It is actually people who carry invasive species with them from one place to another, sometimes without even knowing it. For example, a mosquito or other sort of bug my fly onto an airplane while it is loading passengers, and then be passed from one country or continent to another.
For big ships, when they do not have cargo they are too light to travel well in the water. So what they do is take up some water or other materials to make the ship heavier. This is called ballast. When the ship returns to harbor, it has to dump the ballast water or materials so that it can take up cargo. The animals and living things in the ballast water or soil or other materials then are introduced to that new area and are given the opportunity to expand and reproduce in a new environment.
Forgot to answer your second part of the question. To counter this, ships can be required to release their ballast a certain distance from the harbor and at different intervals.
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