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Census of gorillas in northern Republic of Congo.

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American Journal of Primatology 27275-284 (1992)
CONSERVATION NOTE
Census of Gorillas in Northern Republic of Congo
J. MICHAEL FAY' AND MARCELLIN AGNAGNA'
I Wildlife Conservation International, New York Zoological Society, Bronx; 'Ministsre de
l'Economie Forestiere, Brazzaville, Republic of Congo
We censused gorilla populations in northern Congo from February to April
1989 and June 1990. The objective was to provide the first quantitative
data from a variety of sites on gorilla populations from a country that had
unknown but potentially high populations. The method used was a census
of nests along strip transects. A total of 401.0 km of transects was sampled
in four different study areas. The overall density calculated for all
transects was 0.4nesting gorillaskm'. The highest density, 1.2 nesting
gorillas/km2,was found in the vast Likouala swamp area of north central
Congo. The two northern sites showed low densities of 0.1 and 0.2 nesting
gorillas/km', respectively. The northwestern site showed an intermediate
density of 0.6 nesting gorillaskm'. The vegetation type with the highest
density was swamp forest with 2.4 nesting gorillas/km2.The limited sample presented shows that gorillas are widespread and common in northern
Congo, even in the swamp forests previously considered unsuitable as
gorilla habitat. It is probable that Congo holds the largest population of
gorillas in Africa after Gabon. o 1992 Wiley-Liss, Inc.
Key words: western lowland gorilla, density, Republic of Congo
INTRODUCTION
While several studies have addressed various aspects of the ecology, habitat
preference and density of the western lowland gorilla (Gorilla gorilla gorilla) [Tutin and & Fernandez, 1984; Calvert, 1985; Carroll, 1988; Rogers et al., 1988;
Williamson et al., 1988, 1990; Fay, 1989; Fay et al., 1990; Rogers et al., 19901, our
knowledge of the present range and densities of this subspecies remains inadequate. One potential reservoir of gorillas that has received little attention is the
Republic of Congo (Congo). Gorillas have been known from various locations
throughout Congo for some time [Cousins, 19781 but information on their distribution and population density is sparse [Regusters, 1983; Kuroda, 1990; Mitani,
1990a,b; Nishihara, 1990; Oko 19901.
In this paper we present gorilla density estimates from four localities in northern Congo along with notes on habitat preference, nesting behavior and feeding
ecology, and discuss the distribution and conservation status of G. g . gorilla in
Congo.
Received for publication April 5, 1991; accepted September 21, 1991.
Address reprint requests to J. Michael Fay, Wildlife Conservation International, New York Zoological
Society, Bronx, NY 10460.
0 1992 Wiley-Liss, Inc.
276 I Fay and Agnagna
MATERIALS AND METHODS
Study Area
The Congo is a large country (342,000 km') with a low human population
density (ca. 6 peoplekm'). About one-half of the people live in the towns and cities,
leaving the countryside relatively unpopulated (0.9/km2 inhabitants in study
sites). Nearly two-thirds of the country (222,300 km') is forested, the majority in
the north, which remains largely undisturbed.
In February through April 1989 and June 1990, transects were run in four
areas of northern Congo to census gorilla populations (Fig. 1).The south of the
country was subsequently surveyed, and only the salient points of this study will
be presented here [Agnagna et al., 19911. The four sites surveyed differed in vegetation and in human impact on mammal populations, as summarized below.
The Mboukou site was located in the vast swamp forests of northeastern Congo
found between the Oubangui and Sangha Rivers. Transects were run to the south
ofthe Likouala aux Herbes River near Lake Mboukou (0"56", 17'25'E) in an area
where a spit of terra firma juts into the surrounding swamp forest. The soils here
are typically saturated, with up to 1 m of standing water during the wet season.
The vegetation is dominated by Raphia spp., Trichilia sp., Guibourtia demeusii,
Uapaca spp., Mitragyna stipulosa, Garcinia sp., Symphonia globulifera, Manilkara
sp., Alstonia congensis, Klainedoxa sp., Pandanus candelabrum, Aframomum angustifolium, and Lasiomorpha senegalensis. Flooded forests, also present, generally have a closed canopy, seasonally saturated soils, and lack the dense understory of monocotyledonous herbs of the swamp forest. Flooded forests are
dominated by Guibourtia demeusii, Uapaca spp., Mitragyna stipulosa, Garcinia sp.,
and Diospyros spp. The terra firma has a dense forest cover dominated by Pentaclethra macrophylla, Tetrapleura tetraptera, Macaranga sp., Angylocalyx pynaertii,
Milicia excelsa, Milletia sp., Klainedoxa sp., and Panda oleosa.
