While teaching land-use planning at Williams College, professor Sarah Gardner realized her students had a very narrow idea of what farming in New England looks like. She had asked them to go talk to a farmer for an assignment. “They all went to one of the three new food farmers that go to the farmers’ market or sell at the co-op — the ones that were easy and comfortable for them to talk to.’’ Gardner was shocked that none of them went to one of the large-scale dairy farms in town, which control the majority of the area’s farmland.

This realization that conventional dairy farms were practically invisible to a new generation spurred her to partner with director Dave Simonds to make “Forgotten Farms,’’ a documentary chronicling the plight of New England dairy farms. It screens Oct. 2 as part of the GlobeDocs Film Festival taking place this weekend.

Gardner doesn’t shy away from the grim statistics: According to the Forgotten Farms website, New England has lost 10,000 dairy farms in the past 50 years, and fewer than 2,000 farms remain. She says the reasons are a complicated mix of economic, political, and environmental, but the plummeting price of milk seems to be the biggest culprit.

And it’s not just local milk that’s lost when dairy farms shutter. “If you care about having a more robust regional food system, that can’t happen if you don’t have farmland. So every time a dairy farm goes out, we are losing 300 hundred acres,’’ says the filmmaker.

Building a local food system was the reason Gardner became interested in making the movie. “The places we are importing most of our foods from are becoming deserts. Not to be overly dramatic, but California isn’t really a sustainable place to grow most of our fruits and nuts. And the Midwest is becoming drier and drier,’’ says the professor. She acknowledges climate change will bring its own challenges — more pests, for instance — but believes New England’s best option is to take advantage of the longer growing season and produce more food close to home.

And the professor reminds us that one can’t live on artisanal cheese and baby greens alone: “I also touched on this larger issue of class, because I feel like there are people who are very supportive of the new food movement and farming, and these people, I think, many of them are not predisposed to want to support conventional farming. These dairy farmers are just practically, right now, living in poverty, and the new farmers are growing arugula for rich people, let’s face it.’’

The professor doesn’t want you to think she is against anyone taking their liberal arts degree and moving to the country to start a small farm. “I think it’s important. I don’t want to insult it,’’ she says. “It’s gotten people more interested in farming, more interested in local food, and dairy is the biggest local food product in New England.’’ Catherine Smart

“Forgotten Farms’’ screens Oct. 2 at noon at the Brattle Theatre in Cambridge, followed by a Q&A with Gardner and Simonds. For more information on the GlobeDocs Film Festival, or to buy tickets, go to www.filmfest.bostonglobe.com.

Catherine Smart can be reached at cathjsmart@gmail.com.