Think of the worst invasive species you know. Kudzu: smothering trees and houses, growing a foot a day. Burmese pythons: stripping the Everglades of small animals. Asian carp: hoovering streams clean of plankton and swimming toward the Great Lakes.
They all came from somewhere else, arrived with no natural predators, outcompeted local flora and fauna, and took over whole ecosystems. But they all have their limitations: Kudzu dies in a hard freeze, carp can’t tolerate salt water, and pythons can’t cover long distances very fast. (Thankfully.)
Now imagine a species with all those benefits—foreign origin, no enemies—and no roadblocks to dominance: One that is indifferent to temperature, comfortable in many landscapes, able to run a lot faster than you, and muscular enough to leave a big dent in your car. That describes any of the possibly 6 million feral hogs in the United States, the most intractable invasives that most people have never heard of.