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In Deep Guano

Christopher Wurster digging a guano pile in Gomantong Caves, Sabah, Malaysia.

Christopher Wurster digging a guano pile in Gomantong Caves, Sabah, Malaysia.

By Christopher Wurster

Deep deposits of guano are revealing why South-East Asia is a biodiversity hotspot.

We arrived at mid-day after following an obscure dirt track somewhere in the middle of Sonora, Mexico. The plan was to find the cave with the help of some coordinates I had grabbed from a scientific paper.

After arriving at the spot using the latest in technology at the time, a bulky hand-held GPS, the three of us looked around. We were standing on flat, low ground containing desert scrub, and could see around us fairly well. The nearest hill rose just a little above the horizon.

“Well, the coordinates are wrong,” Matt smiled as he spoke.

“Let’s get looking,” I said.

We moved to the nearest hill, split up and clambered about. I scrambled up the top and almost immediately caught a strange odour. “There is a strange smell about!” I yelled with some excitement. Following my nose, I came to the cave and almost immediately spied my companions following a well-made dirt road leading directly to the mouth of “Cueva del Tigre”.

I chose this cave because it is a roost for millions of Mexican free-tailed bats during the summer. Collectively, the bats drop faecal pellets (guano) and urinate on the floor of the cave in huge quantities. This was the source of the strong odour. If the cave served continuously as a roost and erosion was limited, there ought to be a record of past environments stored in the guano sediments.

The plan was simple. Find the cave, wait until the bats exited during the night, enter it and then bang in a core tube in the thickest deposit of guano we could find. We had found the cave so we waited, using the time to prepare our equipment.

Millions of bats roosting in tight conditions can produce lethal levels of ammonia for humans, and bacterial activity in the large guano deposits can result in dangerously low quantities of oxygen and high amounts of carbon dioxide. It has been reported that the ammonia levels in some Mexican free-tailed bat colonies reach levels sufficient to bleach the hair of inhabiting bats!

Then there are the flesh-eating beetles, which feast on the occasional bat that slips off the roost and falls on the floor. The hungrier beetles can be so impatient that they once climbed a cave wall and took an unfortunate youth who wasn’t resting high enough.

Taking the guano is easy enough. We bring in some PVC tubes and a mallet to bang them in.

However, just entering the cave demands the most preparation. We need full-face shield gas masks with ammonia filters, and white contamination suits to keep hungry beetles at bay as well as any histoplasma spores, which can infect human lungs.

Once all of our equipment was ready, we waited for the bats to exit the cave.

The sun began to dip low in the horizon – and then it happened. The bats began exiting in a trickle, with a few daring bats leading the charge through the cave’s mouth. More and more began to follow, finally reaching a climax as thousands of bats streamed out like a river, moving and swaying as one snake-like body in the dusky horizon heading south before breaking up to find the night-flying moths and beetles that will fill their bellies.

It was incredible to watch as a million or more bats left the tiny cave for their nightly feeding ritual. A full 10 minutes went by before the final trickle exited, and it was time to work.

We entered the hot and stuffy cave fully garbed in our protective gear. Now it was time to find the guano piles!

We entered the cave and walked about, and finally realised why there were nylon bags strewn about and a nice road leading directly to the cave’s mouth: there were no deposits anymore.

Large-scale guano deposits have more value than what they can reveal about past climate and ecosystem changes. It is an economically valuable crop fertiliser.

“Back to the drawing board”, I thought.

Guano as a Depositional Record
Since that first experience, Michael Bird and I have been more successful at finding locations where thick deposits of guano lay undisturbed. The thickest so far is in Niah Great Cave in Borneo, the site where the oldest modern human remains in South-East Asia had been discovered. Here, up to 7 metres of stratified guano have been accumulating over 100,000 years.

Why are we so interested in finding these locations? Any accumulation of material over time has the potential to archive conditions of the past through various “proxies”. A proxy is anything that can be used to infer something else – in this case past environments. For example, by recovering and measuring the relative abundance of pollen in old lake sediment, scientists can discover which plants surrounded the lake in the past.

So guano is just another type of accumulated sediment containing proxies. If left undisturbed it follows that the deeper the guano, the older the guano.

By measuring proxies in discrete layers we can understand how environmental conditions changed in the past, and place those changes in a timeframe through carbon dating. As guano sediment contains many types of proxies, we can gain an understanding of the natural variability in the environment and climate over time.

Guano is simply the faecal droppings from bats or birds. In insect-eating bats and birds, guano is dominated by the exoskeletons of insects.

On the cave floor, the organic guano pellets interact with dripping water and weathered rock to form unique minerals that are found only in such locations. Fungi, bacteria and entire cave communities live off the energy packaged in the guano, thus breaking it down.

Old guano is often a clay-like phosphatic rock that can be hard after drying. Much of the organic material breaks down over a short time, but not everything, and we can still process the guano sediment to recover tiny bits of insect exoskeletons. Unfortunately these bits are no longer recognisable as species or classes of insects, but the chemical signature is still intact and is a proxy for the type of vegetation and climate in which the insects were living at the time they were eaten by the hungry bats or birds.

