Absolute extinction is not the only danger, serious disruption to an ecosystem occurs much more often than the obliteration of a single species.
When I finished college in 1983, I left Wisconsin and headed out to build a career in other parts of the county. The woodlands were notable for their lovely displays of spring wildflowers like trout lilies, dogtooth violets, jack-in-the-pulpet and lady slipper orchids. Upon my return in 1997, I found that nearly every woodland in southern Wisconsin was ringed by dense stands of honeysuckle bushes, buckthorn and biennial called garlic mustard. These three plants had completely changed the character of the woodland environment across an entire region. Woodlands chocked by these invasive plants saw almost no sunlight getting to the soil. Slopes and small ravines lost tremendous amounts of topsoil, eroded because the native plant community had been replaced by a small number of plants, none of which was good holding or building the soil.