The benefits for non native species of plants are actually quite numerous. Habitat, food, nesting materials etc.... The problem with most introduced plant species is their ability to spread. Plants like blackberries, loosestrife, scotch broom, butterfly bush, english ivy spread by different methods. MOST spread by seed and do it very well. Birds and wind deposit these easily germinated seeds into areas that creep farther and farther into natural areas. Some spread by seed and by stolons (modified roots) that search out the right conditions for starting a new clone. Some spread by layering, a process which a branch comes into contact with the soil and sprouts advantageous roots that then form a new replica of this plant. Some spread by all 3! The problem with these plants is that they are highly efficient at spreading and taking advantage of very specific habitats. Our native plants have evolved to live in these situations, these plants crave it! They are extremely aggressive and most natives are no match. The insects that have adapted to specific blooms, fruits or shelter are no longer able to count on them. A food source has been altered and the chain is broken. Salmon and birds suffer. The new invader river bamboo is rapidly displacing native streamside plants such as our native Dogwood and Elderberry. Many birds seek out the fruits of these natives and fatten up on them before migration. Some require it on their migration. Without it they are hungry and stressed and many will die or simply adapt to newer areas for a food source...again the chain is broken.
Simply removing blackberries in a city setting is a different matter. More people are there and it is less likely for native birds to visit these places....but it is more than just a non-native plant removal. It is a new view in our local cultures. Respect and understanding. These are exactly the kind of places where we can make huge changes in our practices. Kids and moms visit these areas, they see the signs and efforts being made and learn that the natives are important in our ecosystem. They have summer cabins and go camping with the rest of their families and friends......with a different level of respect for the natural world around them. Perhaps more importantly is the shift in local governments and their views of native landscaping. Funding is available and put into these areas. It may not make a difference right away but after a few years the net effect of this will be huge! All new buildings now in King County have a minimum % of native plants that must be used. Coalitions and watch dog organizations are now reviewing growth and managing the plants used in sensitive areas. Large developers are being held to strict landscaping codes and requiring set backs for streams and buffer zones for wildlilfe. The issue of fragmentation still exists but it is a good start.
Simply removing blackberries, ivy etc...is not enough, you are correct. It is a great first step though!

Bryan LaComa
Shoreline, Wa.
jennandbryan at msn.com