SummaryUnderstanding the effects of forest fragmentation on biodiversity is essential for successful and efficient forest conservation. Four factors may cause loss of biodiversity in forest fragments: the effect of non-random sampling of the original forest, reduced forest size, isolation and edge effects. A review of 58 papers on effects of forest fragmentation reveals that general conclusions from fragmentation research are biased due to a focus on birds, on size-effects rather than isolation, and on species presence rather than population sizes. Perhaps the most important finding is that current knowledge on fragmentation effects is based mainly on studies in small fragments (<10 ha). These are dominated by edge effects, can not contain viable populations for many species and are rarely the focus of conservation programmes. Studies of small fragments can not be extrapolated to larger-sized, protected areas, and do not necessarily support the case for needing extremely large, protected areas. Conservation of medium-sized, strategically-located areas may be a more efficient option for biodiversity conservation, given financial, social and logistic limitations. More research is needed on forest fragments that arc representative of the sizes of real-world protected areas (i.e. >10 000–100 000 ha) and should focus on the biological and human-induced processes which determine species persistence.