Invasive Species

A species rarely causes trouble on its own turf. Millions of years of shared evolution with predators, parasites, and competitors keep its numbers in balance. Move that species somewhere its natural enemies don't exist, and the brakes come off. Nothing eats it, nothing infects it heavily, and nothing outcompetes it for food or space, so a population that was background noise at home becomes an unstoppable surge somewhere else.

Zebra mussels are the textbook case. Native to the Black and Caspian Seas, they got picked up as larvae in the ballast water of cargo ships, carried across the Atlantic, and dumped into the Great Lakes when ships released that water before loading cargo. With no native predators or parasites waiting for them, and with abundant algae to filter, the mussels multiplied explosively, blanketing surfaces at densities exceeding 100,000 individuals per square meter in some spots and clogging water intake pipes. As filter feeders, they strip phytoplankton out of the water column, which starves the zooplankton that depend on it, which then starves the small fish that eat zooplankton, rippling damage up through the whole food web while also making the water clearer, letting light penetrate deeper and changing which plants can grow.

Not every newcomer does this. A species only earns the label invasive if it spreads aggressively and causes real ecological or economic harm. Plenty of introduced species settle into a small niche, grow slowly, and never outcompete anything, so they stay merely non-native. This distinction connects invasive species to the bigger theme of the unit: human-driven global change, like ozone depletion and warming, often works by removing or altering the natural checks that kept a system stable.