Why plant native?
Locally adapted native plants are the most ethical and most economical choice when making landscape decisions. Planting native is ethical in the sense that it restores habitat previously degraded by human activity, allowing wildlife and humans to coexist on a single piece of land. In most human dominated areas this has traditionally been impossible, as your local ecosystem rests on a foundation of native plants that have been cleared away to make room for human settlements and the non-native plants humans have brought with them. These native animals are often unable to use these non-native plants (and if they are, are then poisoned and killed by humans) and thus become relegated to the shrinking patches of wildland, often not large enough to sustain wildlife in the long run. Small, patchy habitat spaced far apart leaves animal and plant populations vulnerable to extinction, as even a small disturbance can eradicate a small population that has nowhere else to run.
Insects in particular are intimately associated with native species of plants, with herbivorous insects completely unable to use non-natives as a source of food. Native pollinators are often closely tied to only a few species of natives and do not pollinate other species at all. This includes the most charismatic of insects – bees and butterflies often earn our admiration for their beauty and industriousness. What is less understood is that they absolutely require native plants to survive and without them will quickly disappear from our landscapes.
The destruction of native habitat leads to a cascading ecological catastrophe, where the loss of native fauna leads to the loss of native insects and the loss of the birds, bats, reptiles, and amphibians that feed on them. Replacing non-native landscape plants with natives provides a massive ecological benefit, by turning barren suburban and urban environments into valuable wildlife habitat. Enough native plantings can create large continuous areas of habitat where wildlife can thrive, essentially resurrecting the a part of the original ecosystem of an area (with some modifications, to be sure) and increasing the overall biodiversity of an area. Habitat in general becomes more resilient and wildlife less likely to become extinct.
Planting native is also economical. Native plants are adapted to their environments and are often able to thrive without soil amendments, labor, pesticides, herbicides, or irrigation. They have coevolved with local insects and pathogens and are less likely to succumb to pest pressure, instead sustaining insect populations without suffering heavy damage. They have direct human benefits as well – they provide beauty and even food to people. An example would be camas in the PNW – it is a beautiful blue flower and a delicious food source.
When are natives not the best choice?
Natives are almost always the most appropriate landscape choice, however there are a few circumstances where non-natives are appropriate. The first is possibly the most obvious – most of our food plants are domesticated and non-native. Though I do encourage the use of plants native to our regions as a part of our diets, I recognize that they are not always practical as a main source of calories. Even if they are, a diet full of variety is important and life is made all the more exciting by having access to a wide variety of food plants. This is not even mentioning the importance of certain foods and herbs on a cultural level – people want to grow the plants they grew up with and are culturally significant to them.
Another important application of non-native plants occurs with a process called assisted migration. Many native species are only marginally adapted to the farthest borders (particularly the southernmost parts) of their range. As the climate shifts and the native species begin to struggle, species native to nearby areas that might be better adapted to the region can be grown. An example is growing some native Californian/Southern Oregonian species in Northern Oregon or Western Washington, like replacing a dying bigleaf maple with an Umbellularia tree. This is of course a controversial idea and should only be used with a great amount of care in the landscape. Please do not ever introduce non-native plants into a natural area.