Abstract: High rates of human‐mediated mortality on animals are frequently reported inside protected areas, especially near their borders, so the overall effect of reserves on animal conservation is not usually known. To determine the effect of a traditional reserve (Doñana National Park, southwestern Spain) on the Eurasian badger ( Meles meles ), a medium sized carnivore, we studied mortality causes and rates, with the aid of radiotelemetry, in two populations inside the reserve, one closer to the border than the other. Human‐related mortality ( poaching and road kills) was by far the most common cause of mortality (85% of deaths recorded). The average annual mortality rate due to poaching was high (0.48 ± 0.08) for the population close to the border of the park, whereas none of the radiomarked badgers in the core of the reserve died during the study period. A logistic model that included distance from the border of the park, sampling effort, and the local area (i.e., edge and core populations) indicated that the difference between both sites was due to the effect of distance from the border on survival probability. On a regional scale, badger density at the core of the park was 3.16 times higher than outside. Overall, the effect of the reserve was positive, but edge effects reduced reserve effectiveness by 36%. Edge effects in close proximity of the border were strong, reducing badger density even below the expected density outside the reserve. Edge effects should be considered carefully when reserves are implemented because they can greatly reduce reserve effectiveness and influence the viability of the populations inside. Enlargement of reserves and control of human activities that promote edge effects, both inside and outside the reserve, are the two management actions that can most effectively mitigate edge effects.