Stanford, California
Biological invasions are one of a set of ongoing global changes caused directly by humans, i.e. CFCs causing ozone depletion, increases in CO2 and methane, harvesting of natural populations of predators, biological invasions, land use changes, etc. These then have further effects, causing secondary changes, i.e. climate change and biodiversity loss (from land use and land cover changes).
Human activity is systematically breaking down the distinctiveness of ecosystems by accelerating the transport of organisms. People have accelerated the mobility of species, blurring the distinctiveness of biota. Winners and losers emerge from this process; some species will extend their range and populations while others will move toward extinction or be constrained. Charles Elton's 1958 book on species invasions is evidence that this is not a new concept.
Evidence for change in every aspect of how communities work is especially apparent on islands, though it is important everywhere. Examples include: California's Eurasian grasses/weeds that make the hills golden; fauna of aquatic systems in which native species are profoundly endangered as one set of invaders replaces another; the rebounding forests in Eastern North America (often cited as examples of the resilience of systems) are affected dramatically by exotic invasions. Wave after wave of invaders devastate natural ecosystems, yet the damage caused by invaders isn't seen as a serious threat. Why? No end to this process of blurring the biota of the Earth is in sight. How important are these changes?
The loss of biodiversity is perhaps the least reversible of all global changes, and it is dominantly caused by land use and land cover changes. The second least reversible global change is biological invasion, Vitousek says, and this breakdown of biogeographic barriers drives more extinction of species and reduction of diversity than any other factor except land use changes. And invasions interact with the other global changes.
Introduction of exotic grass species, particularly those that carry fires, are themselves an agent of land use change because they enable fires and lead to a large scale change from woody vegetation to grassland savanna. Even in the much longer term, beyond such direct reactions, human decisions to clear land can be predicated on the availability of exotic species to replace them.
Invasive animal species are also agents of disturbance that drive land use changes after their introduction. Why don't invasions get the attention they deserve, compared to, say, acid rain? Are they too familiar? Are there no disaster stories about them? Why is biological invasion treated as distinct from other components of global change when it could, in fact, be more important than many of the other factors?
Discussion
Most people don't know if species are native or exotic and are essentially unaware of the invasions problem. Chestnut, elm and fraser fur declines in eastern U.S. forests due to exotic pathogens was largely known about. But public perception of the magnitude of the problem is not great. Also, the issue of "blame" is a problem; there's no one to directly blame, as in issues like acid rain. Also, there are questions about what is natural and beautiful in an environment? Exotics such as calla lilies, blackberries, and pheasant, are seen as nice additions, not invaders. Is calling them invaders a form of racism? Biotic impoverishment of many parts of the Earth is largely due to invasions by exotics. It helps to stress the positive, i.e., protecting and supporting native species, rather than the negative, i.e., excluding exotic invaders.