I used to select the plants for my garden based on their appearance (and maybe price). As a Master Gardener I've gradually learned more about the role of our plant choices in providing habitat that is being lost at alarming rates. Though our part of the world often looks very green, the diversity and quality of habitat is in decline. Most of our wooded areas, for example, are full of invasive species. Often new trees are not able to grow due to heavy deer browsing. If each of us tries to plant natives -- canopy trees, understory trees, shrubs, perennials and groundcover -- we can help mitigate native plant losses.
Why plant a native tree? Here's just one example. The native dogwood (Cornus florda) supports 117 species of native moths and butterflies. The Asian dogwood (Cornus kousa) supports no native insect herbivores (from Dr. Doug Tallamy, University of Delaware entomologist). Often there are very complex relationships between plants and insects, birds and other animals. When a native plant is no longer growing in an area, creatures dependent on that plant are also lost. The web of life unravels.