I'm not sure as in "from the textbook" but if you look at many invasive species, they are often spread by humans.
For example, when Europeans first landed in the Americas, they brought horses along with them, which are not native to America at all. Soon after that, some horses got loose and they bred in America very successfully since they had no natural predator.
Other examples are like earth worms, European crops, Asian carp, etc.
Humans often bring "new and improved" species to different countries not realizing that the new species may not have a natural predator to keep the population under control.
Basically, what this was all saying, is that most invasive species do spread due to the help of humans.
