Pulau Maratua is the largest island of a group of  c. 20 small isolated islands known as the Derawan islands, c. 51  km off the cost of East Borneo  at the junction of  3 separate floral regions  (1) Borneo/Sundaland,  (2) Sulawesi to New Guinea -Wallacea and  (3) The Philippines.

Each region has  distinctively different plants and animal life. Borneo is by far the closest large island to Maratua,  yet 5 out of the 9 species of figs present on Maratua and the Derawan islands must have originated from Sulawesi or the Philippines

  Fig species Distance from Maratua   Origin of Maratua figs
Maratua 9 0 km all
Borneo 141 51 km 4
Sulawesi 81 270 km 1 + 4 ?
Philippines 87 284 km 1+ 4 ?
Maratua and Wallaces Line.jpg
According to  traditional placement of  Wallace’s Line and Huxley’s Line Pulau Maratua is part of the Borneo/Sundaland bio-geographic region.

Maratua to Borneo 51 km .png

Pulau Maratua is an atoll, the remnant of an extinct volcano and has never been joined by a land bridge to Borneo. The shortest  sea crossing  from Borneo to Maratua is approx. 51 km. One would expect that the figs of Maratua  would all be Bornean in origin but this is not the case.

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Pulau Maratua above and the much smaller Pulau Kakaban (which has an internal lake) below.

According to Berg’s (2005) Flora Malesiana monograph on figs, Sulawesi hosts 81 species of native figs and Borneo hosts  141 native figs.

The relatively small island of Maratua (c. 2,000 ha)   hosts at least 9 species of figs  of which one  Ficus glandifera  can only have come from Sulawesi.

Another 4  Maratua figs obviously originated from Borneo including, Ficus annulata, Ficus microcarpa, Ficus septica and Ficus caulocarpa. 

Another 3 species are small island specialists which probably arrived from either the Philippines or Sulawesi Ficus tinctoria tinctoria,  Ficus nervosa and Ficus prasinicarpa.

Finally there is a curious species which may be wrongly identified but  which is very unlikely to have arrived from Borneo.  

IMG_1802.jpg
The Island Flying Fox  Pteropus  hypomelanus is common on Pulau Maratua and the islands in the Maratua lagoon. This bat appears to feed mainly on coconut flower nectar and figs. Flying foxes are by far the most important disperser of figs between the  small islands surrounding Borneo including the islands east of Borneo which stretch all the way east to New Guinea and the   Pacific Islands.

 

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Two  species of Imperial Pigeons are common on Maratua. The Grey Imperial Pigeon above and the Pied Imperial pigeon below.  Both pigeons  are small island specialists dispersing fruit seeds from one island to another.

One would expect that  as Maratua is relatively much closer to Borneo  (51 km) compared to Sulawesi (270 km)  and the Philippines (284 km)  that the Maratua figs would all have originated from Borneo but in fact over half appear to have arrived from either Sulawesi or the Philippines.

CONCLUSION

One  possible explanation is that the most important dispersers of island figs,  imperial pigeons and flying foxes are nomadic  island specialists  and are more likely to fly between islands than from the islands to the mainland.  Thus very long distance dispersal is  mostly from island to island not from the mainland to islands.

An alternative (more likely) explanation is that many additional species of Bornean forest figs have arrived on Maratua from Borneo over  the last many thousands of years transported by bats and pigeons  but they have been unable to to survive the harsh island conditions including periodic droughts. Only figs and fig wasps which are island specialists are tough enough to survive.

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