response to external markets for gorilla heads--preferably of silverbacks--and live young. We can only speculate about sources of the demand: perhaps unscrupulous zoo and museum directors, private collectors, and/or curio dealers. It was definitely satisfied, however, at the expense of a significant

Gorilla population dynamics

357

portion of the Virunga population. In 1975, dozens of skulls were confiscated by Rwandan officials (A. Monfort, pers. comm.) and, in 1978, we were personally aware of at least 6 gorillas killed in the Mt Visoke region alone.

Group size If the earliest records of gorilla population structures for different areas are assumed to be normal, mean group size has traditionally varied greatly among the three principal sectors of the Virungas (Fig. 5). This variation is consistent with the differential availability and type of food resources, as suggested by Schaller (1963). During the time intervals for which data are available, however, variability within regions has also been detected. 20

'58-'60 MIKENO

MIKENO 16.9 K-V-M[ 9.11

w N

i0,

15

'71275 '76"78 11.'5 7.7

9.0 9.4.

5,

6.8

(30

I0

K-V-M ~

~.~.e

S-G-M

~

5

0

I 1960

I

I 1970 YEAR

I

I 1980

Fig. 5. Changes in mean group size in different sectors of the Virungas. Sector division and labels as in Fig. 3. i 1967 value = mean of 1967 Karisoke Research Center study group size times 1971-72 mean of censused groups divided by 1971-72 mean of KRC study group size (Fossey, 1974; Harcourt & Fossey, 1981). 2 Mean of all values reported 1957 59 (Donisthorpe, 1958; Kawai & Mizuhara, 1959; Emlen & Schaller, 1960; Osborn, Bolwig as reported by Schaller, 1963).

In the Mikeno sector, mean group size dropped 33 ~o, from 16-9 to 11.3, between 1960 and 1973. This parallels the rapid decline in total population size during the same period and is undoubtedly an added consequence of intensive poaching and perhaps cattle disturbance in the region. An acceleration of this decreasing group size to 9.0, however, occurred despite a slight increase in total gorilla numbers between 1973 and 1976. This trend suggests that forces such as poaching continue to threaten the Mikeno population.

358

A . W . Weber, A. Vedder

In other sectors of the Virungas, habitat loss and degradation were equally important factors in the declining group sizes observed. In the eastern sector, where more than 1000 ha were cleared on the Ugandan side, mean group size declined from 7.0 to 5.1 between 1960 and 1972. Figures for the K - V - M region show that mean group size decreased from 9.1 to 7-7 between 1967 and 1972: the period in which 10 000 ha were withdrawn for agricultural purposes. At the same time, disturbance from cattle grazing was a factor in both areas. The diminished quantity and quality of available gorilla habitat could explain the observed decrease in group size prior to 1972. Groups containing fewer members may travel through less area daily in order to fulfil all individuals' nutritional needs. With reduced travel costs, therefore, gorillas in small groups could feed more efficiently than in large groups under conditions of limited resources. Decreased group size would thus be expected to occur when these benefits outweigh those associated with large group living. The fact that mean group size increased in areas outside of Mikeno between 1972 and 1978, apparently as a result of consolidation, is perhaps a function of the decreased habitat disturbance observed during that period. To the extent that this trend continues, it is thus a possible sign of health in the population. Population age structure Age class distinctions from Schaller's 1960 data are not comparable with those of later censuses, so that the following discussion is limited to changes observed between the 1971-73 and 1976-78 counts. The 1976-78 census showed an increased number of infants and nonsilverback adults, but a lower number of young adults and juveniles than found in the 1971-73 survey (Fig. 6). Some of these changes would be expected, as shown by a comparison of the 1976-78 results with numbers projected from the simulation model of population growth from 1971 -73 (Table 4). The unusually high number of young adults in the first census would be expected to move into the adult population by the time of the censuses conducted 3 to 7 years later, and thereby to result in an increase I 1971

-

YA

AD

SB

:! ::][1[i1111[,I 1111111111111 I

o

Fig. 6.

