Counting Animals for Conservation

Innovations in Measuring Wildlife Density and Abundance

By Samantha Strindberg and Fiona Maisels | September 14, 2022

White-bellied duiker (Cephalophus leucogaster) caught on camera in the Congo forest. Photo credit: SWM/WCS.

Wildlife conservation management aims to halt or reverse the ongoing biodiversity loss and ecosystem collapse engendered by the crushing weight of human pressure during the “Anthropocene” — the name for the current geological epoch, so named because humans are having a profound effect on our planet.

Each specific conservation action that protects, maintains, or recovers wildlife and wild lands is dictated by the type of threat. But for all types of conservation action, we need to know whether, and by how much, the action has been effective. The best evidence of successful conservation management is stable or increasing wildlife populations.

Usually it is impossible to count every individual in a population. Instead, wildlife monitoring repeatedly estimates population distribution, density, and numbers through a sample survey. The relationship between conservation interventions and trends in animal populations over space and time can then be tracked, and management strategies and actions adapted as a consequence. Each survey method is selected to suit the characteristics of the species, their habitats, and the spatial scale of the surveyed area.

Black-legged mongoose (Bdeogale nigripes) caught on camera at night in the Congo forest. Photo credit: SWM/WCS.

The human and material resources required — and available — for monitoring vary according to the method chosen, the desired metrics, and local geographic and social landscapes. Often a balance has to be struck between extremely precise, but costly, population estimates or slightly less precise, but still meaningful, estimates or metrics that correctly reflect the scale and direction of management action.

“Often a balance has to be struck between extremely precise, but costly, population estimates or slightly less precise, but still meaningful, estimates or metrics that correctly reflect the scale and direction of management action.”

Methods such as occupancy sampling that rely on species detection and non-detection data can be very cost-effective and have the flexibility to suit many monitoring situations, but usually only provide results on the changing distribution of the target species. Methods that provide estimates of population density and size are the most informative about the effectiveness of conservation interventions, and include methods such as capture-recapture and distance sampling.

Capture-recapture estimates population size by identifying the same individuals on several sampling occasions either through natural markings such as spots and stripes (think zebras and tigers), facial characteristics (think chimpanzees or condors), DNA, individually recognizable calls, or capture of individuals and physically marking them (usually small mammals or birds) during the survey.

Yellow-backed duiker (Cephalopus silvicultor) caught on camera in the Congo forest. Photo credit: SWM/WCS.

The proportion of already marked individuals captured in the second and subsequent sampling occasions is used to account for the capture probability of animals when estimating the total population size — very simply, if the survey sample includes 100 animals in a given area and the capture probability is 20%, it is likely that the true population in the area is 500 animals.

“Ongoing scientific research continually improves wildlife survey methods, technological tools, and analytical methods that provide fresh options for the conservationist’s survey toolkit.”

Distance sampling can be used for surveys of unmarked individuals, where it is difficult or too resource-intensive to distinguish one animal from another. During distance sampling the distances to observations are recorded and these are used to estimate the proportion of animals detected during the survey and this is accounted for during density estimation.

The method is also used to estimate density of animal signs (dung, nests) if the animals themselves are rare, nocturnal or elusive. The density of these signs is later converted to animal density, but this requires additional, and often costly, information regarding the rates of production and decay of these signs. Survey data may be collected by human observers or by digital devices such as camera traps.

Bongo (Tragelaphus eurycerus) caught on camera at night in the Congo forest. Photo credit: SWM/WCS.

Ongoing scientific research continually improves wildlife survey methods, technological tools, and analytical methods. These constantly provide fresh options for the conservationist’s survey toolkit. An excellent recent example is the rapid evolution of survey methods for animals that are individually unidentifiable.

“Evolving camera trap technology and image processing techniques, combined with appropriate analytical updates, now allow density estimation of apparently identical animals.”

Evolving camera trap technology and image processing techniques, combined with appropriate analytical updates, now allow density estimation of apparently identical animals. To date, camera trap density estimation methods have mainly been applied to species where individuals were uniquely identifiable — the new methods allow us to use camera traps for a much wider range of animal species.

With the advances in approaches, technology, and analysis, conservationists can now estimate density for a wide suite of species that are particularly important for ecosystem function: those that constitute prey for animal predators. These same species usually form an important animal protein source for rural people who rely on natural ecosystems.

Agile mangabey (Cercocebus agilis) caught on camera in the Congo forest. Photo credit: SWM/WCS.

One of these new techniques, currently applied at several conservation sites across the globe, is camera trap distance sampling (CTDS). This innovation uses camera traps to collect data within the tried and tested distance sampling framework. Density and abundance estimates can now be obtained for any species that spends part of the day on the ground. These include species for which almost no density information previously existed.

Abundance estimates can now be made for a complete A to Z of species — from aardvarks, anteaters, Congo peacock, and elephant shrews to great apes, pangolins, semi-terrestrial monkeys, and all ungulates (including Zebra duikers). This exciting conservation innovation opens a window on species assessments that were previously impossible.

Samantha Strindberg is a wildlife statistician and conservation scientist at WCS (Wildlife Conservation Society). Fiona Maisels is a conservation scientist at WCS and an affiliate of the University of Stirling in Scotland, UK.

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