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).