From UF/IFAS
A University of Florida research team has developed a more accurate way to count wildlife using drones. This innovation helped confirm the world’s largest known nesting site for a threatened turtle species.
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By combining aerial imagery with statistical modeling, the researchers documented more than 41,000 Giant South American River Turtles gathered along the Amazon’s Guaporé River. Their findings, published in the Journal of Applied Ecology, offer a new tool for conservationists seeking to monitor vulnerable animal populations more precisely.
“We describe a novel way to monitor animal populations more efficiently,” said lead study author Ismael Brack, a post-doctoral researcher within the UF Institute of Food and Agricultural Sciences’ (UF/IFAS) School of Forest, Fisheries and Geomatics Sciences. “Although the method is used to count turtles, it could also be applied to other species.”
Aerial photo of a sandbank in the Amazon’s Guaporé River where female Giant South American River Turtles congregate each year to nest.
The project began with Wildlife Conservation Society (WCS) researchers in Brazil, Colombia, and Bolivia. One of the species the New York-based organization monitors is the Giant South American River Turtle, which is threatened by poachers who sell its meat and eggs. The turtles are exceptionally social creatures, and females congregate each year in July or August to nest in the Guaporé River sandbanks between Brazil and Bolivia.
Brack met WCS scientists at a conference, and they shared how they use drones to count the turtles, he said. They create orthomosaics, which are highly detailed, high-resolution composite images made by stitching together hundreds of overlapping aerial photographs.
Counting the animals shown in ortho mosaics is a quicker, more accurate, and less invasive approach than counting animals from the ground. However, this method alone doesn’t account for the fact that animals sometimes move during observation.
Aerial photo of female Giant South American River Turtles congregating on a sandbank in the Amazon’s Guaporé River to nest.
Together, researchers from UF and the WCS developed a method that improves counting accuracy by eliminating multiple sources of error, including double counts (the same individual counted multiple times) and missed individuals.
According to the study, researchers used white paint to mark the shells of 1,187 turtles gathering on an island sandbank within the Guaporé River. Over 12 days, a drone flew overhead four times a day on a meticulous, back-and-forth path and snapped 1,500 photos each time. Using software, scientists stitched the photos together, and researchers reviewed the composite images.
They recorded each turtle, whether its shell was marked, and whether the animal was nesting or walking when photographed. Equipped with this data, they developed probability models that account for individuals entering and leaving the area, observed turtle behaviors, and the likelihood of detecting an identifiable shell mark.
Giant South American River Turtles nesting on a sandbank in the Amazon’s Guaporé River.
According to the study, the models revealed several potential sources of error that could arise from traditional ortho mosaic counts. Only 35% of the turtles that used the sandbank, for example, were present during drone flights. On average, 20% of the detected walking appeared multiple times in ortho mosaics, some as many as seven times.
According to the study, observers on the ground counted about 16,000 turtles. Researchers who reviewed the ortho mosaics but didn’t account for animal movement or shell markings counted about 79,000 turtles. When they applied their models, however, they estimated about 41,000 turtles.
“These numbers vary greatly, and that’s a problem for conservationists,” Brack said. “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?”
The study describes ways to adapt and apply the approach to conservation efforts involving other species surveyed by drone-derived orthomosaics. Past monitoring studies, for example, have involved clipping seals’ fur, attaching high-visibility collars to elk and marking mountain goats with paintball pellets to keep track of animal movement during counts.
Brack said the research team plans to perfect its monitoring methods by conducting additional drone flights at the Guaporé River nesting site and in other South American countries where the Giant South American River Turtle gathers, 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,” he said.
ABOUT UF/IFAS
The mission of the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) is to develop knowledge relevant to agricultural, human, and natural resources and to make that knowledge available to sustain and enhance the quality of human life. With more than a dozen research facilities, 67 county Extension offices, and award-winning students and faculty in the UF College of Agricultural and Life Sciences, UF/IFAS brings science-based solutions to the state’s agricultural and natural resources industries, and all Florida residents.
For more information, go to ifas.ufl.edu.
