Beyond the invasive species itself, the community it is introduced to has characteristics determining how susceptible it is. Darwin himself wrote, in On the Origin of Species, “High species diversity should result in limiting resources being used more completely, thereby preventing invasion by potential competitors.” While this statement is correct, the real situation is a little more complicated. High species diversity does indeed indicate that more of the possible ecological niches are exploited, and exploited more fully, and in small, species rich areas invasion is harder. At the larger scale, however, high species diversity is more indicative of simply more favourable resources. In this case, the invasive species may actually do better, as it too has a more advantageous environment, while the other abilities which allow it to invade remain undiminished.
The fact that the members of the community being invaded have evolved together may also work to the invader’s advantage. Many predator/prey groups show co-evolutionary ‘arms-races’, in which they both become more specialised – the prey become harder for the predator to catch and/or eat, and so the predator in turn becomes better at catching and/or eating it! For instance, many creatures have tough shells to avoid being eaten. Birds which prey upon such animals have evolved various beak forms, some of them very specialised for certain prey. It may be so specific to one type of shell, that when a more basic form arrives it is unable to eat it, so the new species is not eaten. The more basic form could even be at an advantage over the native prey, for it may not be investing so much in building up defences.
This type of interaction also occurs for diseases - as a plant evolves to produce certain molecules for defence against a pathogen, the pathogen may then evolve to start its attack by blocking this defensive component. The host then begins its defence by indentifying this new pathogenic compound, a cycle which culminates in very specific compounds produced by the host being required before the pathogen can attack. As a result, a new species, which even though it has not evolved any defences for the new diseases it encounters, may be able to avoid attack. The converse of these predator and pathogen specialisations is true for the invading species; by moving it has escaped from predators and parasites which had evolved to attack it efficiently, and from any competitors which may have adapted specifically to outcompete it.