The Motaba site was located in the forest block in the northeastern part of the
country bordered on the west and the north by the Central African Republic
(2"32'N-3"05'N, 17"10'E-17"20'E). Transects were run between the two principal
rivers in the region, the Motaba and Ibenga, and the villages of Makao and Berandjoko. The vegetation is primarily semi-deciduous terra firma forest with large
areas of Raphiu swamp along the major rivers and forests dominated by Gilbertiodendron dewevrei (Malapa) along upland watercourses. The terra firma forest is
dominated by Entandrophragma spp., Polyalthia suaveolens, Pachyelasma tessmannii, Autranella congolensis, Pterocarpus soyamii, Piptadeniastrum africanum,
Combretodendron macrocarpum, Celtis spp., Canarium schweinfurthii, Strombosia
spp., Irvingia spp., Funtumia elastica, and Erythrophleum suaveolens.
The Ndoki site was also located in the forest block in the northeastern part of
the country near the border with southwestern Central African Republic (2'12"2"30", 16"10'E-l6"35'E). Transects were run in the headwater area of three major
tributaries of the Ndoki River. The vegetation is very similar to that of the Motaba
site other than a block of secondary forest, exploited for sapeli (Entandrophragma
cylindricum) in the late 1970s, west of the western tributary of the Ndoki River.
Sapeli is the principal tree species that is commercially exploited in northern
Congo.
The Mbomo site was located in a large tract of uninhabited forest in northwestern Congo bordering Gabon, northwest of Odzala National Park (0'32"1"40'N, 14"32'E-l4"38'E). Transects were run along a compass bearing between the
villages of Mbanza and Sembe. The vegetation is primarily semi-deciduous terra
firma forest dominated by Klainedoxa gabonensis, Coula edulis, Zrvingia spp.,
Fig. 1. Map of northern Congo showing the four principal study sites and important geographic localities.
278 I Fay and Agnagna
TABLE I. Vegetation Types and the Dominant Angiosperms
Recorded on Transects
Vegetation type
Dominant angiosperms
Terra firma
Entandrophragma spp.
Polyalthia suaveolens
Pachyelasma tessmannii
Autranella congolensis
Swamp forest
Raphia spp.
Trichilia sp.
Pandanus candelabrum
Aframomum angustifolium
Flooded forest
Guibourtia demeusii
Mitragyna stipulosa
Garcinia sp.
Diospyros sp.
Hyparrhenia spp.
Andropogon spp.
Gilbertiodendron dewevrei
Savannah
Malapa
Dacryodes spp., Parkia spp., Pachyelasma tessmannii, Daniellia sp., and Panda
oleosa.
At the southern end of the Mbomo site there is a n old road, abandoned for over
30 years. The vegetation along this road is secondary, dominated by Zanthoxylum
macrophylla, Pentaclethra macrophylla, Uapaca sp., Macaranga spp., Tabernaemontana crassa, Tetrapleura tetraptera, Lophira alata, Pycnanthus angolensis,
Barteria fistulosa, Megaphrynium macrostachyum, Aframomum spp., Sarcophrynium prionogonium, Hypselodelphys poggeana, and H . uiolacea. North of the
Ekoutou River the vegetation is primary forest.
CENSUS METHODS
The methods for a quantitative census of gorilla populations used in this study
were modified from Tutin and Fernandez [1984]. These were based on linear
transects of fixed width (20 m), run on fixed compass bearings, and employed two
to four observers. Transect width was set a t 20 m because visibility in dense forest
is limited beyond 10 m on either side of the transect. Distances were measured
using pedometers. A topofil (hip chain) was used at a rate of lo%, or 20% in the
case of the Ndoki site, of total transect distance, in order to calibrate pedometer
readings. Along transect lines, all signs of gorillas were recorded, including feeding sites, tracks, trails, dung, and nests. Gorilla feeding sites were distinguished
from those of chimpanzees principally by tracks around the feeding sites, scent,
and signs on feeding remains. When a nest group was encountered on a transect,
age class, height, plant species used in construction, construction type for each
nest, and nest group size were recorded [after Tutin & Fernandez, 19841.Gorilla
nests were distinguished from chimpanzee nests by construction type, height distribution of nests within nest group (gorilla nest groups often have one or more
nests on the ground, and gorilla nests are rarely found above 15 m), and gorilla
scent, dung, or hair in nests. If more than half of the nests in a group fell outside
the strip width, the group was not included in the census.
