At the same time, the Court of Appeal said the tribunal needs to take another look at whether there might be ways to deal with the potential harm to the Blanding's turtle other than by scuttling the project entirely.
"I would allow the appeal on the merits and restore the tribunal's conclusion that the project will cause serious and irreversible harm to the Blanding's turtle," Justice Russell Juriansz wrote on behalf of the court.
"I would remit the matter back to the tribunal to address remedy after giving the parties the opportunity to be heard."
At issue in the case, which is being closely watched for its ramifications on how endangered species and unspoiled areas are protected, is a provincially approved nine-turbine wind farm on 324 hectares at Ostrander Point south of Belleville, Ont.
The Prince Edward County Field Naturalists Club successfully challenged the approval before the environmental review tribunal based on expert views that the project would harm the area's turtles. Among other things, experts testified that access roads needed for the turbines would mean increased traffic that would further threaten the turtles.
Ostrander Point Wind Energy, the turbine proponents, turned successfully to Divisional Court, which restored the original approval, prompting the field naturalists to head to the Court of Appeal.
In its decision, the Appeal Court said the tribunal had properly weighed the expert opinions and other evidence in siding with the naturalists, and the Divisional Court was wrong to find otherwise.
The field naturalists were thrilled with the ruling.
"We are finally vindicated in showing that the environment matters," said spokeswoman Cheryl Anderson.
Nature Canada, which intervened in the appeal, called the ruling "a huge win for nature."
"Other proposed wind projects will need to consider impacts on birds and bats as well as turtles much more seriously than they have done previously, especially where proposed wind project road networks fragment habitat for species at risk," Stephen Hazell, a lawyer with the group, said in a statement.
However, the top court also found that the tribunal unfairly denied Ostrander a chance to show that closing the access roads to the public would mitigate the risk to the turtles.
The Appeal Court refused Ostrander's request to approve the project, saying the review tribunal was the appropriate forum to hear those submissions.
"That the tribunal will allow the project to proceed upon the roads closure should not be regarded as a foregone conclusion," Juriansz said.
"I note the tribunal had evidence of how the proposed access roads would cause harm to the turtle's habitat quite apart from collisions with motor vehicles."
Anderson said the eco group was confident it can convince the tribunal that Ostrander's idea of putting up gates to close off the roads is no solution.
"The remedy is no roads," she said.
Ostrander did not immediately return calls seeking comment.