New Gorilla Population Estimates in Northern Republic of Congo
On 5 August, 2008, at the XXII International Primatological Society
Congress in Edinburgh, the Wildlife Conservation Society (WCS)
announced the results of a ground-breaking series of western lowland gorilla
surveys conducted in the northern Republic of Congo, one of the last remaining
strongholds for this species. The results estimate a total population
of 125,953 individuals (90,325-161,545) living in 47,444 km² of contiguous
lowland and swamp forest. This estimate exceeds the previous global estimate
for the species and confirms the Republic of Congo as harbouring arguably
the largest remaining populations of western gorillas in Africa.
Surveys in the 1980s indicated that healthy populations of western gorillas
existed in many areas remote from human settlements (Harcourt 1996). In
spite of legal protection across their geographic range, recent evidence
has indicated that western lowland gorillas are declining rapidly, through
a combination of commercial hunting, the spread of logging, which alters
forest structure and opens up remote forest blocks to facilitate poaching,
and Ebola hemorrhagic fever (Tutin et al. 2005). Commercial hunting and
Ebola alone are thought to have reduced the western gorilla population
by over 50% in the last three decades (Walsh et al. 2003), to the extent
where the species was recently upgraded to "Critically Endangered"
in the IUCN Red List of Threatened Species (Walsh et al. 2007).
Most of the world's remaining western lowland gorillas are found in the
Republic of Congo and Gabon (Harcourt 1996). Low human population densities
(< 1/km²) combined with relatively poorly developed infrastructure
and access has, until recently, largely protected these forests and their
wildlife from human disturbance and depletion.
In the 1980s and 1990s, surveys in the northern Republic of Congo indicated
that the region was of great importance for western lowland gorillas (Gorilla
gorilla gorilla) and other endangered large mammals. Since 1991, WCS
has worked together with the government of Congo and other partners to
establish an effective conservation program in the region. In 2006 and
2007 WCS implemented a series of great ape surveys across northern Republic
of Congo (see map). The surveyed area represents one of the most important
remaining blocks of habitat for western lowland gorillas, covering 47,444
km² of contiguous lowland forest. The surveys were conducted in three
phases, corresponding to three adjacent survey zones.
Adapted from an original map by WCS
Ndoki Likouala Landscape (27,970 km²): The Ndoki-Likouala
landscape includes two protected areas (the Nouabalé-Ndoki National
Park and the Lac Télé Community Reserve), and several surrounding
logging concessions. In collaboration with the government of Congo, WCS
have been working in this landscape for the past 17 years. The primary
objective of the surveys across this zone was to assess the population
status of great apes in response to conservation interventions. This was
the first systematic and standardized survey of the entire zone.
Batanga swamps (1,029 km²): The Batanga swamps represent a
largely unexplored area of Raphia swamp forest between the Ndoki-Likouala
Landscape to the west, and the Batanga River to the east. Reconnaissance
missions conducted in the 1990s indicated these swamps support large populations
of gorillas (Blake 1995; Fay & Agnagna 1992; Fay et al. 1989), but
these forests have never previously been systematically surveyed. The
primary objective of the surveys here was to assess the population status
of gorillas and the potential for expansion of conservation activities
into this area.
Ngombe-Ntokou-Pikounda Landscape (18,455 km²): The Ntokou-Ngombe-Pikounda
landscape includes the Ngombe timber concession, and the remote Ntokou-Pikounda
forests to the south and east. The site is covered by very large tracts
of Marantaceae forest to the west and north and by swamp forest to the
east and south. To the west lies the Odzala-Koukoua National Park. It
was the site of the last recorded Ebola outbreak in May 2005, which affected
both human and gorilla populations (Caillaud et al. 2006; Devos et al.
2008). Prior recent outbreaks to the south and south west of the park
were known to have devastated the previously high density gorilla populations
(Bermejo et al. 2006). Reconnaissance missions conducted in the Ntokou-Pikounda
forests in 1999 had suggested that it harboured large populations of great
apes (Quammen 2001), but these forests have never before been systematically
surveyed. The objectives of surveys in this zone were therefore to assess
the population status of great apes in order to provide a baseline for
effective protected area planning and management, and to identify evidence
for Ebola impact on great ape populations in the landscape.
Line transect distance sampling was used to estimate gorilla density from
nest counts (Buckland et al. 2001). Production and decay rates for nests
were estimated and used to convert nest density into gorilla density (for
details of survey design and analysis see Stokes et al. 2008). A total
of 265 transects of 2 km in length were placed across the three survey
zones.
