The Philippines is one of those remarkable playgrounds of speciation. Its high rate of both endemicity and biodiversity can be traced partly to its unique biogeographic history. Take for example these 4 birds of the family Pycnonotidae (bulbuls). The Philippine Bulbul (blue) can be found throughout the Philippines except for Palawan. The Yellow-vented Bulbul (white), is found in most of Southeast Asia and the Philippines, again save Palawan. The Sulphur-bellied Bulbul (red) is found only in Palawan, while the Black-headed Bulbul (Green) is found in most of SEA and Palawan only. (For the purposes of this discussion, we are simply showing distribution patterns, without necessarily implying that these 4 bulbuls share a common ancestor and/or biogeographic history).
And this is a pretty common distribution pattern for many of the Philippines’ insanely awesome fauna (but note that not all have been illustrated)!
Figure 1. Present day distribution of 4 species of bulbuls. Points do not depict exact coordinates (Blue: Hypsipetes philippinus; White; Pycnonotus goiavier; Red: Iole palawanensis; Green: Pycnonotus atriceps. Plates from hbw.com; map from ALearningFamily.com)
You see, most of the other Philippine islands are volcanic in origin, meaning they rose from the water millions of years ago. Palawan, on the other hand, begs to differ. It sits on the Sunda Shelf together with many other present-day countries of Southeast Asia (Figure 2, see Palawan on the pale yellow region).
Figure 2. Reconstruction of the formation of the Philippine islands. Estimated million years shown on top right corner (source: Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations (Hall, 2002))
About 20, 000 years ago, an ice age rendered sea levels roughly 120m lower than they are now. This exposed more land above water, allowing animals from mainland SEA and the Philippine islands to spread to Palawan (Figure 3, dark grey areas). One can say Palawan served as a “melting pot” of species from mainland SEA and the rest of the Philippines.
As global temperatures warmed, the glaciers melted, eventually bringing the seas to present-day levels (Figure 3, light grey areas). Land bridges that linked the volcanic Philippine islands, Palawan, and Borneo, were submerged. This trapped the plants and animals within their respective islands, forming geographically-isolated populations.
Figure 3. Partial map of Southeast Asia showing current vs. Late Pleistocene land area. (source: philippines.fieldmuseum.org)
Each population faced a new set of local conditions. Nature selected the best combinations of traits that gave these populations the advantage to thrive in their respective habitats. These traits were then passed on to their offspring, and as time progressed, made the organisms so different from those in other islands (although they had the same ancestor), giving rise to the Philippines’ current biodiversity.
On behalf of all our non-human friends, welcome to Palawan!
2002. Hall, R. Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations. JAES 20 (2002), 353-431.
2010 The Field Museum (http://archive.fieldmuseum.org/philippine_mammals/introduction.asp)
2018 HBW Alive (https://www.hbw.com/species)
What do you think?