Uganda, Kampala, March 12, 2025—WCS scientists and partners have developed a new method for monitoring wildlife that can be applied when direct sampling is not possible. In a recently published scientific paper, researchers from WCS, Michigan State University, and Birdlife International describe a novel approach that involves recording “indirect counts” of animal signs such as dung of elephants and nests of chimpanzees.
Wildlife monitoring is an important first step of conservation. But, counting how many animals are in one place is challenging in the best of circumstances, and downright impossible in dense habitats, rugged terrain, or when species exhibit elusive behaviors. Indirect count data have been used for some time, but the new paper develops a new way of analyzing the data that produces more robust estimates of animal numbers than was previously possible.
"This new approach to wildlife surveys gives conservationists a big leg up as they work to survey populations of elusive species,” said lead author Samuel Ayebare, who holds affiliations with both WCS and Michigan State University. “Our modeling approach integrates across spatial and temporal variability in habitat use of hard-to-survey species, improving the reliability of population density estimates.”
Chimpanzees construct nests in trees for sleeping each night. Once constructed, these nests can take several days or months to deteriorate. Similarly, elephants produce large dung piles that take a long time to decay. The dung and nests of these animals have a higher rate of being detected than the animals themselves.
WCS scientists in Uganda conducted indirect surveys of chimpanzee nests and elephant dung in Maramagambo Central Forest Reserve within Queen Elizabeth Conservation Area and Kalinzu Central Forest Reserve western Uganda. The work was completed between September and November of 2007, and then again between January and March of 2021. The team used distance sampling methodology, walking line transects across the forests and measuring the distance from the line to the signs.
“The novel modeling approach enables the estimation of parameters that were not possible using traditional approaches,” Ayebare added.
A more precise density estimate model
The study applied a modified hierarchical distance sampling model in the analysis of indirect count data to account for spatial variation in animal density, estimate the decay rate of animal signs and assess population change between survey periods. This modeling approach aims to improve the estimation of population parameters for hard to survey species using all the available monitoring data.
“Following the marked sign count method, traditional approaches of analyzing sign count data such as chimp nests and elephant dung, involve discarding data from the first visit because the number of days in which signs were produced is unknown,” said Andrew Plumptre, a co-author on the study. “The novel approach we used to analyze repeated counts of line transect data,” added Plumptre, “incorporated both data from the first pass and subsequent counts to produce a more robust estimate of animal numbers.”
Where repeated counts of transects are being made, this method makes it possible to use the data collected on the first count as well as the subsequent counts and by doing so the results are more robust and errors are smaller.
Key findings and implications for conservation health
The study found that chimpanzee populations declined by at least 10 percent across the study area from 2007 to 2021, signifying urgent action is needed to enhance protection of these endangered animals. The estimated decline is hypothesized to be due to increased human activities such as cutting poles and trees, laying of snares, and pit sawing. On the other hand, the population of elephants increased by 50 percent from 2007 to 2021 in the Maramagambo Central Forest Reserve.
The increase in elephants is attributed to immigration from the surrounding savanna habitat, possibly for thermal regulation, escape from bush fires, and/or movement triggered by a decrease in food resources during the December–February dry season. Elephant numbers have been increasing in Queen Elizabeth Conservation Area and it is likely they are now using more forest habitat to find food. Although it is difficult to estimate elephant densities in forested habitats, this analysis highlights the importance of Maramagambo habitat for elephant populations in the landscape.