The Sumatra, Borneo, and Java islands have once been part of a much larger Earth-related mass land called Asia, called Sunland.
But there are some species that are unique to every island today – like the two orangutan species – so in the research published today in the scientific reports, we looked at what could stop them.
And this includes looking at old samples.
Exposed to land
Sunandland was the largest at the lowest sea level when it was larger than the whole of modern Europe.
More recently, it was about 20,000 years ago at the peak of the last Ice Age. Ice Age (Ice Age) periods are much longer than intermediate (warm-weather).
This means that Sundaland has been exposed above sea level for about 90% of the time over the last few million years, and it seems today it makes about 10% of that time.
But what about the ancient landscapes of this vast – now largely underwater continent?
Drop what you eat
To understand this, we looked at the thick bats of birds and birds in caves in the region.
Bats for insect feeding and birds live in caves. Every night millions leave their eating places, eat insects from the landscapes around the cave.
Upon their return to sleep, bats and birds "do their business", standing on the floor of the cave. Piles of excrement are predominantly insect skeletons. So, bats act as mini-scientists, taking "samples" of the insects that were around the cave during each meal.
As time goes on, in bags of thickness meters accumulate bags, which contain many thousands of millennium insects.
Although we can not identify insects as they are too destroyed, we can look at chemical fingerprints to understand what plants the insects fed. This is because insects that feed on tropical grasses leave a very different chemical footprint for those who eat trees.
So these habitats tell us what kind of vegetation is around the cave and how it has changed over time. This is a fortune for us because many other types of records from past environments just do not exist in the region or are now under the sea.
Because there are not many other sources of information, there is no consensus about what the landscapes were all over the Sunland in the past.
Some argue, and many models support this idea that tropical forests have always covered the entire region, similar to what exists today on the islands.
But there is another idea: a savanna to cross Sundaland from north to south. This is accompanied by an eastern and western wetland tropical forest that serves as a shelter for rain forests and plants during glacial eras.
The entire Indonesian region is a hot spot for biodiversity with many species that occur only on certain islands and nowhere else. Why? Consider the two types of orangutan, one of which only occurs on Sumatra, and the other – only Borneo. Why are there two Sunda subtypes, blurred leopards, each unique for Borneo and Sumatra? What about the little Indian peppermint found in Continental Asia and Java, but mostly from Borneo and Sumatra?
This is curious, given that most of the time they were not really islands. So how do these species develop individually if, for the most part, they had to move freely from Borneo to Sumatra through the tropical forests?
The answer to this question is important for the conservation of many species in the region.
We need more caves
We have looked around Malaysia and Indonesia for deposits caves that can answer this question. So what does the cave say?
In our latest published paper we present the results of a 3-meter pile of ancient excrement, covering nearly 40,000 years.
Saleh Cave is located in the southeastern corner of Borneo and at the southern equatorial end of the Savannah Corridor, if it exists. Today the lush tropical forests cover the region.
The chemical footprint in the cave is clear. Tropical grasses are the dominant part of the landscape during the glacial era until recently – geological.
Putting this in the context of our earlier work in Malaysia, we come to the conclusion that there may be a corridor of savannah north of the equator. In other words, the tropical rainforest retreated to the summits of Sumatra and Borneo, and did not cover Sundaland during the Ice Age.
Other ocean records also show that tropical grasses have expanded, but these data are to the south and east rather than the heart of the savannah corridor.
The savannah corridor acted as a barrier for tropical forest professionals who wanted to move through Sundaland. On the other hand, the corridor of the savanna serves as a bridge for species adapted to the open non-wood environments north and south of the equator.
This clearly explains many of the strange patterns of animal, insect and bird distribution that we see in a region of great importance as a hot spot for biodiversity.
This could also partly explain how people managed to move to the region so quickly in Sahul (Australia and New Guinea) – the ice age in Sundaland – more than 50,000 years ago.