Author: Ishika Ramakrishna

Key Highlights:

  • Large herbivores are often considered ecosystem engineers, and their presence in an ecosystem dictates its entire functioning. It is, therefore, of utmost importance to accurately quantify their population densities in a particular habitat.
  • This, however, is not an easy task, as direct sightings of these animals are not always possible. There is, thus, the need for alternate methods of counting deer and other herbivores, and the reliability of these methods needs to be tested before being used on a large scale.
  • This team of researchers compared how effective it was to estimate the population densities of 6 herbivorous mammals in Nagarahole National Park, through dung-counts and direct- sightings. Their paper talks about the complexities of both methods, and how using dung counts might still be problematic at times.

 

Large herbivores, like elephants, gaur, pigs and various deer species, are integral to an ecosystem. They are the primary consumers of terrestrial food chains, and knowledge of their population sizes can tell us a lot about the health and resilience of the wild spaces they occupy. Unfortunately, trying to count these large mammals is no easy task. As they often live in dense forests, which also have tall grasses or other shrubberies, it’s not always easy to spot them directly, or count them accurately. Apart from this drawback, doing a complete count of all the deer, elephants and pigs would require immense time, manpower and financial support. Researchers who were interested in estimating how many herbivores are present in the forest, thus, had to go a step further and be creative!

All animals poop, and herbivores do so in distinct and conspicuous ways. Why not, then, try to arrive at a population estimate for each species by looking out for their dung? This was an idea that struck many researchers, starting all the way back in the 1930s, but not all were convinced about how reliable this approach really was. There are many factors at play here as well: How often does each species defecate? And how much? How rapidly does their dung decay? How does one avoid counting the dung of the same animal twice? And finally, a crucial question asked by Dr. Farshid Ahrestani and his peers including Dr. Ullas Karanth: Are dung counts a viable option to assess population densities of various large herbivores in tropical forests, where direct sightings are particularly challenging?

This team of passionate field biologists and statisticians came together in Nagarahole National Park in Karnataka to determine whether dung counts could work while counting the number of elephants, gaur, sambar deer, spotted deer, muntjac and wild pig, present in both the dry and moist deciduous forest habitats of this area. They began by performing ‘dung decay rate’ experiments, where they studied how long it took for each species’ dung to decompose, and these results helped them during their actual count. They also read all of the literature available on herbivore defecation rates. They then went into the field and sampled the landscape for the dung from these 6 species, knowing now exactly at what stage of decay these piles were. They then used all of this information in a software called ‘DUNGSURV’ which statistically analysed the amount of dung found, the stage of decay they were in, and the defecation rates of each species present. The model, which was carefully designed in 1991 for this precise purpose, then provides an estimate for how many herbivores are in that area.

Dr. Ahrestani and team now had both the results from DUNGSURV, and some population data from direct sightings after having conducted extensive field surveys in the National Park using a method called ‘distance sampling’. They found that they ended up with dung-based estimates which were far greater than the population estimates from direct sightings. They felt that there was high uncertainty in how reliable the dung counts were, and also felt that the defecation rates obtained from literature may not have been accurate for the landscape they were working in. After this mammoth effort and painstaking fieldwork, they concluded that sighting-based surveys, albeit effort-intensive, are still more dependable than the dung-based approach. After all, comprehensive results from these studies directly inform conservation action, and poop can be very serious business in the natural world!

Original Article: Ahrestani, F., Kumar, N.S., Vaidyanathan, S., Hiby, L., Jathanna, D., Karanth, K.U. (2018) Estimating densities of large herbivores in tropical forests: Rigorous evaluation of a dung‐based method. Ecology and Evolution. doi: https://doi.org/10.1002/ece3.4227