Biological pollution – the spread of invasive species of plants and animals – is one of the hardest problems to prevent, as this article illustrates. Zebra mussels may rank among the most important threats to Cascadia’s hydroelectric and freshwater ecological systems. These fast-breeding bivalves can spread with astonishing rapidity, encrusting pipes, gates, and other underwater hardware of the hydropower infrastructure. So far, the Northwest has avoided an introduction of the pesky mollusks.

Other invasive species—green crabs, spartina grass, Scotch broom, and English ivy—have long since established themselves. (Read a regional summary in Sightline’s State of the Northwest, which you can download here.) They’re hard to get rid of. In the city of Seattle, English ivy has overrun more than a third of public-land forests. And the ivy is choking off forest succession, threatening to deforest these lands over the next 30 years, as the city’s already mature trees die off. The city and Cascade Lands Conservancy have launched a program to uproot the ivy.