Invited invaders
Species introduced in ballast water are inadvertent hitchhikers, but other invaders are introduced on purpose. Like agriculture on land, the industry of aquaculture ships marketable species worldwide. Thus, Japanese oysters have been grown in Europe, Canada, the U.S., China, Korea, New Zealand and Australia. Mediterranean mussels are grown in both the Atlantic and Pacific oceans. In the U.S., Atlantic lobsters were introduced to the Pacific coast and Pacific clams to the Atlantic.
For commercial and recreational fisheries, the story is similar. Atlantic shad were shipped to San Francisco in the 1870s and spread rapidly up the coast to British Columbia. Pacific salmon now thrive in the Great Lakes. Atlantic salmon, which failed to establish despite repeated introductions in the early 1900s, have now escaped from net pens and are reproducing in the rivers of Vancouver Island. In the late 1800s and early 1900s, European carp were shipped enthusiastically to ponds and streams across North America; today, brown trout and rainbow trout are still stocked in non-native locations.
Along with these intentionally introduced species have come a host of hitchhikers: for example, some fifteen species of clam, snail, and other invertebrates were inadvertently introduced to the Pacific Northwest with Atlantic and Japanese oyster shipments, including at least three oyster predators and parasites.
On land, invasion pathways are broadly similar: inadvertent introductions, intentional introductions, and their hitchhikers. Plants and insects travel in shipments of wood and produce, on car tires, in train cars. Ornamental plants are introduced, and soil nematodes come with them. Invaders sneak in as unseen pests in the fruit in our luggage, and are imported for biocontrol of agricultural pests, which are often invaders themselves as well.

Reducing the risk
Around the globe, humans are, inadvertently and intentionally, smearing the world's species across their natural boundaries and homogenizing communities that were once distinct. In the process, we are generating a host of health and environmental risks. How can we slow this onslaught? First off, not all introduced species survive. Of those that do, not all spread vigorously or reach pest proportions, and many go unnoticed for decades.
The trouble is, the science of predicting which invasions will be successful is only just beginning. Once a species has arrived, controlling it is typically neither pretty or, often, effective. It may take years of physical work, chemical application, and sometimes even the introduction of other, non-native predator species.
Since we do not yet know reliably which invasions will be benign and which disastrous, and since control efforts are at best challenging and at worst cause even further problems, the best way to avoid harmful and expensive future problems is to prevent species from invading in the first place. Luckily, since humans are doing the introducing, humans can also stop it.

(photo:Japanese oysters have been introduced in Europe, Canada, the U.S., China, Korea, New Zealand, and Australia)