The relationship between the abundance of the Nigeria-Cameroon chimpanzee (Pan troglodytes ellioti) and its habitat: a conservation concern in Mbam-Djerem National Park, Cameroon by Kamgang, SA, Bobo, KS, Maisels, F, Ambahe, RDD, Ambassa Ongono, DE, Gonder, MK, Johnson, P, Marino, J, Sinsin, B