Final answer:
The second factor that assists introduced species in establishing large populations in new communities is the presence of naive prey, which are vulnerable to predation.
Step-by-step explanation:
The question addresses the factors that help introduced species survive and reproduce successfully in a new community. When considering the two primary factors for an introduced species' success, it is evident that these species often find themselves in environments where there are naive prey and absent predators. The absence of natural predators allows them to reproduce without check, and the availability of naive prey that have not evolved defenses against these newcomers means there is ample food source. Therefore, based on the information provided, the second factor that helps introduced species to establish big populations in their new environment tends to be the naive prey (b) they find in the community, which are unused to the presence of the new predator and thus vulnerable to predation.