Across the Far North, populations of caribou — an indispensable source of food and clothing for indigenous people — are in steep decline. Scientists point to rising temperatures and a resource-development boom as the prime culprits.

A battle is quietly being waged between the industry that produces genetically modified seeds and scientists trying to investigate the environmental impacts of engineered crops. Although companies such as Monsanto have recently given ground, researchers say these firms are still loath to allow independent analyses of their patented — and profitable — seeds.

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.