Just one example: somewhere around 1995 a nursery in California, most likely in Marin or the North Bay, imported some plants from Asia. What they didn't know at the time was the leaves of those plants were infected with the fungus-like organism Phytophthora ramorum. That organism would eventually escape and infect tanoak and coast live oak throughout central California. A few years later it was found in Oregon. That organism causes the disease called sudden oak death. While it is difficult to make predictions, the impact on oaks in California will likely be profound. One study predicted that 90% of the coast live oak and black oak in California will be dead within the next 50 years. Animals that use acorns from these species for food will probably experience a decline, and one could imagine further ripple effects throughout the ecosystem.
If you live or work in Silicon Valley, and you look to the west at the coast range, most of that green stuff that you are seeing on the mountains is coast live oak woodland. Now imagine 30%, 60%, 90% of those trees dead.
(The above information is based on a talk by Matteo Garbelotto, a plant pathologist at UC Berkeley.)
By the way, you may of heard of California's "golden hills"? Non-native annual grasses. Brought to California by the Spanish when they first explored the state. Over time, the non-native grasses crowded out most of California's native perennial grasses. If you had lived in California 700 years ago, many of the hills wouldn't be golden, they would have been tinged green.