There are several reasons why scientists have thought invasives are able to do this, most of which involve how a particular species interacts with its new environment. First, invasives may be highly adaptable to a variety of climates, and can thus live pretty happily even when they’re displaced to a new region. Second, they are usually quite fecund, which ensures that their populations can grow exponentially. Third, they are usually aggressive when it comes to resources like nutrients and space, easily crowding out their native counterparts. Because those native species did not evolve together with these interlopers, they haven’t developed defenses against them. An example from Hawaii’s forests: the nasty invasive strawberry guava gives off a chemical that prevents all other plants from growing nearby, and it also sucks up more water than native species. And finally, invasives thrive when there are no predators, pests, or pathogens in their new space, which would normally keep their numbers down by killing them off or weakening them so they can’t reproduce.