The distinction between a “naturalized”, “native”, “non-native”, “introduced”, “exotic” and “invasive” species can be a bit fuzzy. In fact, there are some articles in scientific journals that talk about the definitions and how these terms have been, and currently are, used as the building blocks of the emerging field of invasive species ecology. These definitions and uses can be problematic, and even confusing. There are many characteristics of invasive plants that make them "invasive" but the official definition comes from the federal government and is as follows:
USDA National Agricultural Library
What is an Invasive Species?
As per Executive Order 13112 an "invasive species" is defined as a species that is:
- non-native (or alien) to the ecosystem under consideration
- whose introduction causes or is likely to cause economic or environmental harm or harm to human health.
I personally like the terms - native vs. exotic coupled with invasive. As I believe that a native species, given the right conditions, can be rather invasive. And exotic is pretty easy to define as a plant that comes from a different geographical region and would not have gotten there without some help by humans. However, I subscribe to the idea that an "invasive" plant is one that grows and spreads aggressively and will out-compete and/or cause harm to native vegetation in the environment in which it is found.
To me, naturalized means that the species grows, spreads, and reproduces on its own and (most importantly) has little impact, especially economic impact. on the environment in which it lives. Dandelions and western salsify are commonly considered "naturalized" plant species, for example, and they are commonly found in Midwest yards and along roadsides.
Can plants be both naturalized and invasive? Consider that most of us don't like dandelions in our yards - the American public spends millions of dollars a year on controlling dandelions in yards. In this example, dandelions may be considered both "naturalized" and invasive. However, plants can be considered native (common to an area prior to European settlement) and still be invasive. An example of a “native” species that I find to be also invasive is black locust (Robinia pseudoacacia). It is native only to the southernmost region of Indiana, was spread by European settlers for the flowers and likely for fence posts and firewood, and now commonly invades (grows and spreads aggressively) grassland and prairie habitats. But again, that is the problem with definitions.
Visit the following articles for a more in depth look at this discussion:
"A Neutral terminology to define 'invasive' species"
by Robert I. Colautti and Hugh J. MacIsaac
Great Lakes Institute for Environmental Research, University of Windsor
"Naturalization and invasion of alien plants: concepts and definitions"
by David M. Richardson, Peter Pyšek, Marcel Rejmánek, Michael G. Barbour, F. Dane Panetta and Carol J. West
Institute for Plant Conservation, Botany Dept., University of Cape Town
Matthew S. Kraushar, Habitat Specialist
Forestry & Natural Resources, Purdue University