Native Plants. How do we define a native plant?

All we grow are California native plants. Have for almost forever. That means we grow plants that grew within the boundaries of the state line in the year 1540 A.D. The Native Americans didn't move plants about much; a little, but not much. The first Europeans brought wild oats, filaree and other weeds and contaminated vast areas of the state in a few decades. Many of the images of California taken by the first cameras show that in the late 1800"s the hillsides were nothing but black from fires. No trees or shrubs left. It's hard to figure how anything has survived our onslaught on nature.
For your purposes, native plants are plants that lived on or within a few miles of your site in 1400. It's really hard to be site specific in much of California as some areas were completely  replaced by weeds, fire, and other activities of man. 
The closer you can get to what should grow there, the less problems you'll have with, herbivores like deer, soil conditions, rainfall, temperature, humidity, etc. Even though Redwood trees, (Sequoia sempervirens) are native to California, they can only exist without water in the areas where they historically grew. Now that is much of the coastal areas of California from about San Luis Obispo north, but it is not Bakersfield, Barstow or even Los Angeles. It gets even more ridiculous when planting a native from Michigan in your California garden and declaring it native. No, it's not. But plants like some of the Ceanothus (Mountain lilac), Arctostaphylos(manzanita),and  Quercus (oaks) range into neighboring states, and although the genetics are not a perfect match and the plants may not do as well as plants from your area, they are still closer than plants from other areas that never grew there. That is, a Black Oak (Quercus kelloggii) from Eugene, Oregon is going to have different tolerances and requirements than one found in Mesa Grande in San Diego County. But if you want to plant a Black Oak on a north slope in Redlands and you can't find a nursery that grows any in your area, either the Oregon or San Diego tree would be better than a Northern Red Oak(Quercus rubra) from Michigan.
Even if the plant is native from your area, it still could be the wrong plant for the site. A creek side plant will not live up on the hill without extra water, an upland plant will not live in the creek. A plant that grows in shade will probably fail in sun and visa versa. It could still be native there but, not from there, there.
The wildlife are much better supported if you use plants from your flyway or area. An example is the Buckeye (Aesculus californica). Many of the native bees, bumblebees, flies and butterflies use the nectar and develop protection from the nectar, where the European honey bees die from the nectar. AND, Buckeyes have a reputation for poisoning animals and people, but that rarely happens, as in almost never (once?), but alien filaree  has been documented in killing 60 head of cattle (McClintock, 1998). California native plants have been accused of being highly flammable, but most of the watered plants that Californians replace the natives with are easier to burn than the native species.
Science is not absolute, it is your best estimate of the facts. The only things that are absolute are death and religion, and they're beginning to question what is death.