Another reason why it's important to have Native California plants in your garden, is to prevent the possibility of genetic erosion with nearby Natives. Basically non-native gardens can affect the genetic integrity of native plants that were meant to survive in our region of Southern California...so the overabundance of non-native gardens' adverse impact surpasses the metes and bounds of those specific properties, largely due to the birds & the bees.
"If the urban
plants are chosen from geographic
sources that do not reflect local conditions,
this interbreeding could undermine
the local adaptations and long
term viability of natural populations."
And we all know how important our natural populations are: especially since they are the one's most genetically able to soak up rain water and prevent unwanted stormdrain discharge into our oceans!
This is yet another reason we want to encourage land-owners and landscapers to choose, not only water-absorbing plants, but native strains of plants, that were grown naturally in open space. (If you just buy at a commercial nursery, you may be getting a genetic strain that is not native and may not survive in your garden, and may even contaminate the local species.)
Learn more on urban landscaping's effects on genetic erosion here: