Physical laws shape biology.

PRIMARY TROPICAL FORESTS ARE POWERhouses of biodiversity (1) but are rapidly declining in extent and are threatened even within some protected areas (2). As a result, non-primary forests, especially those that have been selectively logged, are becoming more important to conservation efforts. In the tropics, logging is almost always selective, targeting only certain commercially valuable tree species above a minimum size and leaving other species unharvested. More than 400 million hectares of tropical forest are now in permanent timber estates (3), and at least 20% of all tropical forests were logged from 2000 to 2005 (4). Biologists have often emphasized the deleterious impacts of selective logging for disturbance-sensitive wildlife [e.g., (5, 6)], but recent evidence suggests that logged forests can have surprisingly high conservation values. In a meta-analysis across four tropical regions, selectively logged forests were by far the most biologically similar to primary forests, compared with agricultural and agroforestry systems (1). Even after repeated, intensive logging, the biodiversity of native forests in Borneo is roughly comparable to that after the fi rst cut (7). Logged forests also store considerable carbon (8) and maintain most hydrological functions of primary forests. Hence, while they cannot replace primary forests, logged forests have great potential to enhance conservation at landscape and regional scales, act as buffer zones around protected areas, and help maintain forest connectivity for wildlife. Unfortunately, logged forests are more likely than primary forests to be cleared or burned in some (9) but not all (10) nations. For this reason, we do not advocate an expansion of logging—although this will surely continue in many regions. Rather, we assert that logged forests are too vast, vulnerable, Physical Laws Shape Biology