The surveys were conducted between February 2006 and May 2007. A total
of 3,815 great ape nests were counted along 526 km of transects. 2,550
nests were assigned to gorillas. The gorilla population across all three
survey zones was estimated at 125,935 individuals (95% CI: 90,325 to 161,545).
The significance of this finding is two-fold. Firstly, it emphasizes the
importance of scientifically rigorous survey methods in assessing and
evaluating the conservation status of key species. Prior to these surveys
the global estimate of wild western lowland gorillas was approximately
95,000 individuals (Butynski 2001). This estimate was based on range-wide
extrapolation of data collected in the 1980s from a number of discrete
survey locations in Gabon and the Republic of Congo (Harcourt 1996). However,
in the light of both recent advances in survey techniques (Kühl et
al. 2008) and, importantly, the recent documented declines in western
lowland gorilla populations (Walsh et al. 2003; Devos et al. 2008; Bermejo
et al. 2006; Huijgbregts et al. 2003), the validity of this figure is
questionable (Walsh et al. 2007). Whilst we are unable to provide a revised
global estimate of western lowland gorillas from our surveys, we are able
to provide a revised baseline estimate for northern Congo, which represents
a significant contribution to our knowledge of the current status of western
gorilla populations and which provides a basis for monitoring the impact
of future conservation efforts in what is undoubtedly one of the highest
priority areas for the future survival of this species.
Secondly, the findings from these surveys highlight a number of key conservation
issues and priorities for future conservation action. The majority of
western gorilla habitat, and the majority of our surveyed gorilla population,
lies outside of protected areas. In 2006, the government of Congo announced
the proposed Ntokou-Pikounda Protected Area. We are keen to use our findings
to work together with the government to catalyze the process of legally
gazetting this protected area and promoting effective on-the-ground protection.
Many of the areas containing high densities of gorillas lie within sites
managed in partnership between the government of Congo, WCS and other
partners, including the private sector. The partnership with the timber
concession holder CIB (Congolaise Industrielle de Bois) has yielded improved
wildlife management and protection in over 13,000 km² of production
forests surrounding the Nouabalé-Ndoki National Park. In June 2006,
CIB's Kabo concession was the first timber concession to be awarded Forest
Stewardship Council (FSC) certification in Central Africa. CIB's FSC-certified
forests have now been expanded to 7,500 km² (Tropical Forest Trust
2007). These forests support some of the highest recorded densities of
western lowland gorillas. Given the rate of expansion of commercial logging
and mining operations across northern Republic of Congo, and indeed across
much of Central Africa, we support the use of auditable timber certification
schemes in production forests and the integration of best practices for
reducing the impact of logging on great apes across their geographic range
(Morgan & Sanz 2007) to ensure protection of gorilla populations in
key habitats that lie outside of protected areas.
These findings are encouraging for the conservation of western lowland
gorillas. The results indicate healthy populations in numbers higher than
previously thought. The persistence of these populations can be largely
attributed to the remote and inaccessible nature of the Ntokou-Pikounda
forests and the Batanga swamps, combined with successful conservation
investment in the Ndoki-Likouala landscape.
At the same time, however, northern Congo is subjected to similar threats
to those faced by western gorillas across their range. Remote and inaccessible
forests are fast becoming opened up and commercially exploited for natural
resources. The result is escalating hunting pressure to meet the growing
demand for bushmeat. Our surveys did not indicate evidence of recent Ebola
outbreaks in the gorilla population. Nevertheless, expert opinion and
research findings to date all point to this zone for the next predicted
epidemic. The recent upgrading of the western gorilla to Critically Endangered
on the IUCN Red List of Threatened Species was primarily due to
the rapid rate of decline of the subspecies over the past 20 years in
response to increasing threats from poaching and disease. We do not advocate
for western lowland gorillas to be down listed. Rather, we support a renewed
focus on addressing and mitigating the threats that endanger these last
remaining populations and on preventing further population decline of
this species.
Emma Stokes, Richard Malonga and Hugo Rainey
Dr. Emma Stokes is a conservation scientist with
the Wildlife Conservation Society. She spent 10 years in Africa working
on great ape research and conservation, and coordinated the monitoring
program in northern Congo. She is currently coordinating tiger monitoring
activities for WCS in Asia.
Richard Malonga is a conservation scientist with the Wildlife
Conservation Society. He is the Director of the TRIDOM - Odzala Project
in northern Republic of Congo.
Dr. Hugo Rainey is a conservation scientist with the Wildlife Conservation
Society. He has studied African birds since 1996, and was the Director
of the Lac Télé Community Reserve Project in northern Congo.
He is currently a technical advisor for WCS in Cambodia.
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