Almost a decade ago, 183 countries signed the Convention on Biological Diversity to guard against exploitation of their genetic resources. Now scientists say the treaty, although well intentioned, effectively prevents scientists from studying the natural bounty it is designed to protect. The many national bureaucracies spawned by the treaty do not easily distinguish between bioprospecting and basic science, and the system of permits that must be obtained is so daunting that many scientists have simply given up trying to work in species-rich areas. The problem is particularly acute in South America, where researchers have sometimes been chased out of forests by locals, been detained by the police, or had their work destroyed. In February, 13 countries that control about 70 percent of the world’s biological diversity formed the Group of Allied Mega-Biodiverse Nations, which plans to draw up clearer guidelines for studying and possessing biological materials.