I agree with Richard about the issue of non-native species in largely natural plant communities. This is not simply a problem for urban to wild contact zones. The dominance of natural areas, and invasion by aggressive non-native species is widespread. In my area, some of the most sensitive natural areas are in the process of being destroyed by aggressive non-native species. These include tidal freshwater wetlands, and high elevation meadows in the Coast Ranges of SW WA and NW OR, regionally these are called balds. Several native bulbs live in these balds, and are being crowded out by introduced daisies, grasses and other perennials. Meanwhile, the forestlands are progressively being occupied by woody shrubs, including gorse, Scots broom, and Atlantic ivy. We lost the dunes along the Pacific Ocean long ago to beach grasses in the genus Ammophila. The dunes are the only plant communities that are close to or in urban areas, and were also formerly home to several bulbs. Kathleen PNW Coast, zone 8, during a wet and windy Thanksgiving week.