Christmas Island and the Cocos Keeling islands are two very remote small islands in the Indian Ocean under the jurisdiction of the Australian Government. The nearest large land mass to Christmas Island is the island of Java which is approximately 350 km north. The nearest landmass to the Cocos Keeling islands is Sumatra c. 900 km north.
GOOGLE MAP Christmas Island and Cocos Keeling Islands
Christmas Island hosts two species of figs Ficus microcarpa and Ficus saxophila, both of which are abundant on the island and 10 species of endemic mammals and birds including an endemic Flying Fox fruit bat and an Imperial Pigeon. Cocos Keeling has no figs and no endemic birds or mammals. What can these two isolated islands tell us about fig dispersal and ecology ?
In summary there are a number of land bird and bat dispersed plants native to Christmas Island including Pipturis argenteus and Cinnamomum iners as well as the 2 figs which could only have arrived as seeds in the gut of a bird or bat. However there are no land-bird or bat dispersed plants on the Cocos Keeling Islands.
All the native plants of the Cocos Keeling islands are either sea or sea bird dispersed. The most common native tree on the Cocos Keeling Islands is Pisonia grandis which is dispersed by seabirds particularly Frigatebirds.
CONCLUSION: Most of the small islands of SE Asia host both Imperial Pigeons and Flying Fox fruit bats of many different species. These animals are capable of dispersing figs very long distances between islands. In the case of Christmas Island and Cocos Keeling islands at least 350 km but less than 900 km.
The question then arises – if the sea gap between Borneo and Sulawesi is less than 150 km why are not more Bornean figs found in Sulawesi and more figs from Sulawesi not found in Borneo ?
It is commonly assumed that because figs originated in NE India that fig migration was exclusively West to East from Borneo to Sulawesi across Wallace’s Line and onto the Pacific Islands and Australia. In fact there are quite a number of figs which having crossed from Borneo to Sulawesi evolved into different species east of Borneo. These figs then crossed Wallace’s Line East to West back into Borneo almost certainly in the gut of a flying fox or imperial pigeon. These figs include;
For a possible explanation of of why Borneo and Sulawesi do not share more fig species and why Wallace’s Line has proved to be more effective in restricting fig migration East to West and not West to East see Figs and Islands 02: Maratua, Sulawesi and the Philippines