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Gorilla gorilla ssp. gorilla 

Scope: Global
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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Mammalia Primates Hominidae

Scientific Name: Gorilla gorilla ssp. gorilla
Species Authority: (Savage, 1847)
Parent Species:
Common Name(s):
English Western Lowland Gorilla
Taxonomic Source(s): Mittermeier, R.A., Rylands, A.B. and Wilson D.E. 2013. Handbook of the Mammals of the World: Volume 3 Primates. Lynx Edicions, Barcelona.
Taxonomic Notes:

The Western Lowland Gorilla (Gorilla gorilla gorilla) is one of two recognised subspecies of Western Gorilla (Gorilla gorilla) (Groves 2001). Genetic data suggest that Cross River Gorillas (Gorilla gorilla diehli) diverged from Western Gorillas approximately 18,000 years ago (Thalmann et al. 2011). Western Lowland Gorillas are genetically separated into subpopulations by major rivers (Anthony et al. 2007, Fünfstück et al. 2014).

Assessment Information [top]

Red List Category & Criteria: Critically Endangered A4bcde ver 3.1
Year Published: 2016
Date Assessed: 2016-01-29
Assessor(s): Maisels, F., Strindberg, S., Breuer, T., Greer, D., Jeffery, K. & Stokes, E.
Reviewer(s): Williamson, E.A. & Mittermeier, R.A.
Contributor(s): Baillie, J., Butynski, T.M., Gatti, S., Tutin, C. & Walsh, P.D.
Justification:
Western Lowland Gorillas have a large geographic range, covering almost 700,000 km². Between 1983 and 2000, the country of Gabon lost half its Gorilla population (Walsh et al. 2003). A recent evaluation estimated population declines using a predictive model that incorporated survey data collected across the range of the taxon between 2003 and 2013. The results show an 18.75% decline between 2005 and 2013, corresponding to an annual loss of ~2.56% (Strindberg et al. in prep). These population decreases were driven by a combination of poaching and disease. The current size of the population is being evaluated, but thought to be in the order of a few hundred thousand.

Despite their abundance and wide geographic range, in 2008 Western Lowland Gorillas were listed as Critically Endangered under criterion A: a population reduction of more than 80% over three generations (one generation is ~22 years). This listing was based on population losses due to poaching, disease and habitat loss. Illegal hunting has not ceased despite intense anti-poaching efforts and the threat of Ebolavirus has not been removed: poaching is intensifying with the expansion of access routes into forests and Zaire Ebola virus is still present in the forests of the region. In addition, habitat loss due to industrial agriculture is increasing, and the effects of climate change will become more evident. At a conservative rate of reduction (2.56% per year rather than 4%, calculated from Walsh et al. 2003), the reduction in the Western Lowland Gorilla population is predicted to exceed 80% over three generations (i.e., 66 years, 2005–2071). Gorilla gorilla gorilla thus qualifies as Critically Endangered.
For further information about this species, see 9406_Gorilla_gorilla_gorilla.pdf.
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Previously published Red List assessments:

Geographic Range [top]

Range Description:

The total range of Gorilla gorilla gorilla covers close to 700,000 km². The northwestern limit of their distribution is the Sanaga River in Cameroon; the northern limit is the forest-savannah boundary to a maximum of roughly 5°N; the eastern limit is the Ubangi River; and the Congo River south of its confluence with the Ubangi forms the southeastern and southern limits. Most Western Lowland Gorillas are found below 500 m asl, but some reach elevations of 600–800 m asl in the mountains of Gabon.

Countries occurrence:
Native:
Angola (Angola, Cabinda); Cameroon; Central African Republic; Congo; Equatorial Guinea (Equatorial Guinea (mainland)); Gabon
Additional data:
Continuing decline in number of locations:No
Extreme fluctuations in the number of locations:No
Upper elevation limit (metres):800
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:

Western Lowland Gorillas are found in almost all of the protected areas in their geographic range and many of the logging concessions; the majority (78%) live outside the protected area network (Strindberg et al. in prep). They can reach high densities (3–5 individuals/km²) over large areas of both swamp and terra firma forests (Blake et al. 1995, Rainey et al. 2010) and can persist at high densities in well-managed logging concessions (Morgan et al. 2013). Gorilla population estimates are made using a standard index of abundance: night nest abundance and distribution, sometimes combined with predictive modelling. Extensive surveys carried out since the mid-2000s have suggested that 150,000–250,000 Gorillas occur in the areas surveyed (Williamson et al. 2013, Sop et al. 2015), but the total population size is currently being re-evaluated. Based on surveys carried out between 2003 and 2013, most Western Lowland Gorillas are now known to reside in the Republic of Congo (60%), followed by Gabon (around 27%) and southwestern Cameroon (10%) (Strindberg et al. in prep).

