A number of biologists are challenging the long-held orthodoxy that alien species are inherently bad. In their contrarian view, many introduced species have proven valuable and useful and have increased the diversity and resiliency of native ecosystems.

Introduced more than a decade ago, genetically modified crops are now planted on millions of acres throughout the world. But the fundamental questions about them remain — both about their safety and their long-term impact on global food security and the environment.