Geographical barriers
The forest contraction that occurred during the Pleistocene radically changed the landscape. Additional landscape changes occurred in the region at the end of the last glacial period (ca. 10,000 years ago) when a rise in sea level isolated Bioko from the African mainland (Jones 1994). A shallow channel separates Bioko from the Cameroon coast by 32 km (Schabetsberger et al. 2004). We do not have evidence of recent dispersal and gene flow between North Bioko and mainland Cameroon.
The central depression with lowland forest (0-500 masl) between North and South Bioko (Schabetsberger et al. 2004) exemplifies another geographic barrier that probably maintains reproductive isolation between the populations of L. columnaris in Bioko Island. The central depression consists of lowland forest vegetation.
The expansion, colonization, and recolonization of L. columnariswas possible by wind dispersal. Wind played an essential role in dispersing tiny seeds altitudinally and latitudinally across sky islands. In Cameroon, high ridges act as a natural forest corridor connecting sky islands, and facilitating dispersal and gene flow among contemporaneous populations (Smith et al. 2000).