I found a great definition of 'native plants'.
University of Maryland Extension:
Native to the ecoregion where it has evolved naturally in concert with soils, climate, fauna and other members of the plant community. This process of adaptation and evolution is ongoing and helps to perpetuate species even as conditions change. Even within its home eco-region, a species is native to a specific set of conditions.
Pretty much covers all the parameters that cause confusion. It's those 'specific set of conditions' that are key to identifying and growing natives. Some are more adaptable than others and have a wide native range, while others are more specific in their requirements and only grow in small areas. Then there are those whose 'specific set of conditions' (habitat) have been obliterated by human development or exploitation. California Coastal Sage Scrub is a big eco-region that stretches from San Diego to Marin County, but most of it is occupied by homes and businesses. That's what makes the species native to that eco-region so threatened.
