The major threats to Western Lowland Gorillas are:
- 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).
- 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). Ebolavirus 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, Ebolavirus 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 Ebolavirus 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. Ebolavirus thus remains a highly significant threat in this region.
- 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.
- 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.