J

73

I

I

4o

I

I

80

I

I

,&o

I

200

I

240

Group composition: 1971 73, 1976-78. Age classes in Table 3i

359

Gorilla population dynamics

TABLE 4 Comparison of Census Results and Computer Projections Age class a

I J YA AD SB Total

1971-73

33 27 38 105 43

(13.4) b (11-0) (15.4) (42-6) (17.5) 246

1976-78

42 23 23 118 40

(17.1) (9-3) (9.3) (48.0) (16.3) 246

Projected number

SAS

46.7 (18.0) 31.9 (12-3) 23.2 (8.9) 119-2 (45.8) 39.0 (15.0)

(17.0) (12.8) (9-5) (42.6) (18.3)

260

a Age classes as in Table 3. b All values in parentheses are percentages.

in the latter age class. Likewise, the lower number of young adults found in the 1976-78 census would be expected given the number of infants from the preceding count. There is, in fact, a good fit between the 1976-78 findings and the results projected from the earlier census for most age classes, which supports the validity of the parameters used in the simulation. The lower than predicted numbers of juveniles and infants in 1978 are exceptions to this good fit, and account for 97 ~o of the discrepancy between projected and observed results. The infant age class numbers indicate that some disturbance continued to affect the youngest segment of the gorilla population throughout the mid-1970s. During the period of only a few months prior to the 1978 census, four infants were known to have died as a result of poaching. These four alone, added to the 1978 results, would have equalled the number of infants expected from the simulation. Mortality in a young age class will have a compound effect on population growth, since these individuals are eliminated before reaching reproductive age. Therefore, the killing and capture of infants continues to have a disproportionate impact on the future of the population. Most of the shortfall from projection estimates, however, is in the juvenile age class, which is only 72 ~o of the expected value. This would indicate unusually high infant mortality 3 to 6 years prior to the 1978 census, i.e., during the years 1972 to 1975. As previously described, these years included a period of renewed poaching which would differentially affect the infant class in three principal ways. First, it is these youngest gorillas who are the primary targets for the live animal trade. Second, infants are those most likely to be caught in bamboo spring traps, which

360

A . W . Weber, A. Vedder

are set for other species but which are indiscriminant in their catch. Finally, the poaching of silverbacks for trophy skulls has an additional indirect impact on infant mortality through the possibility of infanticide whenever new males attempt to take control of a group following the killing of its leader (Harcourt et al., 1980). Silverback mortality itself, however, is only slightly reflected in group composition analyses, since most are replaced by individuals from the pool of solitary males. Poaching pressure during the 1972-75 period is therefore the most likely cause of the depressed number of juveniles found in the 1978 census. There remains the possibility, however, that habitat losses from earlier periods could have had a delayed effect, particularly in the western Rwandan and eastern Ugandan sectors. If the loss of range caused a higher density of gorillas in the remaining Virunga forest zone, food resources may have been overexploited. Resource depression would not be expected immediately, but might instead occur after several years. Lactating females would be most stressed by limited food availability or quality (Vedder, in preparation) and this, in turn, would increase infant mortality. If this were the case, it proved only a temporary condition, since the number of infants in the subsequent 3-year period again increased. Finally, comparison of age-class percentages can be made with those of the stable age structure (SAS) derived from the simulation (Table 4) in order to assess intensity of disturbance. Although the proportion of juveniles departs from the SAS value for the 1976-78 census, infant and young adult classes in the latter match the respective SAS percentages more closely than those found in 1971-73. Adults differ little from the predicted SAS. Despite the extrinsic disturbances described above, the Virunga population appears to be approaching a more stable age structure, and thus shows signs of intrinsic health. Most encouraging is the fact that the number of infants has increased from 33 to 42 since the 1971-73 counts (Table 5). While this remains 7 ~ lower than expected, the trend is nevertheless toward greater realisation of the population's reproductive potential, particularly in the Rwandan sector of the Virungas. Future prospects In the absence of extrinsic disturbance (poaching, grazing, encroachment), the Virunga gorilla population should be increasing. Given the

Gorilla population dynamics

361

TABLE 5 Observed and Expected Number a of Infants

Mikeno Obserred Expected 1971 73 1976-78

8 12

14.5 14.0

Excluding Mikeno Obserred Expected 25 30

28.9 31.0

Total Observed Expected 33 42

43-4 45.0

Number of infants expected based on the number of adult females counted, and the fertility and infant mortality rates as used in the computer simulation.

fertility and mortality schedules employed above, growth would stabilise at a finite rate of 1.7% per year. This is significantly higher than previously reported rates of 0.02 to 0.6~o per year (Harcourt et al., 1981), due to dependence of the latter on a smaller data base and subsequent errors in the derivation of birth rates from previous census results (see Methods). Projections using the 1976-78 census counts as a conservative starting point indicate that the sectors excluding Mikeno could harbour a gorilla population of 206 by 1988. This represents an increase of 41 individuals over a ten-year period and results in a total greater than Schaller's 1960 estimate for the same area. Moreover, the entire Virunga population could theoretically return to the 1960 level of 450 gorillas by the year 2010! Several important caveats must be kept in mind with respect to the above projections, however. First, results of the model are based on the assumption that fertility and mortality rates remain constant. In fact, one would expect that after an initial period of rapid population increase, the growth rate would decline as resources become limiting. Furthermore, given past irreversible losses of gorilla habitat, it is likely that the Virunga carrying capacity has been permanently reduced and that pre-Sehaller population levels will never again be attained. Finally, it must be recognised that the model takes no account of extrinsic disturbances and, unless these are either eliminated or significantly lessened in coming years, these projections will not be realised. The true value of the present analysis lies in its application to the conservation of the Virunga gorillas. Specifically, it indicates that intensive management and manipulations in the form of translocations, captive breeding, and subsequent reintroductions are neither necessary nor desirable, as the gorilla population in most sectors shows signs of