Current Population Trend:Decreasing
Additional data:
Continuing decline of mature individuals:Yes
Extreme fluctuations:NoPopulation severely fragmented:No

Habitat and Ecology [top]

Habitat and Ecology:Western Lowland Gorillas inhabit both swamp and terra firma lowland forests throughout Western Equatorial Africa, occupying a variety of forest types, including open- and closed-canopy moist mature, seasonally-inundated, and disturbed and secondary (regenerating) forest (Williamson and Butynski 2013). They occur at high density in vast swamps in northern Republic of the Congo. They eat fruit, seeds, leaves, stems, bark, shoots, roots, petioles, bracts, vine tendrils, invertebrates and earth (ibid.) Staple and fallback foods are pith and shoots of the families Marantaceae and Zingiberaceae, whereas fruit consumption varies greatly between seasons according to availability (Rogers et al. 2004). Some populations feed on Gilbertiodendron dewevrei seeds during mast fruiting (Blake and Fay 1997). Gorilla g. gorilla is sympatric with Pan troglodytes troglodytes throughout its range, and the two species show a high degree of dietary overlap. Annual home range may be as large as 25 km², but is more usually 10–15 km² and the ranges of neighbouring groups’ overlap extensively (Cipolletta 2004). Western Lowland Gorillas live in relatively stable groups composed of 10 individuals, on average, with one “silverback” adult male, several adult females and their offspring (Parnell et al. 2002).

Western Gorilla reproductive rates are lower than those of Eastern Gorillas due to a later age of parturition and longer interbirth intervals between surviving offspring (Breuer et al. 2009). Life history data are available from a few habituated Western Lowland Gorilla groups and 20 years of observations in the Republic of Congo, and demonstrate a slower life history than Mountain Gorillas (Breuer et al. 2009). Males reach full maturity at 18 years and age at first reproduction of females is around 11 years (ibid.). Inter-birth interval averages 5.2 years for surviving births with offspring being weaned at four years of age (Stoinski et al. 2013). Generation time is estimated to be 22 years (see Appendix I).
Systems:Terrestrial
Continuing decline in area, extent and/or quality of habitat:Yes
Generation Length (years):22
Movement patterns:Not a Migrant

Use and Trade [top]

Use and Trade:

Gorillas are completely protected by national and international laws in all countries of their range, and it is, therefore, illegal to kill, capture or trade in live Gorillas or their body parts.

Threats [top]

Major Threat(s): The major threats to Western Lowland Gorillas are:

