Preservation through legal protection of natural areas is costly and limited in its extent. A much cheaper strategy to preserve natural areas could be to simply identify areas of ecological interest and let it be known. A survey was launched in France to identify such areas (called ZNIEFF) in the early 1980s. Since then, municipalities have had to account for ZNIEFFs in their formal land planning schemes, though they are not under any obligation to protect them. In this study, we tested the effectiveness of ZNIEFFs as a conservation tool in an area of high growth near Paris. Using GAM modeling, we compared the rate of urbanization inside and outside ZNIEFFs in the 17 years following designation, accounting for the share of farmland, the overall rate of urbanization and ZNIEFF proportion in the municipalities, and also accounting for demography and physical constraints (hydromorphy and slope). Overall, there was less urbanization inside ZNIEFFs, but this varied depending on the context. Surprisingly, they were better preserved in areas of more intense urbanization. This effect was increased if farmland area was already reduced (<30% of the municipality area). In contrast, when farmland was still predominant and urbanization rates were lower, ZNIEFFs tended to be more urbanized than the areas outside. This shows that the value of remnant natural areas varies considerably, perhaps as a function of the value attributed to farmland.