362

A . W . Weber, A. Vedder

vitality and the capacity to expand naturally. At the same time, analysis of the most recent census results lends support to the view that external disturbances of human origin--particularly poaching--are primarily responsible for the gorilla population decline. The future of the Virunga population depends entirely on efforts to control these external forces.

Conservation of the Virunga gorillas Efforts to reduce human impact on the gorillas and their montane habitat have thus far been most intensive, and moderately successful, in Rwanda. These began in 1974 with the cattle removal programme, and an increase in the guard force of the PNV. International reaction to renewed killing of gorillas in 1978, however, resulted in the formation of a consortium of conservation groups dedicated to dealing with the problem. In full cooperation with the Rwandan government and its Office of Tourism and National Parks, the Mountain Gorilla Project (MGP) was launched in 1979. While an intensified anti-poaching effort was led by MGP adviser J. P. von der Becke, we were charged with the conceptualisation and implementation of two additional projects: a tourism programme centred on the gorillas to increase park revenues and thereby provide incentive for continued Rwandan preservation efforts; and a conservation education programme to spread awareness of both the mountain gorilla and more general principles of conservation to the Rwandan population. The M G P has been quite successful (Harcourt, 1980; Weber, 1981). Since the implementation of better-equipped, trained and organised patrols, poaching in the PNV has been reduced to a minimum, with only one confirmed killing of a gorilla in 2½ years. In addition, the tourism project has resulted in more than a 1200 ~o increase in park revenues over the same period, while gross foreign revenue receipts have more than tripled to $6.3 million, making tourism Rwanda's fourth major revenue earner (PCRB, 1981: Weber, 1981). The education programme has succeeded in distributing audio visual materials to all Rwandan secondary schools, while wtrious displays and radio broadcasts have been used to reach the extrascholastic population. Finally, a mobile education unit has been employed to give slide and film presentations to nearly 100 000 Rwandans, with special attention given to schools and villages in the region around the Virungas (Aveling & Aveling, 1981 ; O'Keefe, 1981). In spite of these early successes, however, much remains to be done to

Gorilla population dynamics

363

assure adequate protection of the Virunga gorillas. Security forces must continue to patrol the forest, or poachers will soon return. The tourism programme must be carefully managed so that tourist impact itself does not have a negative effect on the park and gorillas. And education work of any kind requires many years of continuous inputs to be successful. Most importantly, the MGP must expand these programmes into Zaire and Uganda to be fully effective. This will require the full cooperation of host government authorities, as well as continued financial support from M G P consortium members. Ultimately, efforts are needed to modify those conditions which have forced local human populations into competition with the gorillas for the same land (Weber, 1979, 1981). Rwanda is the most densely populated country in Africa as well as one of the poorest, with 95 ~ of its people living as subsistence farmers. There are indications that Rwanda's traditionally high population growth rate of nearly 3.7 ~ is now declining (Niyibizi, 1979) and that agricultural practices may also be changing in response to population pressures (Morris, 1979). Still, with the growing population, demand for land will continue to increase well into the next century. While the M G P has helped to stem the tide of human impact, failure to deal with these issues of population and land use could ultimately trigger another human wave powerful enough to wash the Virunga forest and all its gorillas away forever.

CONCLUSION Predictions of the continued sharp decline of the Virunga gorilla population have not held up in light of the most recent census results. We believe the reasons for this lie in the nature of the primary causes of the dramatic post-1960 decline: heavy hunting pressure during a period of heightened civil disturbance in Zaire and, to a lesser degree, large-scale habitat destruction in the Rwandan and Ugandan sectors of the range. These were localised, albeit large-scale, phenomena and their effects diminished over time--although they continue to be reflected in the current population dynamics of group size and composition. The 1976-78 census results show that the gorilla population did not continue its marked decline, but rather stabilised just below the 1971-73 level. The population is healthier in many respects than before, and has a greater than predicted potential for growth.

