The main reason that a newly introduced species would likely increase in number when placed into a new habitat can be explained by referencing the concept of niche. What is a niche? A niche can be understood as the way an organism makes a living. It is not a place, but rather an "occupation." It encompasses habitat, of course, but it includes everything about the life of the organism: how it gets it food, how ...
This solution provides an explanation for why a newly introduced species be likely to increase in numbers in a new habitat.