  1. Poaching - Illegal hunting for bushmeat is a serious problem across the whole of Central Africa and the primary cause of Western Lowland Gorilla decline. Until the mid-1990s, much of their range included vast, roadless blocks of forest that were extremely difficult to access, where human population densities were very low, and Gorilla densities were high. In the last quarter century, however, almost all of the terra firma forest in this region outside the protected area network has been attributed as industrial logging concessions (Global Forest Watch 2016). This means that most of the once-remote, previously inaccessible forests are now covered by a network of logging roads (Laporte et al. 2007), which gives easy access to both hunters entering the forest and to traffickers taking consignments of bushmeat (and ivory) out of the forest to distant destinations – often towns and cities where meat fetches high prices. Throughout Central Africa, mining permits for prospecting or resource extraction are being issued over an increasingly large surface area. Apart from direct removal of Gorilla habitat (depending on the type of extraction), mining leads to high rates of human immigration and the creation of yet more access roads, which are then used for poaching (Edwards et al. 2014, White and Fa 2014). The region’s governance is typically poor (Transparency International 2016), and the weak law enforcement and corruption that enable poaching are evident in ivory trafficking (Bennett 2014), but also widespread elsewhere (Bour et al. 2013). In addition, huge road projects are currently underway (Laurance et al. 2015), which will substantially fragment Gorilla habitat. The ease and speed of transport are now orders of magnitude higher than before the existence of such roads, and hunting pressures are much higher. Across their range, Gorilla densities are lowest near roads, except where forest guards operate control points or roadblocks (Strindberg et al. in prep). In addition, human populations in these once remote areas have increased, as people migrate to the new employment opportunities offered by logging operations (Wilkie et al. 2000, Poulsen et al. 2009). This phenomenon is not confined to Central Africa, but is global (Laurance et al. 2014).
  2. Disease - The second major driver of rapid Western Lowland Gorilla decline is infectious disease, specifically the Ebola virus disease (EVD). Surveys show that there have been a series of die-offs of tens of thousands of Gorillas and chimpanzees since the early 1990s in a large, mostly intact forested area straddling northeastern Gabon and northwestern Congo, comprising about 14% of their total range (Walsh et al. 2003, Maisels et al. 2004, 2013). Ebola virus outbreaks started in 1994 in the northeast of Gabon, moving through Minkébé and Mwagne national parks and their surroundings, and western Republic of Congo (including parts of the Odzala-Kokoua National Park and the Lossi Gorilla Sanctuary), continuing until the mid-2000s (Walsh et al. 2003, 2005; Maisels et al. 2013). Three quarters of the Gorilla population in six protected areas with low hunting pressure died between 1995 and 2000, presumably from EVD (Walsh et al. 2008), with mortality rates reaching 95% during the worst disease outbreaks (Bermejo et al. 2006, Caillaud et al. 2006). Populations in the protected areas hit by disease began to recover within a decade (e.g., Genton et al. 2012); however, total recovery would take 75–131 years in the (unlikely) absence of poaching (Ryan and Walsh 2011). Although rivers and forest fragmentation seem to present a partial barrier to its spread, Ebola virus can cross rivers and has already been detected to the east of the Mambili River, a major river barrier between Odzala-Kokoua National Park and the Sangha River (Reed et al. 2014), so a future outbreak in the largest known Gorilla population, in the Ngombe logging concession, is a possibility. The Ngombe concession and Ntokou-Pikounda National Park together contain about 80,000 Gorillas, a population which has remained stable since 2007 (Maisels et al. 2015). In Southeast Cameroon across the Ngoko River in the north, around 40,000 Gorillas are found in logging concessions and a series of national parks (Blake et al. 2012). The Ndoki-Likouala landscape, on the eastern side of the Sangha River, harbours another 50,000 or so Gorillas (Maisels et al. 2012). Since large areas can be affected by a single Ebola virus outbreak and transmission between individuals is rapid (Walsh et al. 2007, 2009), large numbers of Gorillas could be wiped out in a short space of time. Ebola virus thus remains a highly significant threat in this region.
  3. Habitat degradation and destruction - Habitat loss is a major emerging threat to Western Lowland Gorillas. In the past, habitat loss (as opposed to degradation) was not an issue in this region. However, as oil-palm plantations are reaching capacity in Asia, Africa has become the new frontier for this crop that offers excellent economic prospects in countries with appropriate rainfall, soil and temperatures (Rival and Lavang 2014). Unfortunately, these areas coincide with good Gorilla habitat: 73.8% of the Western Lowland Gorilla's range is considered suitable for oil palm (Wich et al. 2014). Similarly, the recent expansion of industrial-scale mining and the creation of open-pit mines are of great concern (Edwards et al. 2014, Lanjouw 2014). Extractive industry also leads to the establishment of development corridors, which can be several kilometres wide, and add to areas of “lost forest” (Laurance et al. 2015). There is a disconnect between the various bodies responsible for land-use planning in the realms of conservation, mining and agriculture in all Western Lowland Gorilla range states except Gabon. Consequently, there will be increasing competition for land between long-term conservation needs and immediate financial gain as governments explore the potential of clearing natural habitat in favour of economic development. Without very careful and immediate land-use planning that involves cooperation between the government bodies responsible for protected areas and wildlife on one hand, and economic agricultural development on the other, large areas of Western Lowland Gorilla habitat could be cleared within a few decades.
  4. Climate change - Climate change is already thought to be affecting the Central African tropical moist forests (Lewis et al. 2013). Although the likely impacts of global climate change in Western Equatorial Africa are not yet known, there are predictions of drying of this region with potentially-negative consequences for forest ecology, such as changes in forest productivity and fruit and flower phenology, increased vulnerability to fire, and even forest retreat (James et al. 2013). Seasonal changes in precipitation and temperature, and weather extremes are likely already ongoing and set to continue during the next few decades (Lovett 2015). Negative consequences for great apes in this region have already been predicted (Lehmann et al. 2010), particularly along the coast (Korstjens et al. 2010). Climate change is the least likely factor for which effective action for great apes, and for African tropical forests in general, can be taken in a timely manner. Although climate talks in 2015 resulted in international cooperative agreement between most of the world’s nations regarding the need for action, the task of reversing, or even flattening current temperature trends will be extremely challenging. Nonetheless, there is potential for mitigating against the impacts of habitat degradation and conversion, which would otherwise exacerbate the effects of climate change on both the short- and long-term prospects for Gorilla survival.

