How many elephants in Uda Walawe?

This is a question that has been frequently put to us. Typically, Asian elephant numbers have to be estimated through indirect evidence, usually dung. This is because they are often found in dense habitats where visibility is poor and tracking is difficult. In other locations, such as dry reservoir beds, elephants may be plainly visible but only at certain times of year. But Uda Walawe is an exception. Encompassing large tracts of savannah-like grassland as well as forest and scrub, and having a fairly well-maintained road network thanks to tourism, the park allows researchers to watch elephants directly all-year round. This is what has given the project an unparalleled view of the lives of wild Asian elephants as they naturally live.

Age classes by size, based on calves of known age. The ‘newborn’ in this picture is actually several months old, but less than one year (click for larger image).

In the past five years the Uda Walawe Elephant Research Project has been dedicatedly following the moves of practically every elephant ever to visit the national park. It is an ongoing collaboration between Dr. Devaka Weerakoon at the University of Colombo, and Dr. Shermin de Silva, formerly at the University of Pennsylvania and now full-time director of the research program. By painstakingly photographing and identifying each and every adult, the study keeps track of who was seen, where, and when. This has given us a detailed picture of not only how many elephants there are, but approximately how many individuals there are of different ages, and at which times of year they are present.

The jagged saw-like pattern of this graph shows that many animals tend to be seen more often at particular times of year, suggesting they are they use the park primarily in the wet season or dry season but not both (click for larger image).

The result is the astonishing finding that there are nearly 600 adult males and females using UWNP, published recently in the journal Biological Conservation [1].   If we add calves, the total estimate lies between 800-1160 animals using an area of just over 300 square kilometers.  This population seems to be remaining at a stable number for the moment.  This is still more surprising since the park was previously thought to ‘contain’ between 500-700 animals, so the estimate is nearly double what anyone expected!  What’s more, it appears that all these elephants don’t occupy the observable portion of the park at the same time – instead, only about 200-275 adults make use of a particular portion of the park any given time.  This seasonal rotation means that the density remains almost constantly at around 200 adults per 100 square kilometers.  Where do they go when they’re out of sight?  No one knows – they could be in far corners of the park with no roads, or they may go out of the park entirely.  Without collaring and tracking at least a few of the ‘seasonal’ animals, there’s no way to know.

What does this mean for the ecology of Uda Walawe and the conservation of elephants?  Should we celebrate because elephant numbers are higher than we expected?  The study warns that that would be a mistake.  First, imagine that you have a well full of water with a single small opening connecting it to another larger tank.  The water level in the well can change two ways – either the water level in the ground itself changes, or it can flow in and out of the other tank.  If the water in the ground changes, the well water can spill into the second tank and appear as though it hasn’t changed.  Similarly, if the second tank is squeezed and made smaller, water from it can spill into the well and disappear into the ground. This is similar to what happens to a population of animals in a fenced area like Uda Walawe National Park.

• One possibility is that there is a constant exchange between the well and the tank – that is, some elephants may currently use areas both inside and outside the park whereas others remain inside throughout the year.  There may be as many animals dying as there are babies being born, and over the long run then the population would be larger than the park alone could support, but stable.

• If the animals inside are reproducing successfully, this is like the ground water level rising – eventually the population must either spill out of the park, or some will die for lack of food.  If that happens, it could mean more human-elephant conflict or habitat degradation.

• Similarly, if suitable habitat for elephants outside the park is shrinking pushed into UWNP.  For some period of time even if animals inside are dying at the same rate, it may not look like it since new animals are replacing them from outside.  But once that supply runs out, of course there will be fewer elephants left than there were before.   Personally, we have seen ‘new’ groups from time to time – elephants we don’t recognize – which makes us worry that they may be refugees from other areas who have been evicted due to loss of habitat.

So which is it? This brings us back to the question of “How many elephants are there in Uda Walawe National Park?”  This question is wrong for several reasons, as it should now be clear.  Strictly speaking, the elephants may not all be ‘in’ Uda Walawe, at least not all the time.  The study will next focus on the calves to see how many of each age group are surviving – since elephants have long lives, the true picture may take some time to emerge. Until then, there is still a mystery to be solved and only time will tell.

The full text of the article can be found here.

de Silva, S., Ranjeewa, A., & Weerakoon, D. (2011). Demography of Asian elephants (Elephas maximus) at Uda Walawe National Park, Sri Lanka based on identified individuals. Biological Conservation DOI: 10.1016/j.biocon.2011.03.011