We can also recover bits of charcoal as a proxy for fires, as well as pollen and other chemical indicators of climate.

One advantage of cave guano over other types of sediment traps can be the location where such deposits are found. Large populations of bats live in semi-arid and tropical locations. Many more “traditional” continental records are based in lake sediments. However, there are few lakes in semi-arid regions, and lakes are prone to disappearing entirely during drier times! No lake, no record.

In tropical locations, suitable lakes can be surprisingly difficult to find and accurately date. Often they are missing sediment, or are only preserved in ever-wet swamps that will stay wet even if conditions nearby are dry. Offshore continental records are located near river outlets, and often contain vegetation proxies that are specific to riverine communities rather than the community average.

This is not to say that guano is a perfect lens through which to view the past, but it is only through gathering and interpreting many different types of records that we can piece together a good understanding of past environmental and climate history.

Forest or Savanna in Equatorial South-East Asia?
South-East Asia is characterised by many small and some very large islands separated by shallow continental seas or deep oceanic trenches. Because of this, modern Indonesia is often referred to as the “maritime continent”. It was covered by lush unbroken rainforest until recently.

However, past sea level rises and shoalings have led to drastically different sizes and connections between these islands and mainland Asia. Each time a new land bridge was established, plants and animals could cross into new territories. Each time a connection was sundered, plants and animals were isolated, and evolution acted unhindered by new invasions and migrations. This is part of the reason why “Sundaland” is a biodiversity hotspot containing 25% of all species on only 9% of the available global landmass.

However, considerably more diversity exists in this region than would be expected given that some of the larger landmasses, such as Borneo and Sumatra, were connected for most of the past two million years. Why are there endemics unique to each region?

For example, why is there a separate Sumatran and Bornean orang-utan instead of one species? It could be that these and other forest specialists could not migrate and mix at times when Borneo and Sumatra were physically connected because the connecting land bridge was too dry to support forest vegetation.

In fact, two opposing conceptual models are debated. One hypothesis is that, during the last ice age, Sundaland contained a massive and unbroken tropical rainforest surrounding the Equator. Several computer simulations that reconstruct the vegetation during the last ice age and a few studies of offshore sediments have supported this idea.

A second hypothesis suggests that the landmass had a dry interior and that tropical rainforests were diminished and separated from one another into a few key refugia in Borneo and Sumatra. Biogeographic and genetic studies, as well as some undated geomorphic evidence of dry environments in the core of Sundaland, support this idea.

The problem has been that reliable and well-dated sediment records on the land surface have been scarce. Luckily, after much searching we have found some suitable guano deposits in several key parts of Sundaland that have helped clarify which of the competing hypotheses is more likely.

We have sampled guano deposits dating back over 40,000 years that included the maximum extent of the last ice age. Sites were located in Batu Caves near the capital of Malaysia, Niah Cave in northern Borneo, and a couple of caves in Palawan, The Philippines. We have extracted the insect cuticles and measured the carbon isotope ratios from discrete intervals in each deposit. The results are incredible.

There are two major pathways that plants use to fix inorganic carbon to organic carbon. One is called the C4 photosynthetic pathway, while the other is the more primitive C3 photosynthetic pathway. In the tropics, almost all grasses use the C4 pathway, while everything else (shrubs and trees) uses the C3 pathway.

It turns out that the carbon isotope ratios of these photosynthetic mechanisms are very different, so by measuring soil carbon, for instance, the relative abundance of tropical grasses in that area can be determined. It also turns out that insects feeding on each plant type incorporate the same isotopic ratio as its diet, and it is these insect remains in the guano deposits that we have recovered and measured.

The carbon isotope ratios from each site indicated that the dense tropical rainforest that now covers all the sites has existed for about 10,000 years, but before this time the records at each site diverged substantially. Carbon isotope ratios from Niah Cave indicated that tropical rainforest remained intact for at least 40,000 years. However, to the east of Niah, a switch of vegetation occurred and savanna containing a rich grass understory dominated during the maximum stage of the last ice age in Palawan. To the west of Niah, in Peninsular Malaysia, high carbon isotope ratios during the early part of the record indicated that grasses were in ample supply from at least 35,000 years ago.

These results allow us to conclude that a tropical rainforest refugium existed in northern Borneo near Niah, but much of Sundaland must have been drier, with the modern rainforests replaced by much more open savanna vegetation during the last ice age.

Digging guano has therefore helped to explain the distribution of modern biodiversity in South–East Asia. The results also suggest the tantalising possibility that ancient humans, adapted to the savannas of Africa, could readily traverse Sundaland when they arrived and then move rapidly to the savannas of northern Australia.

We still need to find many more deep guano deposits from sites distributed throughout South-East Asia to determine in detail the distribution of ice age forests and savannas on the drowned continent of Sundaland.

Christopher Wurster is a Senior Research Associate at James Cook University.