A defeat for a small group of Harpswell residents should sound an alarm for anyone in the state who assumes that private property owners will always welcome public access to their land.

In a unanimous opinion issued Tuesday, the Maine Supreme Judicial Court struck down a lower court ruling that had given the public the right to walk down a private road on Bailey Island to access two beaches. The court found that the public can still use the beaches, but only if they get there by boat.

The case was decided on a technical legal issues based on the review of a long history and a complicated set of facts. But there is nothing opaque about the result: Even though generations of neighbors have been able to reach the beaches by walking down the road, the current owners have the right to change the rules.

That should send a message to critics of land conservation programs, especially Land for Maine’s Future, which has been securing public access to special natural places that are currently in private hands for three decades. It should also cause opponents of the possible declaration of a national monument in the Katahdin region to think twice.

What this court case shows is that unless access is secured, the right to use treasured spots can disappear in a wink of an eye.

According to court records, Eugene McCarty owned Cedar Beach Road from 1927 to 1956, and allowed the public to use the road to get to the beach. After McCarty’s death, the land passed through several owners, and there were times when they tried to block or limit public use of the land. In 2011, the owners blocked the road completely. Now the court has upheld that move, making a trespasser out of anyone who used the same road that their parents and grandparents had used to get to the water.

Maine is 90 percent forested, but the vast majority of it is privately owned. Residents and visitors have been able to count on access to private property to hunt, fish, camp, hike, ski and snowmobile. But as large tracts of land have been subdivided, new owners have exercised the right to post their property and keep the public out.

Gov. Paul LePage has repeatedly argued that land conservation is something that benefits only the wealthy, but the Harpswell situation shows how wrong he is.

The people who could afford to buy the land haven’t been denied access to the beach, and neither have the people who have boats. The people who used to walk down the road to get to the beach have been closed out, and the same thing could happen at other cherished spots all over the state.