Vegetation was grouped into five broad categories: savannah, terra firma forest, flooded forest, swamp, and Malapa as described above (see Table I for domi-
Northern Congo Gorilla Census / 279
nant species in each vegetation type). When a qualitative change in vegetation
type was noted, the vegetation type entered and distance along transect were
recorded. This permitted a quantitative assessment of the overall area represented
by each vegetation type in the survey. Species identifications were made by the
principal author in the field for known species and by identification of vouchers a t
the Missouri Botanical Garden herbarium.
The formula used in calculating nesting gorilla (ng) density was (number of
nestdarea sampled)/mean nest life span. Because no in situ data were collected on
nest degradation rates, we used those determined for Gabon by Tutin and Fernandez [19841 that are based on mean nest degradation rates for the different categories of nest construction types as follows: no vegetative construction (no nest structure existed), minimal vegetative construction (few stems and soil used for nest),
herbaceous (herbaceous material is used exclusively and extensively in nest construction), mixed herbaceous and woody (woody material was mixed with herbaceous material), woody (exclusively woody material was used in nest construction).
We believe that the nest degradation processes in Gabon are similar t o those in
Congo because of geographical proximity and similar vegetation and climate. Density calculations reflect the average degradation rate based on the ratio of nest
types noted in each vegetation type for each study site and assume 100%observer
efficiency. No attempt was made to account for non-nesting individuals. The density figures presented in this paper, because of the error involved in the methods,
must be considered to be very broad estimates particularly useful in comparing
different sites. Tutin and Fernandez 119841 used a standard error of 20%for their
density estimates.
RESULTS
The results of the gorilla nest census, for a total of 401.0 km of line transects
(n = 44 transects, mean = 9.1 km, S.D. = 3.2) in the four different study areas,
are presented in Table 11. The overall density calculated for all transects, based
solely on nest census, was 0.4 ng/km2. Gorilla density for the four sites ranged from
1.2 ng/km2 to 0.1 ng/km2.
The vegetation type with the highest gorilla density was swamp forest with 2.4
ng/km2. In terra firma forest, which accounted for 64.7% of transect total area
censused, the densities varied from 0.1 ng/km2 to 0.6 ng/km2 by site.
The average nest group size overall was 3.8 nests (n = 51 sites, S.D. = 2.9
nests), and the average nest group size excluding solitary males was 5.0 nests (n =
36 sites, S.D. = 2.8 nests). The majority of nests (82.6%)were constructed on the
ground using herbaceous material, in particular Aframomum spp. Another 13.3%
of nests were in trees situated between 5 and 15 m above ground level.
The most commonly observed gorilla feeding remains were those of Aframomum spp., Haumania danckelmaniana, and broken termite mounds (Cubitermes
sp.) (Table 111).
DISCUSSION
The overall gorilla density figure obtained for the four sites compares favorably with data from other parts of west Africa [Tutin & Fernandez, 1984; Carroll,
1988; Fay, 19891. Gorillas occur in very high densities in the Likouala swamps.
This population was known only from a few reports [Mackal et al., 1982; Regusters,
19831 and is discussed elsewhere [Fay et al., 19901. Previously it was thought that
the eastern range of G . g. gorilla may have been limited by the Likouala swamps
[Groves, 19711. The existence of high densities of gorillas east of the Likouala aux
Herbes River necessitates a modification of the accepted range limit of the western
280 I Fay and Agnagna
TABLE 11. Gorilla Density for Four Sites in Northern Congo
Site and
vegetation
type
Lake Mboukou
Terra firma
Swamp
Flooded
Savannah
Mboukou total
Motaba
Terra firma
Flooded
Malapa
Motaba total
Ndoki
Terra firma
Swamp
Malapa
Primary pooled
Secondary
Ndoki total
Mbomo
Terra firma
Swamp
Old second
Mbomo total
Overall total
Total
transect
length
Area of
transect
(km2)
Number
of nests
counted
Mean nest
duration
(days)
Gorilla
density
(/km2)
19.20
26.81
16.19
0.80
0.38
0.54
0.32
0.02
88
2
60.2
61.7
0.0
2.7
0.1
0.0
63.000
1.26
90
60.3
1.2
127.15
10.60
3.75
2.54
0.21
0.08
15
57.4
2
33.0
0.1
0.0
0.8
141.50
2.82
17
54.5
0.1
34.3
5.0
45.5
8
1
0
18.6
61.7
16.2
0.69
0.10
0.91
1.70
0.32
14
60.9
0.6
0.0
0.0
0.2
0.7
101.0
2.02
23
46.8
0.2
79.15
2.35
14.00
1.58
0.05
0.28
59
3
4
60.8
35.3
61.7
0.6
1.7
0.2
95.50
1.91
66
59.7
0.6
401.00
8.02
196
58.0
0.4
0
0
0
9
lowland gorilla as well as a reassessment of suitable gorilla habitat. The probable
reason for the success of this Likouala aux Herbes gorilla population is the abundance of monocotyledons in this swamp, dominated by Pandanus sp., Aframomum
angustifolium, and Raphia sp., which are important dietary items of the gorilla at
this site [Fay et al., 19901. The distribution of the gorilla within the Likouala
swamp area is not known [Fay et al., 19901, but it is probable that gorillas occur
throughout the swamp forest area where there is enough terra firma to which the
gorillas can retreat in the wet season. According to local informants, gorillas reach
the Oubangui River in the area of Impfondo and Doungou [Fay et al., 19901.