364

A. W. Weber, A. Vedder

While these are positive indicators, we remain painfully aware that the Virunga population is 40 ~o below its 1960 level ,and rests near a critical lower limit which has already been surpassed in certain sectors. Furthermore, despite its evident growth potential, the gorilla population has not increased since the last census, indicating the continued negative impacts of poaching and disturbance. Still, we remain guardedly optimistic with respect to the future of the Virunga gorillas. The population is viable and adequate critical habitat areas remain protected. Moreover, Rwanda has recently contributed significantly to the preservation of the gorillas through its role in the successful anti-poaching, tourism, and education programmes of the Mountain Gorilla Project. There is an urgent need, however, to extend this project into Zaire and Uganda to have a similar impact on their administrative sectors of the Virunga environment. Serious problems remain in the areas of human population growth, land use, and development. While the resolution of these issues is beyond the capabilities of any conservation effort like the MGP, it is essential that conservationists continue to stress their importance to relevant national and international authorities and institutions.

ACKNOWLEDGEMENTS We wish to thank the New York Zoological Society for its primary financial support of our research. Additional funding for census work came from the Fondation pour Favoriser la Recherche Scientifique en Afrique, and logistical support from the Karisoke Research Centre was provided by the National Geographic Society. Our conservation work with the Mountain Gorilla Project was funded by the Fauna Preservation Society and the People's Trust for Endangered Species. We are particularly grateful to the Rwandan Office of Tourism and National Parks for their authorisation to work in the Parc National des Volcans. Many individuals helped with various stages of this project, to whom we would like to express our appreciation: the 1978 census would not have been possible without the invaluable skills of the Rwandan guides Nemeye, Rwelekana and Vatiri; R. Palm and I. Redmond assisted with the census count in certain sectors; Drs A. and N. Monfort provided valuable advice and personal support throughout our stay in Rwanda; Dr A. H. Harcourt was consistently supportive of our efforts, open in his

Gorilla population dynamics

365

discussion o f ideas, and provided unpublished data for our use; Dr A. Palloni made himself available for long discussions on the subject of demography; and, finally, this manuscript benefited from comments and suggestions by Drs T. Vale, T . C . M o e r m o n d , and W. M. Denevan, as well as M. DeJong and K. N o o n a n .

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Harcourt, A. H. & Groom, A. F. (1972). Gorilla census. Oryx, 11,355-63. Harcourt, A. H., Stewart, K. J. & Fossey, D. (1976). Male emigration and female transfer in wild mountain gorilla. Nature, Lond., 263, 226-7. Hedberg, O. (1951). Vegetation belts of the East African Mountains. Svensk. bot. Tidskr., 45, 140- 202. Kawai, M. & Mizuhara, H. (1959). An ecological study on the wild mountain gorilla (Gorilla gorilla beringei)--report of the Japan Monkey Center second gorilla expedition 1959. Primates, 2, 1 42. Leslie, P. H. (1945). On the use of matrices in certain population mathematics. Biometrika, 33, 183-212. Morris, W. H. M. (1979). A report on agricultural production, marketing, and crop storage in Rwanda. Kigali, Rwanda, USAID. Niyibizi, S. (1979). Population et developpement. Kigali, Rwanda, Bureau Nationale de Recensement. O'Keefe, J. (1981). Nature and environment in Rwanda. Final Report. Gland, WWF/IUCN. PCRB (1981). Tourisme et protection de la nature. Brussels, Projet de Cooperation Rwando-Belge. Poole, Robert W. (1974). An introduction to quantitative ecology. New York, McGraw-Hill. Schaller, G. (1963). The mountain gorilla. Chicago, University of Chicago Press. Spinage, C. A. (1972). The ecology and problems of the Volcanoes National Park, Rwanda. Biol. Conserv., 4, 194-204. Stott, K. W., Jr (1981). A suitable vernacular name for an intermediate gorilla. Mammalia, 45, 261. Verschuren, J. (1972). Contribution 6 l'kcologie des Primates, Pholidota, Carnivora, Tubulidentata, et Hyracoidea, Part 3. Brussels, Fondation pour Favoriser la Recherche Scientifique en Afrique. Weber, A. W. (1979). Gorilla problems in Rwanda. Swara, 2, 28-32. Weber, A. W. (1981). Conservation of the Virunga gorillas: a socioeconomic perspective on habitat and wildlife preservation in Rwanda. MS thesis, University of Wisconsin, Madison, WI. White, F. (1981). The history of the afromontane archipelago and the scientific need for its conservation. Afr. J. Ecol., 19, 33-54.