A seemingly innocuous invader, the zebra mussel, has devastated fisheries and industries in its host lands. Source: USGS
The invasive snakehead. Source: USGS, Artist: Susan Trammell
Invasive species are often transported via the ballast water ships sometimes carry to help keep them level. Water is taken from the starting point of a journey and then released at the end point--along with any organisms living inside. High risk ballast in regards to transport of invasives is water taken onboard in a freshwater or estuarine port as those organisms have a high chance of surviving in their new environment (Portland University, 2006). Invasives are also released intentially-as with unwanted pet release or through aquarium dumping--such as the release of the red-eared slider in the United States.
An important aspect of the invasive species problem is that once an invasive is present in an estem, it is often impossible to remove it; thus, it is essential to prevent invasive species from arriving in the first place.