The forest region where the drill and the ellioti chimpanzee subspecies live has never been well defined.  It is considered part of the Gulf of Guinea forests, which stretch from Sierra Leone in West Africa to the Congo Basin in Central Africa.  In relation to the rest of Africa, the Gulf of Guinea forests are a biodiversity 'hotspot', meaning that they are high in species richness (biodiversity) as well as high in endemic species.

In relation to the rest of Africa, the forests of the Nigeria-Cameroon border area have high endemism and high biodiversity. Where did this species richness come from?

A growing human population makes demands on its environment.  Most people of Nigeria and Cameroon struggle with some of the things we, in the more developed world, take as basic amenities in our lives. For example, you have probably never considered your refrigerator a luxury item, but most families in Africa have never had one!

The Nigeria-Cameroon forest region is best defined by its rivers. For wildlife, rivers can be a barrier to movement.