Spread by trade and climate, bugs butcher America's forests
Insect pests, some native and others from as far away as Asia, can undermine forest ecosystems. For example, scientists say, several species of hemlock and almost 20 species of ash could nearly go extinct in the coming decades. Such destruction would do away with a critical sponge to capture greenhouse gas emissions, shelter for birds and insects and food sources for bears and other animals. Dead forests also can increase the danger of catastrophic wildfires.
Today's connected world enables foreign invaders to cross oceans in packing materials or on garden plants, and then reach American forests. Once here, they have rapidly expanded their ranges.