To evaluate conservation efforts, scientists and land managers must measure the ability of conservation projects to protect biological diversity. Concord, Massachusetts, home of noted philosopher and naturalist Henry David Thoreau and the first battles of the American Revolutionary War, provides a unique opportunity to examine the ability of protected areas to preserve biodiversity in a suburban landscape. Much of the town's land (35%) has been protected and a further 27% remains undeveloped. Botanists, including Thoreau, have surveyed plants in Concord five times over the last 170 years. Of the species seen by Thoreau in the mid-19th century, 28% are no longer present in Concord and 32% are vulnerable to local extinction, with only one or two populations. Most of the loss appears to have occurred in the past three to four decades. Certain groups, such as orchids and aquatic plants, have suffered particularly severe losses. Non-native species represent an increasing percentage of the flora. The flora has experienced a net loss of species since 1974, with 83 species gained and 243 species lost. Concord is losing species surprisingly quickly given the level of protection. Many factors have likely contributed to the loss of species, including forest succession, pollution, habitat destruction, spread of invasive species, and grazing by deer. Because many species have only one or two populations remaining, local extinctions will likely continue in coming decades. Active management of existing protected areas will be important in future efforts to protect the biodiversity of the historically significant Concord flora.