Hi Wendy, from my reading on the net, I have learnt that due to modern transportation systems and an ever more global economy, our biotic mixing — intentional and unintentional — has increased at a frightening pace.
Some scientists have dubbed the current era the Homogocene, a time when weedy generalist species will take over large portions of the globe, pushing out the specialist species that developed in isolation. For example the quagga mussel, is one of those weedy winners. It has taken to North American waters — and within the last year the lower Colorado River — as if it always belonged here.
So why should we care about yet another species flexing its muscle in the New World? First, as they multiply and clog aqueducts and pipes, the quagga, and its relative, the zebra mussel, could cause an immense and expensive headache for Western water managers. Secondly it is difficult it is to stop the movement of just one invasive species, even when you have decades to prepare, and you mount extensive education campaigns.
If you want any region to support a large proportion of its native flora and fauna, countries will need a much higher level of societal commitment to exotics control than has been shown to date. Otherwise, the West, long seen as a region isolated by its vast distances and harsh climate, will support ecosystems that look no different than any other semi-arid place on Earth.