Most invasive species have a far greater impact in their new home than they did in the community in which they evolved. Why is this?

When a species leaves its community of origin behind, it also leaves behind the competitors, predators, and parasites with which it coevolved and which helped to keep its numbers in check. If the invasive species is a parasite, predator, or herbivore, it also leaves behind hosts or prey that had coevolved with it and had defenses against it. As a result, an invasive species often reaches a higher population density in its new home than it achieved in its old one.