A new study has identified the world’s largest known nesting site of endangered giant South American river turtles – with the help of drones, scientists recorded more than 41,000 of the large reptiles on the Guaporé River along the Brazil-Bolivia border.
The study – conducted by the University of Florida (UF) and the Wildlife Conservation Society (WCS), and published in the Journal of Applied Ecology – shows how combining drone technology with statistical analysis can vastly improve wildlife surveys.
“We describe a novel way to more efficiently monitor animal populations,” says lead author Ismael Brack. “And although the method is used to count turtles, it could also be applied to other species.”
Counting giant river turtles
The giant South American river turtle (Podocnemis expansa) – also known as the Arrau turtle – is one of the largest freshwater turtles in the world, reaching up to 90cm in length and weighing as much as 80kg. It can vary in colour from dark grey to brown or olive green.
The turtle is known for its highly social nesting behaviour. Every year in July or August, females gather in large numbers along the sandy banks of the Guaporé River (one of the major tributaries of the Madeira River in the Amazon basin) to lay their eggs. Yet despite legal protections, the species remains under threat from poaching and habitat loss.
In order to conserve species such as the giant South American river turtle, it's important to work out accurate population sizes – to know if the species is in decline or whether efforts to protect it are successful. But counting wildlife has always posed challenges for scientists, says Brack.
On-the-ground surveys can be time consuming, invasive and inaccurate, he explains. Aerial photos taken by drones provide a faster and less disruptive way to count animals, but even this method has limitations. Animals move around, and it’s easy to miss individuals or count the same one more than once.
- Incredibly rare giant turtle with a face like a frog found in India's tropics
- "The hatchlings' heads were completely separated from their bodies": Australia's ghostly turtle killer caught on camera
To tackle the issue of inaccurate counting, the research team used a more refined approach. At a site on the Guaporé River, they developed a system to account for errors such as double counting or missed animals. The team marked the shells of 1,187 turtles with white paint and monitored their movements over 12 days using drones.
Each day, the drones flew over the site four times, capturing 1,500 images per flight. These were stitched together into detailed orthomosaics – high-resolution composite images created from overlapping photographs. Scientists then reviewed the images, recording whether each turtle was marked and if it was walking or nesting.
Using this information, the team built probability models to better estimate the true number of turtles present. Traditional counts and counts based on drone images had very different results: ground observers logged around 16,000 turtles, while researchers who reviewed drone images and didn’t account for animal movement counted nearly 79,000. The new method produced what the researchers believe is a more accurate estimate of 41,000.
“These numbers vary greatly, and that’s a problem for conservationists,” says Brack. “If scientists are unable to establish an accurate count of individuals of a species, how will they know if the population is in decline or whether efforts to protect it are successful?”
Why the study matters – and what's next
The giant South American river turtle plays a key role in the Amazon’s ecosystem. As seed dispersers and prey for larger animals, they help maintain a healthy balance in the riverine environment. Protecting their nesting sites is therefore vital, and accurate population estimates – such as those provided by this new drone-led approach – are key to this.
Using drones in this way has the potential to improve surveys of other animals, too, such as seals, elk or mountain goats, where visual markings or movements can affect counts.
The research team plans to continue refining their methods, with future surveys in other parts of the Amazon basin where the turtles nest, including Colombia and possibly Peru and Venezuela.
“By combining information from multiple surveys, we can detect population trends, and the Wildlife Conservation Society will know where to invest in conservation actions,” says Brack.
In pictures: giant South American river turtle nest site
Find out more about the study: Estimating abundance of aggregated populations with drones while accounting for multiple sources of errors: A case study on the mass nesting of Giant South American River Turtles
Top image credit: Omar Torrico, Wildlife Conservation Society
More wildlife stories from around the world
- “It was one of the shocks of my life": third species of manta ray confirmed after decades of suspicion
- Colossal flood explodes through Greenland Ice Sheet. What scientists find in aftermath astounds them
- When a diver swam up to a frogfish, they were amazed by what they saw in its mouth
- Science may finally have discovered why humans are so good at drinking booze
