Estimating Carnivore Densities for Conservation Purposes: Indirect Methods Compared to Baseline Demographic Data

1. What they did

Protecting large carnivores needs reliable numbers, but carnivores are secretive, often active at night, and live at low densities. The most direct way to count them is to recognise every individual, which takes years of fieldwork. So managers often rely on quicker, indirect methods. The authors asked how accurate these indirect methods are, using the cheetah as a model.

They tested four indirect methods:

• Interviews. Biologists, park staff, tourists and farmers in ten study areas in Kenya and Tanzania reported their cheetah sightings (date, place, number, age class and sex). Sightings of groups with the same make-up were counted as the same animals, to stay on the cautious side.
• Average density. An average of reported densities from 13 protected areas in Eastern and Southern Africa (0.021 cheetahs per km²), multiplied by the size of each area.
• Prey biomass. A regression linking cheetah biomass to the biomass of their preferred prey (herbivores of 15 to 60 kg) in ten areas.
• Home range. A model built from the home ranges of female cheetahs on the Serengeti Plains in 1988: how big they were (414 km² on average) and how much they overlapped (4.82 at any point), then scaled up to the whole population with a demographic equation.

They compared each method with “reference” densities from three long-term studies where every cheetah was known by their spot and tail-band patterns: Nairobi National Park, Masai Mara National Reserve and Serengeti National Park. They also compared the four methods with each other across all ten study areas.

2. What they found

Accuracy against the reference densities

• Interviews were the most accurate: 75% of the reference density in the Masai Mara, 100% in Nairobi and 90% in the Serengeti. Their average deviation from the reference was 12%.
• Prey biomass came next (average deviation 37%), then the home range model (51%) and the average density method (53%).
• Interviews, prey biomass and average density all underestimated cheetah numbers. The home range model showed no consistent bias: it overestimated in the Masai Mara (146%) and underestimated in Nairobi (44%).
• With only three reference areas, the authors could not test whether these differences were statistically significant.

Agreement between methods

• Prey biomass estimates were significantly higher than home range estimates. No other pair differed significantly.
• After log transformation, interview and prey biomass estimates were correlated.

Why the methods went wrong

• Interviews did best in small, much-visited parks with open country. In the Masai Mara, cheetahs may be harder to spot in the more closed vegetation. In the Serengeti, cheetahs gather seasonally while following migrating prey, which may let people see most of them.
• Prey biomass may fall short because aerial counts often miss small and medium herbivores, and because cheetahs also take the young of larger migrating herbivores that the model leaves out.
• The home range model applied to Nairobi failed mostly in its demographic part: cheetahs there have unusually large litters and subadult groups.
• The average density method underestimated where cheetahs are dense, as in Nairobi and the Serengeti.
• Even the reference counts may be low: new adult males were still being found in the Serengeti after five years, and shy animals that avoid vehicles may never be photographed or drawn.

3. What they concluded

• Each indirect method works best only in certain conditions.
• Interviews give accurate, inexpensive estimates in small, much-visited protected areas, and could be run by reserve staff. The method only suits species that people recognise and remember, and whose individuals live in separate ranges or stable groups so they can be told apart.
• Prey biomass gives a good first estimate if local prey preferences are known and a reliable prey count exists.
• Average density only works across areas with similar ecology and human pressure; densities from cheetah strongholds overestimate numbers in poorer habitat.
• Home range is very “park-specific”, so it is best used to back up interview or average density estimates.
• Interviews and prey biomass can be used together, because the relationship between their estimates is known. Prey biomass and home range estimates should not be combined to monitor changes in numbers.
• The authors encourage researchers and managers to calibrate their methods and, where possible, to use more than one.

6. Other technologies, skills or innovations

• Individual recognition of cheetahs from photographs or drawings of spot and tail-band patterns, used to build the reference counts.
• Radio-collared females (located more than 10 times in 1988) gave the home range sizes.
• Sighting locations of all 51 known adult females on the Serengeti Plains were mapped in Arc Info Geographic Information System (GIS), with home ranges drawn as minimum convex polygons.

7. Implications for conservation (from the paper)

• Conservation planning often needs quick estimates of carnivore numbers, so indirect methods are widely used, yet they have rarely been checked against real numbers.
• Estimates from different methods can disagree, which makes it hard for planners to combine them into one strategy.
• At least one of these methods is available for many carnivore species, and the findings apply to monitoring carnivores generally, not only cheetahs.