Conservation Actions [top]

Conservation Actions: Gorilla gorilla is listed under Class A in the African Convention on the Conservation of Nature and Natural Resources and is on CITES Appendix I. All Gorillas are protected by national and international laws throughout their geographic range, but law enforcement is generally weak. Only 22% of Western Lowland Gorillas live in the protected areas that cover 14% of their geographic range, with a further 21% of of the population in FSC-certified logging concessions, which add up to 8% of their range (Strindberg et al. in prep). Although forest guards work in most protected areas and in the well-managed logging concessions, 58% of Western Lowland Gorillas and 78% of their range are unprotected and highly vulnerable to poachers.

Two targeted conservation action plans for the great apes of Western Equatorial Africa have been produced (Tutin et al. 2005, IUCN 2014). The areas where most of the world's Western Lowland Gorillas occur in geographically distinct blocks have been identified, and a series of actions outlined for each, which can be broadly encapsulated as:
  1. An increase in effective law enforcement throughout the region, not only in protected areas, but also in logging concessions and unprotected swamp forests. Law enforcement needs to be supported by updated regulations and sanctions.
  2. Effective, coordinated land-use planning to avoid the clearing of large areas of intact Gorilla habitat to establish agricultural plantations, for oil palm in particular. Industrial extraction of natural resources, namely timber and minerals, should be included in this holistic, spatially-explicit approach. Such planning needs to be done at both national and regional levels. Several of the most important areas for Gorilla conservation are transboundary, and thus fall within the remit of national agencies of two or three countries.
  3. Awareness-raising among all sectors that deal with land and the protection of natural resources: law enforcement and judiciary; protected area authorities; mining, logging, and agricultural industries; local communities and tour operators. This outreach should include the prevention of human disease transmission to great apes. IUCN best practice recommendations for logging companies regarding management that is compatible with great ape conservation (Morgan and Sanz 2007, Morgan et al. 2013) can be downloaded here: http://www.primate-sg.org/best_practice_logging.
  4. Further research into ways of mitigating the spread and virulence of Ebola virus, such as addressing the possibilities of administering vaccines that are non-detrimental to the target species (great apes) or other species that may come into contact with the vaccine, and that protect a sufficiently large and geographically appropriate proportion of the great ape population to form barriers against its spread. IUCN guidelines on great apes and disease (Gilardi et al. 2015) are available here: http://www.primate-sg.org/best_practice_disease.
  5. Long-term standardised monitoring of law enforcement efforts and effectiveness, of Gorilla abundance throughout their range, and of great ape health, particularly with respect to Ebola virus. A standardised tool for law enforcement monitoring (SMART: http://www.smartconservationsoftware.org) is in use across much of the range; standard methods for surveying and monitoring great ape populations that facilitate more accurate and precise monitoring of changes in abundance have been available for almost a decade (Kühl et al. 2008 http://www.primate-sg.org/best_practice_surveys); and non-invasive diagnosis of a range of pathogens is now possible, for example, detection of Ebola virus in faeces (Reed et al. 2014).
Maintaining large, intact and well-protected areas of forest will be key to maintaining great ape populations in the long term, and this can only be done by a combination of the actions detailed in the 2015–2025 IUCN Action Plan. In both action plans, threats and conservation actions needed are outlined in greater detail than is possible here. These documents can be downloaded at: http://www.primate-sg.org/action_plans.

Citation: Maisels, F., Strindberg, S., Breuer, T., Greer, D., Jeffery, K. & Stokes, E. 2016. Gorilla gorilla ssp. gorilla. The IUCN Red List of Threatened Species 2016: e.T9406A17989591. . Downloaded on 07 December 2016.
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