Gorilla density was low at the Motaba site. This is probably due to two factors.
Herbaceous monocotyledons typical of areas of high gorilla density were sparse
and poaching pressure was high [Fay & Agnagna, in press]. Interviews with local
inhabitants along the Motaba and Ibenga Rivers indicated that gorilla density
decreases from the headwaters of these rivers to the Oubangui River. Discussions
with local loggers, poachers, pygmies, and ethnologists further to the northeast
indicated that gorillas were rare in the extreme northeast (NE of Enneyle). A
similar pattern was noted just north of the border in the Central African Republic
where gorillas were not found east of the Ngoto forest [Fay, pers. obs.].
In the Ndoki site, gorilla density was comparable to the Motaba site. To the
west of the Ndoki River, gorilla density was much higher than to the east of the
Northern Congo Gorilla Census / 281
TABLE 111. Frequency of Gorilla Feeding Remains From Four Sites in Northern Congo
Ranked by Overall Frequency
Site
Food species
Aframomum spp.
Haumania dankelmaniana
Cubitermes sp.
Pandanus sp.
Hyselodelphys poggeana
Black ants
Fern
Megaphrynium macrostachyum
Dig (Thonningia sanguinea)
Pycnobotyra nitida sp.
Palisota sp.
Thomandersia sp.
Hyselodelphys scandens
Palm (rattan)
Klainedoxa gabonensis
Raphia sp.
Laccosperma senegalensis
Ficus sp. (leaf)
Myristicaceae sp.
Renealmia sp.
Wood (dead)
Unk. vine
Mboukou
Motaba
Mbomo
Ndoki
Total
14
0
0
25
0
0
0
0
0
0
0
0
0
0
0
1
1
0
0
0
0
0
2
4
4
0
0
1
0
0
0
5
0
3
2
0
0
0
0
1
1
0
0
0
5
0
28
0
18
0
1
3
8
0
0
0
0
0
0
0
0
0
0
1
1
0
138
89
25
0
0
13
10
6
0
1
6
1
1
2
2
0
0
0
0
0
0
1
159
93
57
25
18
14
11
9
8
6
6
4
3
2
2
1
1
1
1
1
1
1
Ndoki River. This high gorilla density west of the Ndoki River was also found by
Kuroda [19901 and in adjacent Central African Republic [Fay, 19891. It can probably be attributed to an unusually high natural density of herbaceous monocotyledons in this area, in particular Megaphrynium macrostachyum. To the east of the
Ndoki River where there are few gorillas, except in areas in proximity to the river,
herbaceous monocotyledons are sparse, with the virtual absence of M . macrostachyum. This distinct vegetation difference is evident on a 1990 Landsat TM
image of this area (Fay, pers. obs.) produced for World Wildlife Fund-US by
EOSAT, Washington, D.C.
The Mbomo study area overall showed a high gorilla density. It was evident
during the survey at this site that areas with the highest gorilla density were
primary forest that had abundant herbaceous monocotyledons, especially Hypselodelphys poggeana, H . violacea, Megaphrynium macrostachyum, and Aframomum
spp. (>7 spp.). This vegetation was common in the center of the study area and is
discernible on aerial photographs [Letouzey, 19683. Physiognomically this vegetation is very similar to areas in southwestern Central African Republic, and southeastern Cameroon where gorillas also occur in high densities [Letouzey, 1968;Fay,
19891 (Fay, pers. obs.). Areas t o the south of the study site with old secondary
growth have very low gorilla densities. In other areas, secondary growth often
contains high gorilla densities [Schaller, 1963; Tutin & Fernandez, 1984; Carroll,
19881. Poaching associated with long-term human habitation of the area probably
accounts for the low gorilla densities in this vegetation type at the Mbomo site.
