Intensive agriculture causes landscape homogenization and massive defaunation. The creation of Strict Nature Reserves (SNR) where management is proscribed aims to stem this trend; this is the case for SNR forests in Western Europe. Yet, unmanaged forests are subjected to rapid habitat closing; semi-open bushy habitats that are essential to many thermophilic organisms can be squeezed between industrial agriculture and high timber forests. Long-term capture-mark-recapture monitoring of snake populations (~27 years; 3,447 individuals) coupled with canopy closure monitoring conducted in a 2,579 ha forest amid a landscape dominated by intensive agriculture in France provides an illustration of this threat. Prior to SNR classification, logging maintained a mosaic of habitats (e.g., open, semi-open, closed plots) favorable to snakes (Hierophis viridiflavus, Zamenis longissimus). SNR classification promoted the growth of trees and rapid habitat closing, causing the disappearance of open woodlands and a strong decline of snake abundance (~60 % drop). The impact was rapid in the most thermophilic species (H. viridiflavus) and delayed in the less thermophilic species (Z. longissimus). This study provides robust and direct evidence that habitat changes can entail a drastic decrease of snake abundance. Unintendedly, applying the most protective conservation status to temperate forests might be less valuable in terms of snake abundance and diversity than a status where limited conservation management actions are permitted.