A turtle that insists on crossing a road has put a stop to a massive wind-energy development in Eastern Ontario.

The Ontario Court of Appeal ruled on Monday that a 324-hectare, nine-turbine wind farm proposed for the south shore of Prince Edward County puts a population of endangered Blanding's turtles at risk of dying out in that region's wetland. The risk is posed not by the wind farm itself but by 5.4 kilometres of roads to and from the site. Experts said the turtles, which range widely as part of their natural life cycle, would inevitably try to cross those roads, exposing them to vehicles, predators and human poachers.

The ruling restores an environmental tribunal's 2013 decision that the wind farm, while not posing a serious risk to human health, would cause "serious and irreversible" harm to the Blanding's turtle. That ruling had been rejected by Ontario Divisional Court partly because the tribunal did not know how many turtles live in the provincially significant wetland.

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But the Ontario Court of Appeal said the number of turtles at risk does not matter. "The number of Blanding's turtles, no matter what that number is, satisfies the criteria" for being deemed threatened and endangered, the court said in a 3-0 ruling written by Justice Russell Juriansz. It cited testimony from Frédéric Beaudry, a wildlife ecologist at Alfred University in New York State, that the number is "likely small."

The Court of Appeal ruling means the case now goes back to the environmental tribunal to decide what should happen with the project, including whether an alternative plan can be permitted that takes the turtles into account. The company involved, Ostrander Point Wind Energy LP, had proposed at an earlier stage to close the road to public access.

The ruling is a setback for Ontario's multibillion-dollar wind energy business. "It will mean that, in future, wind companies are going to have to pay attention to some of these environmental effects," said Stephen Hazell, director of conservation and a lawyer with Nature Canada, which supported the suit launched by the Prince Edward County Field Naturalists, a local conservancy group.

Mr. Hazell added that other groups with concerns about the impact of wind projects in their own jurisdictions now have "a legal test that in some cases they may be able to meet."

During the initial hearing, conservationists argued that the wind project would have adverse effects on a number of species, including migratory birds, but the final decision came down to the Blanding's turtle alone because of its extreme sensitivity to human activity, particularly roads.

With a bright yellow throat, a gentle disposition and an expression that resembles a perpetual smile, the species makes a tempting target for poaching, even by well-meaning individuals looking for an unusual pet. But Blanding's turtles usually die once they are captured or released in a different location.

Ponderously slow to grow and mature, females of the species generally do not reproduce until they reach 18 years of age. Even then, they may only lay eggs every other year. The turtle's long life span offsets its slow replacement rate – adults may live 90 years or more – but only in places when individuals have a good chance of avoiding lethal encounters along the way.

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"Losing a couple of females can, in the long run, do a population in," said Dr. Beaudry, a world expert on the species.

He added that he had no doubt the turtles would be crossing the roads if the wind project went ahead, as they typically travel for kilometres from the places where they hatch in search of food or mates.

Blanding's turtles are considered globally endangered. Small populations are found in scattered pockets from the American Midwest to Nova Scotia.