A recent survey in the Mayombe and Chaillu regions of southern Congo [Agnagna et al., 19911 indicated that gorillas occur in very low densities but are still
282 I Fay and Agnagna
widespread in southwestern Congo. Based on observations in the field and information gathered in Brazzaville [Agnagna, pers. obs.; Attwater, pers. comm.; Hecketsweiler, pers. comm.1 it is evident that there is still a significant number of
gorillas being killed in southwestern Congo. Many orphaned gorillas came into the
gorilla orphanage in Brazzaville from this area in 1990 [Attwater, pers. comm.].
Interviews with individuals selling gorilla hands and other parts in Brazzaville
markets indicate that most of the parts come from the Mayombe area (Agnagna,
pers. obs.). It is conceivable that exports of gorillas from Cabinda [Anon., 19901
originate in the Mayombe region of Congo.
Results of feeding sites noted in this survey reveal two species not previously
recorded as important gorilla foods: Pandanus sp. and Thonningia sanguinea. The
differences in feeding sites between the four study areas reflect, as far as we can
detect in this general survey, differences in abundance not in preference by gorillas. For example in the swamp forests of the Mboukou site Cubitermes sp. mounds
do not occur or are rare, and thus are not consumed by gorillas.
While the western lowland gorilla (G.g. gorilla) is more frugivorous than other
subspecies of gorilla, the majority of its diet is derived from the stems and leaves
of a limited number of species of monocotyledonous herbs [Sabater Pi, 1977; Tutin
& Fernandez, 1985; Calvert, 1985; Rogers & Williamson, 1987; Rogers et al., 1988,
1990; Carroll, 1988; Fay, 1989; Fay et al., 1990; Williamson et al., 19901. It has
been suggested that primary lowland forests, because they largely lack terrestrial
herbaceous vegetation (THV) are poor gorilla habitat resulting in low gorilla population densities in these forests [Schaller, 1963; Groves, 19711. More recent studies have shown that in the west African lowland forests gorillas attain high densities [Tutin & Fernandez, 1985; Carroll, 1988; Fay, 1989; Fay et al., 19901 and
that THV is often abundant in primary forest [Rogers and Williamson, 1987;
Rogers et al., 1990; Fay et al., 19901 [Fay, unpublished data]. This study supports
the notion that gorilla densities are geographically correlated with the abundance
of THV. Our future studies of gorillas in Congo will address the question of the
quantitative abundance of THV and its correlation to gorilla density on a more
general geographic scale.
Gorillas are currently not threatened with extinction over much of northern
Congo. However, in areas where forest exploitation is taking place, gorilla densities have decreased dramatically (e.g., Kabo) [Fay & Agnagna, pers. obs.]. This
is largely because gorilla meat is a preferred food in the region. In the near future
most of northern Congo will be opened up by logging operations (over 50% of the
area has already been awarded to logging companies). While selective logging may
not inherently cause a decrease in gorilla populations, and may even improve the
habitat for gorillas [Tutin & Fernandez, 1984; Calvert, 1985; Carroll, 19881, it will
most likely lead t o decreased population levels because of increased poaching. In
southern Congo decades of hunting for meat and traditional medicine threaten
local gorilla populations.
CONCLUSIONS
1. The majority of the 170,000 km2 of forests in northern Congo is suitable
gorilla habitat.
2. Gorillas are widespread in northern Congo and occur at high densities in
some areas, even in swamp forests previously considered unsuitable as gorilla
habitat.
3. Gorilla populations in southern Congo have been reduced because of poaching associated with long-term logging operations throughout most of the region,
but gorillas are still present in many areas.
Northern Congo Gorilla Census / 283
4. The limited data set available does not permit a population estimate for the
whole of the country, but it is very probable that Congo holds the largest population of gorillas in Africa, after that of Gabon, estimated at 35,000 & 7,000 [Tutin&
Fernandez, 19841.
5. Gorilla density seems to be correlated with the density of THV.
6 . Opening the vast forest areas of northern Congo to mechanized logging will
most likely have severe adverse effects on gorilla populations there, given the
current management of exploited forests in the country.
ACKNOWLEDGMENTS
This project was carried out by Wildlife Conservation International under
contract to the World Wide Fund for Nature (WWF) as part of the European
Economic CommunityNVWF African Elephant Program and the United States
Agency for International Development. Additional funding was provided by New
York Zoological Society, National Geographic Society, and L.S.B. Leakey Foundation. This survey could not have been a success without the help of the following
individuals and institutions: Richard Barnes (Wildlife Conservation InternaJ. Boukindi
tional), Jacques Kanwe, Ndinga Assitou, Mokoko Ikonga, D. NSOSSO,
(Minist6re de 1’Economie Forestiere, Brazzaville). We thank three reviewers for
their useful comments.
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