How humans effect the environment
How they are introduced to an area.
Most of them were through human activity, deliberate or accidental. Most deliberate introductions were undertaken by people who meant well but didn't know what would happen.
Exotic isn't always bad.
Some non native species actually help the environment like the "invasive" tamarisk shrubs that the endangered southwestern willow flycatcher nests in. Some were introduced deliberately by humans to keep a plant species from going extinct. The Aldabra tortoise is an example of this when it was introduced to the islands of Mauritius after it lost all of its native large tortoises.
However, the same can't be said for the zebra mussels(See the image on the right). They were introduced to the Great Lakes in the US inadvertently in 1988 and has spread dramatically. They can stick to almost anything, even other mussels and crayfish. Because of this, they have caused many problems by clogging water intake and discharge pipes among other this. They have also out competed the native clams.
Should Exotic species be removed?
This is a heavily debated topic. In the case of the zebra mussels, they should at least be contained before they can cause any more damage. However, not all Exotic species are bad to an ecosystem, most are harmless but some may even be beneficial to an ecosystem. For example, in one ecosystem, the "invasive" tamarisk shrubs have become a nesting place for the endangered southwestern willow flycatcher. In this case, removing the shrubs would take away a home for these endangered birds and most likely cause a decline in the southwestern willow flycatcher population. In some cases, these Exotic species are endangered in the places where they are from. For example, the red crowned Amazon parrot flourishes in cities in California and Hawaii as a non native species but in northwest Mexico, where it's from, it's endangered. Whether or not a Exotic species should be removed from where it's been introduced should only be decided once all the facts have been found.
Is the introduction of Exotic species to new ecosystems preventable?
People can make their best attempt but it won't prevent it in the long run. Most introductions of these species to an area is through human activity, accidental or deliberate. But it is also caused by it. As humans improve technology and machines, the pollution these things give off is causing climate changes that in turn cause species to find new homes that they can live in. It is not fully preventable but not much can be done to prevent it. It should accepted that not all Exotic species are harmful and can stay were they were introduced.