The case, which pits turtles against turbines, could have widespread repercussions as to how endangered species are protected across Canada, and raises questions about the protection of unspoiled areas.
At issue is a proposed nine-turbine wind farm at Ostrander Point south of Belleville on the shore of Lake Ontario.
Prince Edward County Field Naturalists Club had successfully argued before the province's environmental review tribunal that the project on the 324-hectare site would threaten Blanding's turtles in the area.
Gilead Power, through its Ostrander Point Wind Energy, had Divisional Court overturn that decision in February. It argued the tribunal had made a half-dozen errors in concluding the project would cause "serious and irreversible" harm to the turtles.
Addressing the panel in the packed courtroom, lawyer Eric Gillespie, who speaks for the naturalists, pleaded with the three justices to defer to the review tribunal.
"The tribunal was squarely within its mandate," he said. "We are simply asking this court to respect and uphold that original decision."
Gillespie noted the tribunal heard evidence over 24 days but essentially based its decision on four duelling experts on the long-lived Blanding's turtles.
The main issue, according to the tribunal, was that five kilometres of access road needed would lead to more turtle road kill, poaching and predation, and degradation of critical habitat.
The turtles, which are already in decline, can take up to 25 years to mature and survive for 75 years, court heard.
"It's a highly vulnerable population," Gillespie said.
Even a single individual matters, he said, when you're dealing with a small local population.
In an interview, Gillespie said the Divisional Court decision — if allowed to stand — would harm protections for Canada's threatened and endangered species given the limited data that exists for most of them.
In court, the justices were clearly troubled by the fact no detailed turtle population study exists.
They repeatedly questioned how the tribunal could rely on expert evidence about the impacts of increased turtle mortality rates without knowing how many of the critters there are.
"What good is the (expert) opinion?" Justice Gloria Epstein asked.
Gillespie warned against turning the Appeal Court into a "science academy."
"We have to rely on the opinions of experts," he said. "The tribunal did exactly what an expert tribunal is entitled to do."
Another lawyer, Chris Paliare, said Divisional Court "subverted" the review tribunal by suggesting, as the project proponents maintain, there was no evidence as to what "irreversible harm" means.
"It's parsing beyond belief," Paliare said.
"The evidence is overwhelming this project will cause serious and irreversible harm."
Gilead, which will continue arguing its case on Tuesday, maintains it can do what's necessary to protect the turtles.
Stephen Hazell, a lawyer with Nature Canada which is intervening in the case, disputed that.
The proposed access roads are "directly" in the turtles' habitat, Hazell said as the justices pored over maps of the area.
"That is irreversible harm."
However, Gilead lawyer Neil Finkelstein said the tribunal didn't conclude that access roads would necessarily be harmful to the turtles.
The tribunal's reasoning, he said, simply didn't hold together.
"Divisional Court was correct that the tribunal's decision was unreasonable and should be set aside," Finkelstein said.