As with any flora and fauna, the decline of a population can usually be attributed to anthropogenic factors.  Pollution, habitat destruction and fragmentation, over harvesting (in the form of illegal poaching), agriculture, and the introduction of diseases are some of the usual suspects; the mountain gorilla suffers from all of these. The entire aforementioned are due to the most significant threat to gorilla survival; human population growth.
Poaching: Mountain gorillas are not usually hunted for bush meat, but they are frequently maimed or killed by traps and snares intended for other animals.

They have been killed for their heads, hands, and feet, which are sold to collectors. Infants are sold to zoos, researchers, and people who want them as pets. The abduction of infants generally involves the loss of at least one adult, as members of a group will fight to the death to protect their young. The Virunga gorillas are particularly susceptible to animal trafficking for the illegal pet trade. With young gorilla’s worth from $1000 to $5000 on the black market, poachers seeking infant and juvenile specimens will kill and wound other members of the group in the process. Those of the group that survive often disband. One well documented case was that known as the ‘Taiping 4’.

In this situation, a Malaysian Zoo received four wild-born infant gorillas from Nigeria at a cost of US$1.6 million using falsified export documents.  Poaching for meat is also particularly threatening in regions of political unrest. Most of the African great apes survive in areas of chronic insecurity, where there is a breakdown of law and order. The killing of mountain gorillas at Bikenge in Virunga National Park in January 2007 was a well documented case.
Habitat loss: This is one the most severe threats to gorilla populations. The forests where mountain gorillas live are surrounded by rapidly increasing human settlement. Through shifting (slash-and-burn) agriculture, pastoral expansion and logging, villages in forest zones cause fragmentation and degradation of habitat. The late 1960s saw the Virunga Conservation Area (VCA) of Rwanda’s national park reduced by more than half of its original size to support the cultivation of Pyrethrum. This led to a massive reduction in mountain gorilla population numbers by the mid-1970s.

The resulting deforestation confines the gorillas to isolated deserts. Some groups may raid crops for food, creating further animosity and retaliation. The impact of habitat loss extends beyond the reduction of suitable living space for gorillas. As gorilla groups are increasingly geographically isolated from one another due to human settlements, the genetic diversity of each group is reduced. Some signs of inbreeding are already appearing in younger gorillas, including webbed hands and feet.

Disease: Despite the protection garnered from being located in National parks, the mountain gorilla is also at risk from people of a more well-meaning nature. Groups subjected to regular visits from tourists and locals are at a continued risk of disease cross-transmission (Lilly et al., 2002) – this is in spite of attempts to enforce a rule that humans and gorillas be separated by a distance of 7 meters at all times to prevent this. With a similar genetic makeup to humans and an immune system that has not evolved to cope with human disease, this poses a serious conservation threat. Indeed, according to some researchers, infectious diseases (predominantly respiratory) are responsible for about 20% of sudden deaths in mountain gorilla populations. It is notable that with the implementation of a successful ecotourism program in which human-gorilla interaction was minimized, during the period of 1989-2000 four sub-populations in Rwanda experienced an increase of 76%. By contrast, seven of the commonly visited sub-populations in the Democratic Republic of Congo (DRC) saw a decline of almost 20% over only four years (1996–2000). From this, we can conclude that the negative impacts of ecotourism on gorilla health can be minimized if proper management is undertaken.

The risk of disease transmission is not limited to those of a human origin; pathogens from domestic animals and livestock through contaminated water are also a concern. Studies have found that water borne, gastrointestinal parasites such as Cryptosporidium species, Microsporidia species, and Guardia species are genetically identical when found in livestock, humans, and gorillas; particularly along the border of the Bwindi Impenetrable Forest, Uganda.  Another example of human induced disease is Tuberculosis; Kabagambe et al found that as high as 11% of cattle in Rwanda suffered from this affliction.

War and civil unrest: Rwanda, Uganda and the Democratic Republic of Congo have been politically unstable and beleaguered by war and civil unrest over the last decades. Simulation modeling, Byers et al. (2003) has suggested that times of war and unrest have negative impacts on the habitat and populations of mountain gorillas. For example, events such as 1994’s Rwanda genocide would take place approximately every 30 years, with each event lasting for 10 years. Due to the increase in human encounters, aggressive and passive alike, this would result in a rise in mortality rates and a decrease in reproductive success. More direct impacts from conflict can also be seen. Kanyamibwa notes that there were reports that mines were placed along trails in the Volcanoes National Park, and that many gorillas were killed as a result. Pressure from habitat destruction in the form of logging also increased as refugees fled the cities and cut down trees for wood. During the Rwandan genocide, some poaching activity was also linked to the general breakdown of law and order and lack of any ramifications.