These strangers are plants, not people, and usually arrive here for their otherwise commendable attributes of good looks, easy adaptability and fast growth. Examples are the numerous varieties of broom, a flowering shrub that turns places like the Nevada County community of North San Juan into a carpet of gold in spring. It's lovely to behold, but it has overwhelmed virtually every other native shrub. When native plants disappear, so do all of the animal species that depend on them for food and shelter. What's left behind are a small number of aggressive non-native plants.
Real evolution occurs over geologic time and creates a greater diversity of species.
Butler says there are others that seem well-behaved at first but display their greedy behavior when released into the wild. But environmentalists in parts of coastal Texas and Louisiana say it's taking over their wetland areas, and it's been banned as a noxious weed in Florida. Now, even though it's widely available in nurseries, the tallow tree is becoming a dominant species along the American River, according to the American River Parkway Exotics Management Project. Two more trees that are widely available in the nursery trade, Chinese pistache and silver maple, have established themselves along the American River Parkway and their level of invasiveness is being studied by the parkway management project. Surprisingly, even wildflower seed mixes have come under scrutiny, since they often contain seeds of plants native to other states or countries.